CHAPTER VIII CARBON AND THE HYDROCARBONS

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It is necessary to clearly distinguish between the two closely-allied terms, charcoal and carbon. Charcoal is well known to everybody, although it is no easy matter to obtain it in a chemically pure state. Pure charcoal is a simple, insoluble, infusible, combustible substance produced by heating organic matter, and has the familiar aspect of a black mass, devoid of any crystalline structure, and completely insoluble. Charcoal is a substance possessing a peculiar combination of physical and chemical properties. This substance, whilst in a state of ignition, combines directly with oxygen; in organic substances it is found in combination with hydrogen, oxygen, nitrogen, and sulphur. But in all these combinations there is no real charcoal, as in the same sense there is no ice in steam. What is found in such combinations is termed ‘carbon’—that is, an element common to charcoal, to those substances which can be formed from it, and also to those substances from which it can be obtained. Carbon may take the form of charcoal, but occurs also as diamond and as graphite. Truly no other element has such a wide terminology. Oxygen is always called ‘oxygen,’ whether it is in a free gaseous state, or in the form of ozone, or oxygen in water, or in nitric acid or in carbonic anhydride. But here there is some confusion. In water it is evident that there is no oxygen in a gaseous form, such as can be obtained in a free state, no oxygen in the form of ozone, but a substance which is capable of producing both oxygen, ozone, and water. As an element, oxygen possesses a known chemical individuality, and an influence on the properties of those combinations into which it enters. Hydrogen gas is a substance which reacts with difficulty, but the element hydrogen represents in its combinations an easily displaceable component part. Carbon may be considered as an atom of carbon matter, and charcoal as a collection of such atoms forming a whole substance, or mass of molecules of the substance. The accepted atomic weight of carbon is 12, because that is the least quantity of carbon which enters into combination in molecules of its compounds; but the weight of the molecules of charcoal is probably very much greater. This weight remains unknown because charcoal is capable of but few direct reactions and those only at a high temperature (when the weight of its molecules probably changes, as when ozone changes into oxygen), and it does not turn into vapour. Carbon exists in nature, both in a free and combined state, in most varied forms and aspects. Carbon in a free state is found in at least three different forms, as charcoal, graphite, and the diamond. In a combined state it enters into the composition of what are called organic substances—a multitude of substances which are found in all plants and animals. It exists as carbonic anhydride both in air and in water, and in the soil and crust of the earth as salts of carbonic acid and as organic remains.

The variety of the substances of which the structure of plants and animals is built up is familiar to all. Wax, oil, turpentine, and tar, cotton and albumin, the tissue of plants and the muscular fibre of animals, vinegar and starch, are all vegetable and animal matters, and all carbon compounds.[1] The class of carbon compounds is so vast that it forms a separate branch of chemistry, known under the name of organic chemistry—that is, the chemistry of carbon compounds, or, more strictly, of the hydrocarbons and their derivatives.

If any one of these organic compounds be strongly heated without free access of air—or, better still, in a vacuum—it decomposes with more or less facility. If the supply of air be insufficient, or the temperature be too low for combustion (see Chapter III.), and if the first volatile products of transformation of the organic matter are subjected to condensation (for example, if the door of a stove be opened), an imperfect combustion takes place, and smoke, with charcoal or soot, is formed.[2] The nature of the phenomenon, and the products arising from it, are the same as those produced by heating alone, since that part which is in a state of combustion serves to heat the remainder of the fuel. The decomposition which takes place on heating a compound composed of carbon, hydrogen, and oxygen is as follows:—A part of the hydrogen is separated in a gaseous state, another part in combination with oxygen, and a third part separates in combination with carbon, and sometimes in combination with carbon and oxygen in the form of gaseous or volatile products, or, as they are also called, the products of dry distillation. If the vapours of these products are passed through a strongly heated tube, they are changed again in a similar manner and finally resolve themselves into hydrogen and charcoal. Altogether these various products of decomposition contain a smaller amount of carbon than the original organic matter; part of the carbon remains in a free state, forming charcoal.[3] It remains in that space where the decomposition took place, in the shape of the black, infusible, non-volatile charcoal familiar to all. The earthy matter and all non-volatile substances (ash) forming a part of the organic matter, remain behind with the charcoal. The tar-like substances, which require a high temperature in order to decompose them, also remain mixed with charcoal. If a volatile organic substance, such as a gaseous compound containing oxygen and hydrogen, be taken, the carbon separates on passing the vapour through a tube heated to a high temperature. Organic substances when burning with an insufficient supply of air give off soot—that is, charcoal—proceeding from carbon compounds in a state of vapour, the hydrogen of which has, by combustion, been converted into water; so, for instance, turpentine, naphthalene, and other hydrocarbons which are with difficulty decomposed by heat, easily yield carbon in the form of soot during combustion. Chlorine and other substances which, like oxygen, are capable of taking up hydrogen, and also substances which are capable of taking up water, can also separate carbon from (or char) most organic substances.

Wood charcoal is prepared in large quantities in a similar manner—that is, by the partial combustion of wood.[4] In nature a similar process of carbonisation of vegetable refuse takes place in its transformation under water, as shown by the marshy vegetation which forms peat.[5] In this manner doubtless the enormous masses of coal were formed[6] which, following the example set by England, are now utilised everywhere as the principal material for heating steam[345]
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boilers, and in general for all purposes of heating and burning.[7] Russia possesses many very rich coalfields, amongst which the Donetz district is most worthy of remark.[8]

During the imperfect combustion of volatile substances containing carbon and hydrogen, the hydrogen and part of the carbon first burn, and the remainder of the carbon forms soot. Tar, pitch, and similar substances for this reason burn with a smoky flame. Thus soot is finely-divided charcoal separated during the imperfect combustion of the vapours and gases of carbonaceous substances rich in carbon. Specially-prepared soot (lampblack) is very largely used as a black paint and a large quantity goes for the manufacture of printers' ink. It is prepared by burning tar, oil, natural gas, naphtha, &c. The quantity of organic matter remaining undecomposed in the charcoal depends on the temperature to which it has been submitted. Charcoal prepared at the lowest temperature still contains a considerable quantity of hydrogen and oxygen—even as much as 4 p.c. of hydrogen and 20 p.c. of oxygen. Such charcoal still preserves the structure of the substance from which it was obtained. Ordinary charcoal, for instance, in which the structure of the tree is still visible, is of this kind. On submitting it to further heating, a fresh quantity of hydrogen with carbon and oxygen (in the form of gases or volatile matter) may be separated, and the purest charcoal will be obtained on submitting it to the greatest heat.[9] If it be required to prepare pure charcoal from soot it is necessary first to wash it with alcohol and ether in order to remove the soluble tarry products, and then submit it to a powerful heat to drive off the impurities containing hydrogen and oxygen. Charcoal however when completely purified does not change in appearance. Its porosity,[10] bad conducting power for heat, capability of absorbing the luminous rays (hence its blackness and opacity), and many other qualities, are familiar from everyday experience.[11] The specific gravity of charcoal varies from 1·4 to 1·9, and that it floats on water is due to the air contained in its pores. If charcoal is reduced to a powder and moistened with spirit, it immediately sinks in water. It is infusible in the furnace and even at the temperature of the oxyhydrogen flame. In the heat generated by means of a powerful galvanic current charcoal only softens but does not completely melt, and on cooling it is found to have undergone a complete change both in properties and appearance, and is more or less transformed into graphite. The physical stability of charcoal is without doubt allied to its chemical stability. It is evidently a substance devoid of energy, for it is insoluble in all known liquids, and at an ordinary temperature does not combine with anything; it is an inactive substance, like nitrogen.[12] But these properties of charcoal change with a rise of temperature; thus, unlike nitrogen, charcoal, at a high temperature, combines directly with oxygen. This is well known, as charcoal burns in air. Indeed, not only does oxygen combine with charcoal at a red heat, but sulphur, hydrogen, silicon, and also iron and some other metals[12 bis] do so at a very high temperature—that is, when the molecules of the charcoal have reached a state of great instability—whilst at ordinary temperatures neither oxygen, sulphur, nor metals act on charcoal in any way. When burning in oxygen, charcoal forms carbonic anhydride, CO2, whilst in the vapours of sulphur, carbon bisulphide, CS2, is formed, and wrought iron, when acted on by carbon, becomes cast iron. At the great heat obtained by passing the galvanic current through carbon electrodes, charcoal combines with hydrogen, forming acetylene, C2H2. Charcoal does not combine directly with nitrogen, but in the presence of metals and alkaline oxides, nitrogen is absorbed, forming a metallic cyanide, as, for instance, potassium cyanide, KCN. From these few direct combinations which charcoal is capable of entering into, may be derived those numerous carbonaceous compounds which enter into the composition of plants and animals, and can be thus obtained artificially. Certain substances containing oxygen give up a part of it to charcoal at a relatively low temperature. For instance, nitric acid when boiled with charcoal gives carbonic anhydride and nitric peroxide. Sulphuric acid is reduced to sulphurous anhydride when heated with carbon. When heated to redness charcoal absorbs oxygen from a large number of the oxides. Even such oxides as those of sodium and potassium, when heated to redness, yield their oxygen to charcoal although they do not part with it to hydrogen. Only a few of the oxides, like silica (oxide of silicon) and lime (calcium oxide) resist the reducing action of charcoal. Charcoal is capable of changing its physical condition without undergoing any alteration in its essential chemical properties—that is, it passes into isomeric or allotropic forms. The two other particular forms in which carbon appears are the diamond and graphite. The identity of composition of these with charcoal is proved by burning an equal quantity of all three separately in oxygen (at a very high temperature), when each gives the same quantity of carbonic anhydride—namely, 12 parts of charcoal, diamond, or graphite in a pure state, yield on burning 44 parts by weight of carbonic anhydride. The physical properties present a marked contrast; the densest sorts of charcoal have a density of only 1·9, whilst the density of graphite is about 2·3, and that of the diamond 3·5. A great many other properties depend on the density, for instance combustibility. The lighter charcoal is, the more easily it burns; graphite burns with considerable difficulty even in oxygen, and the diamond burns only in oxygen and at a very high temperature. On burning, charcoal, the diamond, and graphite develop different quantities of heat. One part by weight of wood charcoal converted by burning into carbonic anhydride develops 8,080 heat units; dense charcoal separated in gas retorts develops 8,050 heat units; natural graphite, 7,800 heat units; and the diamond 7,770. The greater the density the less the heat evolved by the combustion of the carbon.[13]

By means of intense heat charcoal may be transformed into graphite. If a charcoal rod 4 mm. in diameter and 5 mm. long be enclosed in an exhausted receiver and the current from 600 Bunsen's elements, placed in parallel series of 100, be passed through it, the charcoal becomes strongly incandescent, partially volatilises, and is deposited in the form of graphite. If sugar be placed in a charcoal crucible and a powerful galvanic current passed through it, it is baked into a mass similar to graphite. If charcoal be mixed with wrought iron and heated, cast iron is formed, which contains as much as five per cent. of charcoal. If molten cast iron be suddenly chilled, the carbon remains in combination with the iron, forming so called white cast iron; but if the cooling proceeds slowly, the greater part of the carbon separates in the form of graphite, and if such cast iron (so called grey cast iron) be dissolved in acid, the carbon remains in the form of graphite. Graphite is met with in nature, sometimes in the form of large compact masses, sometimes permeating rocky formations like the schists or slates, and in fact is met with in those places which, in all probability, have been subjected to the action of subterranean heat.[14] The graphite in cast iron, and sometimes also natural graphite, occasionally appears in a crystalline form in the shape of six-sided plates, but more often it occurs as a compact amorphous mass having the characteristic properties of the familiar black-lead pencil.[15]

The diamond is a crystalline and transparent form of carbon. It is of rare occurrence in nature, and is found in the alluvial deposits of the diamond mines of Brazil, India, South Africa, &c. It has also been found in meteorites.[15 bis] It crystallises in octahedra, dodecahedra, cubes, and other forms of the regular system.[16] The efforts which have been made to produce diamonds artificially, although they have not been fruitless, have not as yet led to the production of large-sized crystals, because those means by which crystals are generally formed are inapplicable to carbon. Indeed, carbon in all its forms being insoluble and infusible does not pass into a liquid condition by means of which crystallisation could take place. Diamonds have several times been successfully produced in the shape of minute crystals having the appearance of a black powder, but when viewed under the microscope appearing transparent, and possessing that hardness which is the peculiar characteristic of the diamond. This diamond powder is deposited on the negative electrode, when a weak galvanic current is passed through liquid chloride of carbon.[16 bis]

Moissan (Paris, 1893) produced diamonds artificially by means of the high temperature attained in the electrical furnace[17] by dissolving carbon in molten cast iron, and allowing the solution with an excess of carbon, to cool under the powerful pressure exerted by rapidly cooling the metal.[17 bis] K. Chroustchoff attained the same end by means of silver, which dissolves carbon to the extent of 6 p.c. at a high temperature. Rousseau, for the same purpose, heated carbide of calcium in the electric furnace. There is no doubt that all these investigators obtained the diamond as a transparent body, which burnt into CO2, and possessed an exceptional hardness, but only in the form of a fine powder.

Judging from the fact that carbon forms a number of gaseous bodies (carbonic oxide, carbonic anhydride, methane, ethylene, acetylene, &c.) and volatile substances (for example, many hydrocarbons and their most simple derivatives), and considering that the atomic weight of carbon, C = 12, approaches that of nitrogen, N = 14, and that of oxygen, O = 16, and that the compounds CO (carbonic oxide) and N2C2 (cyanogen) are gases, it may be argued that if carbon formed the molecule C2, like N2 and O2, it would be a gas. And as through polymerism or the combination of like molecules (as O2 passes into O3 or NO2 into N2O4) the temperatures of ebullition and fusion rise (which is particularly clearly proved with the hydrocarbons of the CnH2n series), it ought to be considered that the molecules of charcoal, graphite, and the diamond are very complex, seeing that they are insoluble, non-volatile, and infusible. The aptitude which the atoms of carbon show for combining together and forming complex molecules appears in all carbon compounds. Among the volatile compounds of carbon many are well known the molecules of which contain C5 ... C10 ... C20 ... C30, &c., in general Cn where n may be very large, and in none of the other elements is this faculty of complexity so developed as in carbon.[18] Up to the present time there are no grounds for determining the degree of polymerism of the charcoal, graphite, or diamond molecules, and it can only be supposed that they contain Cn where n is a large quantity. Charcoal and those complex non-volatile organic substances which represent the gradual transitions to charcoal[19] and form the principal solid substances of organisms, contain a store or accumulation of internal power in the form of the energy binding the atoms into complex molecules. When charcoal or complex compounds of carbon burn, the energy of the carbon and oxygen is turned into heat, and this fact is taken advantage of at every turn for the generation of heat from fuel.[20]

No other two elements are capable of combining together in such variety as carbon and hydrogen. The hydrocarbons of the CnH2m series in many cases differ widely from each other, although they have some properties in common. All hydrocarbons, whether gaseous, liquid or solid, are combustible substances sparingly soluble or insoluble in water. The liquefied gaseous hydrocarbons, as well as those which are liquid at ordinary temperatures, and those solid hydrocarbons which have been liquefied by fusion, have the appearance and property of oily liquors, more or less viscid, or fluid.[21] The solid hydrocarbons more or less resemble wax in their properties, although ordinary oils and wax generally contain oxygen in addition to carbon and hydrogen, but in relatively small proportion. There are also many hydrocarbons which have the appearance of tar—as, for instance, metacinnamene and gutta-percha. Those liquid hydrocarbons which boil at a high temperature are like oils, and those which have a low boiling point resemble ether, whilst the gaseous hydrocarbons in many of their properties are akin to hydrogen. All this tends to show that in hydrocarbons physically considered the properties of solid non-volatile charcoal are strongly modified and hidden, whilst those of the hydrogen predominate. All hydrocarbons are neutral substances (neither basic nor acid), but under certain conditions they enter into peculiar reactions. It has been seen in those hydrogen compounds which have been already considered (water, nitric acid, ammonia) that the hydrogen in almost all cases enters into reaction, being displaced by metals. The hydrogen of the hydrocarbons, it may be said, has no metallic character that is to say, it is not directly[22] displaced by metals, even by such as sodium and potassium. On the application of more or less heat all hydrocarbons decompose[23] forming charcoal and hydrogen. The majority of hydrocarbons do not combine with the oxygen of the air or oxidise at ordinary temperatures, but under the action of nitric acid and many other oxidising substances most of them undergo oxidation, in which either a portion of the hydrogen and carbon is separated, or the oxygen enters into combination, or else the elements of hydrogen peroxide enter into combination with the hydrocarbon.[24] When heated in air, hydrocarbons burn, and, according to the amount of carbon they contain, their combustion is attended more or less with a separation of soot—that is, finely divided charcoal—which imparts great brilliancy to the flame, and on this account many of them are used for the purposes of illumination—as, for instance, kerosene, coal gas, oil of turpentine. As hydrocarbons contain reducing elements (that is, those capable of combining with oxygen), they often act as reducing agents—as, for instance, when heated with oxide of copper, they burn, forming carbonic anhydride and water, and leave metallic copper. Gerhardt proved that all hydrocarbons contain an even number of hydrogen atoms. Therefore, the general formula for all hydrocarbons is CnH2m where n and m are whole numbers. This fact is known as the law of even numbers. Hence, the simplest possible hydrocarbons ought to be: CH2, CH4, CH6 ... C2H2, C2H4, C2H6, C2H8 ... but they do not all exist, since the quantity of H which can combine with a certain amount of carbon is limited, as we shall learn directly.

Some of the hydrocarbons are capable of combination, whilst others do not show that power. Those which contain less hydrogen belong to the former category, and those which, for a given quantity of carbon, contain the maximum amount of hydrogen, belong to the latter. The composition of those last mentioned is expressed by the general formula CnH2n + 2. These so-called saturated hydrocarbons are incapable of combination.[25] The hydrocarbons CH6, C2H8, C3H10, &c.... do not exist. Those containing the maximum amount of hydrogen will be represented by CH4 (n = 1, 2n + 2 = 4), C2H6 (n = 2), C3H8 (n = 3), C4H10, &c. This may be termed the law of limits. Placing this in juxtaposition with the law of even numbers, it is easy to perceive that the possible hydrocarbons can be ranged in series, the terms of which may be expressed by the general formulÆ CnH2n+2, CnH2n, CnH2n-2, &c.... Those hydrocarbons which belong to any one of the series expressible by a general formula are said to be homologous0 with one another. Thus, the hydrocarbons CH4, C2H6, C3H8, C4H10, &c.... are members of the limiting (saturated) homologous series CnH2n+2. That is, the difference between the members of the series is CH2.[26] Not only the composition but also the properties of the members of a series tend to classification in one group. For instance, the members of the series CnH2n+2 are not capable of forming additive compounds, whilst those of the series CnH2n are capable of combining with chlorine, sulphuric anhydride, &c.; and the members of the CnH2n-6 group, belonging to the coal tar series, are easily nitrated (give nitro-compounds, Chapter VI.), and have other properties in common. The physical properties of the members of a given homologous series vary in some such manner as this; the boiling point generally rises and the internal friction increases as n increases[27]—that is, with an increase in the relative amount of carbon and the atomic weight; the specific gravity also regularly changes as n becomes greater.[28]

Many of the hydrocarbons met with in nature are the products of organisms, and do not belong to the mineral kingdom. A still greater number are produced artificially. These are formed by what is termed the combination of residues. For instance, if a mixture of the vapours of hydrogen sulphide and carbon bisulphide be passed through a tube in which copper is heated, this latter absorbs the sulphur from both the compounds, and the liberated carbon and hydrogen combine to form a hydrocarbon, methane. If carbon be combined with any metal and this compound MCn be treated with an acid HX, then the haloid X will give a salt with the metal and the residual carbon and hydrogen will give a hydrocarbon. Thus cast iron which contains a compound of iron and carbon gives liquid hydrocarbons like naphtha under the action of acids. If a mixture of bromo-benzene, C6H5Br, and ethyl bromide, C2H5Br, be heated with metallic sodium, the sodium combines with the bromine of both compounds, forming sodium bromide, NaBr. From the first combination the group C6H5 remains, and from the second C2H5. Having an odd number of hydrogen atoms, they, in virtue of the law of even numbers, cannot exist alone, and therefore combine together forming the compound C6H5.C2H5 or C8H10 (ethylbenzene). Hydrocarbons are also produced by the breaking up of more complex organic or hydrocarbon compounds, especially by heating—that is, by dry distillation. For instance, gum-benzoin contains an acid called benzoic acid, C7H6O2, the vapours of which, when passed through a heated tube, split up into carbonic anhydride, CO2, and benzene, C6H6. Carbon and hydrogen only unite directly in one ratio of combination—namely, to form acetylene, having the composition C2H2, which, as compared with other hydrocarbons, exhibits a very great stability at a somewhat high temperature.[29]

There is one substance known among the saturated hydrocarbons composed of 1 atom of carbon and 4 atoms of hydrogen; this is a compound containing the highest percentage of hydrogen (CH4 contains 25 per cent. of hydrogen), and at the same time it is the only hydrocarbon whose molecule contains but a single atom of carbon. This saturated hydrocarbon, CH4, is called marsh gas or methane. If vegetable or animal refuse suffers decomposition in a space where the air has not free access, or no access at all, then the decomposition is accompanied with the formation of marsh gas, and this either at the ordinary temperature, or at a comparatively much higher one. On this account plants, when decomposing under water in marshes, give out this gas.[29 bis] It is well known that if the mud in bogs be stirred up, the act is accompanied with the evolution of a large quantity of gas bubbles; these may, although slowly, also separate of their own accord. The gas which is evolved consists principally of marsh gas.[30] If wood, coal, or many other vegetable or animal substances are decomposed by the action of heat without access of air—that is, are subjected to dry distillation—they, in addition to many other gaseous products of decomposition (carbonic anhydride, hydrogen, and various other substances), evolve a great deal of methane. Generally the gas which is used for lighting purposes is obtained by this means and therefore always contains marsh gas, mixed with dry hydrogen and other vapours and gases, although it is subsequently purified from many of them.[31][362]
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As the decomposition of the organic matter, which forms coal, is still going on underground, the evolution of large quantities of marsh gas frequently occurs in coal-mines.[32] When mixed with air it forms an explosive mixture, which forms one of the great dangers of coal mining, as subterranean work has always to be carried on by lamp-light. This danger is, however, overcome by the use of Humphry Davy's safety lamp.[33] Sir Humphry Davy observed that on introducing a piece of wire gauze into a flame, it absorbs so much heat that combustion does not proceed beyond it (the unburnt gases which pass through it may be ignited on the other side). In accordance with this, the flame of the Davy lamp is surrounded with a thick glass (as shown in the drawing), and has no communication whatever with the explosive mixture except through a wire gauze which prevents it igniting the mixture of the marsh-gas issuing from the coal with air. In some districts, particularly in those where petroleum is found—as, for instance, near Baku, where a temple of the Indian fire-worshippers was built, and in Pennsylvania, and other places—marsh gas in abundance issues from the earth, and it is used, like coal gas, for the purposes of lighting and warming.[34] Tolerably pure marsh gas[35] may be obtained by heating a mixture of an acetate with an alkali. Acetic acid, C2H4O2, on being heated is decomposed into marsh gas and carbonic anhydride, C2H4O2 = CH4 + CO2.

An alkali—for instance, NaHO—gives with acetic acid a salt, C2H3NaO2, which on decomposition retains carbonic anhydride, forming a carbonate, Na2CO3, and marsh gas is given off:

C2H3NaO2 + NaHO = Na2CO3 + CH4

Marsh gas is difficult to liquefy; it is almost insoluble in water, and is without taste or smell. The most important point in connection with its chemical reactions is that it does not combine directly with anything, whilst the other hydrocarbons which contain less hydrogen than expressed by the formula CnH2n + 2 are capable of combining with hydrogen, chlorine, certain acids, &c.

If the law of substitution gives a very simple explanation of the formation of hydrogen peroxide as a compound containing two aqueous residues (OH)(OH), then on the basis of this law all hydrocarbons ought to be derived from methane, CH4, as being the simplest hydrocarbon.[36] The increase in complexity of a molecule of methane is brought about by the faculty of mutual combination which exists in the atoms of carbon, and, as a consequence of the most detailed study of the subject, much that might have been foreseen and conjectured from the law of substitution has been actually brought about in such a manner as might have been predicted, and although this subject on account of its magnitude really belongs, as has been already stated, to the sphere of organic chemistry, it has been alluded to here in order to show, although only in part, the best investigated example of the application of the law of substitution. According to this law, a molecule of methane, CH4, is capable of undergoing substitution in the four following ways:—(1) Methyl substitution, when the radicle, equivalent to hydrogen, called methyl CH3, replaces hydrogen. In CH4 this radicle is combined with H and therefore can replace it, as (OH) replaces H because with it it gives water; (2) methylene substitution, or the exchange between H2 and CH2 (this radicle is called methylene), is founded on a similar division of the molecule CH4 into two equivalent parts, H2 and CH2; (3) acetylene substitution, or the exchange between CH on the one hand and H3 on the other; and (4) carbon substitution—that is, the substitution of H4 by an atom of carbon C, which is founded on the law of substitution just as is the methyl substitution. These four cases of substitution render it possible to understand the principal relations of the hydrocarbons. For instance, the law of even numbers is seen from the fact that in all the cases of substitution mentioned the hydrogen atoms increase or decrease by an even number; but as in CH4 they are likewise even, it follows that no matter how many substitutions are effected there will always be obtained an even number of hydrogen atoms. When H is replaced by CH3 there is an increase of CH2; when H2 is replaced by CH2 there is no increase of hydrogen; in the acetylene substitution CH replaces H3, therefore there is an increase of C and a decrease of H2; in the carbon substitution there is a decrease of H4. In a similar way the law of limit may be deduced as a corollary of the law of substitution. For the largest possible quantity of hydrogen is introduced by the methyl substitution, since it leads to the addition of CH2; starting from CH4 we obtain C2H6, C3H8, and in general, CnH2n+2, and these contain the greatest possible amount of hydrogen. Unsaturated hydrocarbons, containing less hydrogen, are evidently only formed when the increase of the new molecule derived from methane proceeds from one of the other forms of substitution. When the methyl substitution alone takes place in methane, CH4, it is evident that the saturated hydrocarbon formed is C2H6 or (CH3)(CH3).[37] This is called ethane. By means of the methylene substitution alone, ethylene, C2H4, or (CH2)(CH2) may be directly obtained from CH4, and by the acetylene substitution C2H2 or (CH)(CH), or acetylene, both the latter being unsaturated hydrocarbons. Thus we have all the possible hydrocarbons with two atoms of carbon in the molecule, C2H6, ethane, C2H4, ethylene, and C2H2, acetylene. But in them, according to the law of substitution, the same forms of substitution may be repeated—that is, the methyl, methylene, acetylene, and even carbon substitutions (because C2H6 will still contain hydrogen when C replaces H4) and therefore further substitutions will serve as a source for the production of a fresh series of saturated and unsaturated hydrocarbons, containing more and more carbon in the molecule and, in the case of the acetylene substitution and carbon substitution, containing less and less hydrogen. Thus by means of the law of substitution we can foresee not only the limit CnH2n+2, but an unlimited number of unsaturated hydrocarbons, CnH2n, CnH2n-2 ... CnH2(n-m), where m varies from 0 to n-1,[38] and where n increases indefinitely. From these facts not only does the existence of a multitude of polymeric hydrocarbons, differing in molecular weight, become intelligible, but it is also seen that there is a possibility of cases of isomerism with the same molecular weight. This polymerism so common to hydrocarbon compounds is already apparent in the first unsaturated series CnH2n, because all the terms of this series C2H4, C3H6, C4H8 ... C30H60 ... have one and the same composition CH2, but different molecular weights, as has been already explained in Chapter VII. The differences in the vapour density, boiling points, and melting points, of the quantities entering into reactions,[39] and the methods of preparation[40] also so clearly tally with the conception of polymerism, that this example will always be the clearest and most conclusive for the illustration of polymerism and molecular weight. Such a case is also met with among other hydrocarbons. Thus benzene, C6H6, and cinnamene, C8H8, correspond with the composition of acetylene or to a compound of the composition CH.[41] The first boils at 81°, the second at 144°; the specific gravity of the first is 0·899; that of the second, 0·925, at 0°—that is, here also the boiling point rises with the increase of molecular weight, and so also, as might be expected, does the density.

Cases of isomerism in the restricted sense of the word—that is, when with an identity of composition and of molecular weight, the properties of the substances are different—are very numerous among the hydrocarbons and their derivatives. Such cases are particularly important for the comprehension of molecular structure and they also, like the polymerides, may be predicted from the above-mentioned conceptions, expressing the principles of the structure of the carbon compounds[42] based on the law of substitution. According to it, for example, it is evident that there can be no isomerism in the cases of the saturated hydrocarbons C2H6 and C3H8, because the former is CH4, in which methyl has taken the place of H, and as all the hydrogen atoms of methane must be supposed to have the same relation to the carbon, it is all the same which of them be subjected to the methyl substitution—the resulting product can only be ethane, CH3CH3;[43] the same argument also applies in the case of propane, CH3CH2CH3, where one compound only can be imagined. It is to be expected, however, that there should be two butanes, C4H10, and this is actually the case. In one, methyl may be considered as replacing the hydrogen of one of the methyls, CH3CH2CH2CH3; and in the other CH3 may be considered as substituted for H in CH3, and there it will consist of CH3CH CH3 / CH3 . The latter may also be regarded as methane in which three of hydrogen are exchanged for three of methyl. On going further in the series it is evident that the number of possible isomerides will be still greater, but we have limited ourselves to the simplest examples, showing the possibility and actual existence of isomerides. C2H4 and CH2CH2 are, it is evident, identical; but there ought to be, and are, two hydrocarbons of the composition C3H6, propylene and trimethylene; the first is ethylene, CH2CH2, in which one atom of hydrogen is exchanged for methyl, CH2CHCH3, and trimethylene is ethane, CH3CH3, with the substitution of methylene for two hydrogen atoms from two methyl groups—that is, CH2 CH2 / CH2 ,[44] where the methylene introduced is united to both the atoms of carbon in CH3CH3. It is evident that the cause of isomerism here is, on the one hand, the difference of the amount of hydrogen in union with the particular atoms of carbon, and, on the other, the different connection between the several atoms of carbon. In the first case they may be said to be chained together (more usually to form an ‘open chain’), and in the second case, to be locked together (to form a ‘closed chai’ or ‘ring’). Here also it is easily understood that on increasing the quantity of carbon atoms the number of possible and existing isomerides will greatly increase. If, at the same time, in addition to the substitution of one of the radicles of methane for hydrogen a further exchange of part of the hydrogen for some of the other groups of elements X, Y ... occurs, the quantity of possible isomerides still further increases in a considerable degree. For instance, there are even two possible isomerides for the derivatives of ethane, C2H6: if two atoms of the hydrogen be exchanged for X2, one will have the ethylene structure, CH2XCH2X, and the other an ethylidene structure, CH3CHX2; such are, for instance, ethylene chloride, CH2ClCH2Cl, and ethylidene chloride, CH3CHCl2. And as in the place of the first atom of hydrogen not only metals may be substituted, but Cl, Br, I, OH (the water radicle), NH2 (the ammonia radicle), NO2 (the radicle of nitric acid), &c., so also in exchange for two atoms of hydrogen O, NH, S, &c., may be substituted; hence it will be understood that the quantity of isomerides is sometimes very great. It is impossible here to describe how the isomerides are distinguished from each other, in what reactions they occur, how and when one changes into another, &c.; for this, taken together with the description of the hydrocarbons already known, and their derivatives, forms a very extensive and very thoroughly investigated branch of chemistry, called organic chemistry. Enriched with a mass of closely observed phenomena and strictly deduced generalisations, this branch of chemistry has been treated separately for the reason that in it the hydrocarbon groups are subjected to transformations which are not met with in such quantity in dealing with any of the other elements or their hydrogen compounds. It was important for us to show that notwithstanding the great variety of the hydrocarbons and their products,[45] they are all of them governed by the law of substitution, and referring our readers for detailed information to works on organic chemistry, we will limit ourselves to a short exposition of the properties of the two simplest unsaturated hydrocarbons: ethylene, CH2CH2, and acetylene, CHCH, and a short acquaintance with petroleum as the natural source of a mass of hydrocarbons. Ethylene, or olefiant gas, C2H4, is the lowest known member of the unsaturated hydrocarbon series of the composition CnH2n. As in composition it is equal to two molecules of marsh gas deprived of two molecules of hydrogen, it is evident that it might be, and it actually can be, produced, although but in small quantities, together with hydrogen, by heating marsh gas. On being heated, however, olefiant gas splits up, first into acetylene and methane (3C2H4 = 2C2H2 + 2CH4, Lewes, 1894), and at a higher temperature into carbon and hydrogen; and therefore in those cases where marsh gas is produced by heating, olefiant gas, hydrogen, and charcoal will also be formed, although only in small quantities. The lower the temperature at which complex organic substances are heated, the greater the quantity of olefiant gas found in the gases given off; at a white heat it is entirely decomposed into charcoal and marsh gas. If coal, wood, and more particularly petroleum, tars, and fatty substances, are subjected to dry distillation, they give off illuminating gas, which contains more or less olefiant gas.

Olefiant gas, almost free from other gases,[46] may be obtained from ordinary alcohol (if possible, free from water) if it be mixed with five parts of strong sulphuric acid and the mixture heated to slightly above 100°. Under these conditions, the sulphuric acid removes the elements of water from the alcohol, C2H5(OH), and gives olefiant gas; C2H6O = H2O + C2H4. The greater molecular weight of olefiant gas compared with marsh gas indicates that it may be comparatively easily converted into a liquid by means of pressure or great cold; this may be effected, for example, by the evaporation of liquid nitrous oxide. Its absolute boiling point is +10°, it boils at -103° (1 atmosphere), liquefies at 0°, at a pressure of 43 atmospheres, and solidifies at -160°. Ethylene is colourless, has a slight ethereal smell, is slightly soluble in water, and somewhat more soluble in alcohol and in ether (in five volumes of spirit and six volumes of ether).[47]

Like other unsaturated hydrocarbons, olefiant gas readily enters into combination with certain substances, such as chlorine, bromine, iodine, fuming sulphuric acid, or sulphuric anhydride, &c. If olefiant gas be sealed up with a small quantity of sulphuric acid in a glass vessel, and constantly agitated (as, for instance, by attaching it to the moving part of a machine), the prolonged contact and repeated mixing causes the olefiant gas, little by little, to combine with the sulphuric acid, forming C2H4H2SO4. If, after this absorption, the sulphuric acid be diluted with water and distilled, alcohol separates, which is produced in this case by the olefiant gas combining with the elements of water, C2H4 + H2O = C2H6O. In this reaction (Berthelot) we see an excellent example of the fact that if a given substance, like olefiant gas, is produced by the decomposition of another, then in the reverse way this substance, entering into combination, is capable of forming the original substance—in our example, alcohol. In combination with various molecules, X2, ethylene gives saturated compounds, C2H4X2 or CH2XCH2X (for example, C2H4Cl2), which correspond with ethane, CH3CH3 or C2H6.[48]

Acetylene, C2H2 = CHCH, is a gas; it was first prepared by Berthelot (1857). It has a very pungent smell, is characterised by its great stability under the action of heat, and is obtained as the only product of the direct combination of carbon with hydrogen when a luminous arc (voltaic) is formed between carbon electrodes. This arc contains particles of carbon passing from one pole to the other. If the carbons be surrounded with an atmosphere of hydrogen, the carbon in part combines with the hydrogen, forming C2H2.[48 bis] Acetylene may be formed from olefiant gas if two atoms of hydrogen be taken from it. This may be effected in the following way: the olefiant gas is first made to combine with bromine, giving C2H4Br2; from this the hydrobromic acid is removed by means of an alcoholic solution of caustic potash, leaving the volatile product C2H3Br; and from this yet another part of hydrobromic acid is withdrawn by passing it through anhydrous alcohol in which metallic sodium has been dissolved, or by heating it with a strong alcoholic solution of caustic potash. Under these circumstances (Berthelot, Sawitsch, Miasnikoff) the alkali takes up the hydrobromic acid from CnH2n-1Br, forming CnH2n-2.

Acetylene is also produced in all those cases where organic substances are decomposed by the action of a high temperature—for example, by dry distillation. On this account a certain quantity is always found in coal gas, and gives to it, at all events in part, its peculiar smell, but the quantity of acetylene in coal gas is very small. If the vapour of alcohol be passed through a heated tube a certain quantity of acetylene is formed. It is also produced by the imperfect combustion of olefiant and marsh gas—for example, if the flame of coal gas has not free access to air.[49] The inner part of every flame contains gases in imperfect combustion, and in them some amount of acetylene.

Acetylene, being further removed than ethylene from the limit CnH2n+2 of hydrocarbon compounds, has a still greater faculty of combination than is shown by olefiant gas, and therefore can be more readily separated from any mixture containing it. Actually, acetylene not only combines with one and two molecules of I2, HI, H2SO4, Cl2, Br2, &c.... (many other unsaturated hydrocarbons combine with them), but also with cuprous chloride, CuCl, forming a red precipitate. If a gaseous mixture containing acetylene be passed through an ammoniacal solution of cuprous chloride (or silver nitrate), the other gases do not combine, but the acetylene gives a red precipitate (or grey with silver), which detonates when struck with a hammer. This red precipitate gives off acetylene under the action of acids. In this manner pure acetylene may be obtained. Acetylene and its homologues also readily react with corrosive sublimate, HgCl2 (Koucheroff, Favorsky). Acetylene burns with a very brilliant flame, which is accounted for by the comparatively large amount of carbon it contains.[50]

The formation and existence in nature of large masses of petroleum or a mixture of liquid hydrocarbons, principally of the series CnH2n+2 and CnH2n is in many respects remarkable.[51] In some mountainous districts—as, for instance, by the slopes of the Caucasian chain, on inclines lying in a direction parallel to the range—an oily liquid issues from the earth together with salt water and hot gases (methane and others); it has a tarry smell and dark brown colour, and is lighter than water. This liquid is called naphtha or rock oil (petroleum) and is obtained in large quantities by sinking wells and deep bore-holes in those places where traces of naphtha are observed, the naphtha being sometimes thrown up from the wells in fountains of considerable height.[52] The evolution of naphtha is always accompanied by salt water and marsh gas. Naphtha has from ancient times been worked in Russia in the Apsheron peninsula near Baku, and is also now worked in Burmah (India), in Galicia near the Carpathians, and in America, especially in Pennsylvania and Canada, &c. Naphtha does not consist of one definite hydrocarbon, but of a mixture of several, and its density, external appearance, and other qualities vary with the amount of the different hydrocarbons of which it is composed. The light kinds of naphtha have a specific gravity about 0·8 and the heavy kinds up to 0·98. The former are very mobile liquids, and more volatile; the latter contain less of the volatile hydrocarbons and are less mobile. When the light kinds of naphtha are distilled, the boiling point taken in the vapours constantly changes, beginning at 0° and going up to above 350°. That which passes over first is a very mobile, colourless ethereal liquid (forming gazolene, ligroin, benzoline, &c.), from which the hydrocarbons whose boiling points start from 0° may be extracted—namely, the hydrocarbons C4H10, C5H12 (which boils at 30°), C6H14 (boils at 62°), C7H16 (boils about 90°), &c. Those fractions of the naphtha distillate which boil above 130°, and contain hydrocarbons with C9, C10, C11, &c., enter into the composition of the oily substance, universally used for lighting, called kerosene or photogen or photonaphthalene, and by other names. The specific gravity of kerosene is from 0·78 to 0·84, and it smells like naphtha. Those products of the distillation of naphtha which pass off below 130° and have a specific gravity below 0·75, enter into the composition of light petroleum (benzoline, ligroin, petroleum spirit, &c.); which is used as a solvent for india-rubber, for removing grease spots, &c. Those portions of naphtha (which can only be distilled without change by means of superheated steam, otherwise they are largely decomposed) which boil above 275° and up to 300° and have a specific gravity higher than 0·85, form an excellent oil,[53] safe as regards inflammability (which is very important as diminishing the risks of fire), and may be used in lamps as an effective substitute for kerosene.[54] Those portions of naphtha which pass over at a still higher temperature and have a higher specific gravity than 0·9, which are found in abundance (about 30 p.c.) in the Baku naphtha, make excellent lubricating or machine oils. Naphtha has many important applications, and the naphtha industry is now of great commercial importance, especially as naphtha and its refuse may be used as fuel.[55] Whether naphtha was formed from organic matter is very doubtful, as it is found in the most ancient Silurian strata which correspond with epochs of the earth's existence when there was little organic matter; it could not penetrate from the higher to the lower (more ancient) strata as it floats on water (and water penetrates through all strata). It therefore tends to rise to the surface of the earth, and it is always found in highlands parallel to the direction of the mountains.[56] Much more probably its formation may be attributed to the action of water penetrating through the crevasses formed on the mountain slopes and reaching to the heart of the earth, to that kernel of heated metallic matter which must be accepted as existing in the interior of the earth. And as meteoric iron often contains carbon (like cast iron), so, accepting the existence of such carburetted iron at unattainable depths in the interior of the earth, it may be supposed that naphtha was produced by the action of water penetrating through the crevices of the strata during the upheaval of mountain chains,[57] because water with iron carbide ought to give iron oxide and hydrocarbons.[58] Direct experiment proves that the so-called spiegeleisen (manganiferous iron, rich in chemically combined carbon) when treated with acids gives liquid hydrocarbons[59] which in composition, appearance, and properties are completely identical with naphtha.[60]

Footnotes:

[1] Wood is the non-vital part of ligneous plants: the vital part of ordinary trees is situated between the bark and the lignin. Every year a layer of lignin is deposited on this part by the juices which are absorbed by the roots and drawn up by the leaves; for this reason the age of trees may be determined by the number of lignin layers deposited. The woody matter consists principally of fibrous tissue on to which the lignin or so-called incrusting matter has been deposited. The tissue has the composition C6H10O5, the substance deposited on it contains more carbon and hydrogen and less oxygen. This matter is saturated with moisture when the wood is in a fresh state. Fresh birch wood contains about 31 p.c. of water, lime wood 47 p.c., oak 35 p.c., pine and fir about 37 p.c. When dried in the air the wood loses a considerable quantity of water and not more than 19 p.c. remains. By artificial means this loss of water may be increased. If water be driven into the pores of wood the latter becomes heavier than water, as the lignin of which it is composed has a density of about 1·6. One cubic centimetre of birch wood does not weigh more than 0·901 gram, fir 0·894, lime tree 0·817, poplar 0·765 when in a fresh state; when in a dry state birch weighs 0·622, pine 0·550, fir 0·355, lime 0·430, guaiacum 1·342, ebony 1·226. On one hectare (2·7 acres) of woodland the yearly growth averages the amount of 3,000 kilograms (or about 3 tons) of wood, but rarely reaches as much as 5,000 kilos. The average chemical composition of wood dried in air may be expressed as follows:—Hygroscopic water 15 p.c., carbon 42 p.c., hydrogen 5 p.c., oxygen and nitrogen 37 p.c., ash 1 p.c. Wood parts with its hygroscopic water at 150°, and decomposes at about 300°, giving a brown, brittle, so-called red charcoal; above 350° black charcoal is produced. As the hydrogen contained in wood requires for its combustion about forty parts by weight of oxygen, which is present to the amount of about 36 p.c., all that burns of the wood is the carbon which it contains, 100 parts of wood only giving out as much heat as forty parts of charcoal, and therefore it would be far more profitable to use charcoal for heating purposes than wood, if it were possible to obtain it in such quantities as correspond with its percentage ratio—that is forty parts per 100 parts of wood. Generally, however, the quantity produced is far less, not more than 30 p.c., because part of the carbon is given off as gas, tar, &c. If wood has to be transported great distances, or if it is necessary to obtain a very high temperature by burning it, then even as little as 25 p.c. of charcoal from 100 parts of wood may be advantageous. Charcoal (from wood) develops on burning 8,000 heat units, whilst wood dried in air does not develop more than 2,800 units of heat; therefore seven parts of charcoal give as much heat as twenty parts of wood. As regards the temperature of combustion, it is far higher with charcoal than with wood, because twenty parts of burning wood give, besides the carbonic anhydride which is also formed together with charcoal, eleven parts of water, the evaporation of which requires a considerable amount of heat.

see caption

Fig. 57.—Apparatus for the dry distillation of wood. The retort a containing the wood is heated by the flues c e. The steam and volatile products of distillation pass along the tube g through the condenser m, where they are condensed. The form, distribution, and dimensions of the apparatus vary.

The composition of the growing parts of plants, the leaves, young branches, shoots, &c., differs from the composition of the wood in that these vital parts contain a considerable quantity of sap which contains much nitrogenous matter (in the wood itself there is very little), mineral salts, and a large amount of water. Taking, for example, the composition of clover and pasture hay in the green and dry state; in 100 parts of green clover there is about 80 p.c. of water and 20 p.c. of dry matter, in which there are about 3·5 parts of nitrogenous albuminous matter, about 9·5 parts of soluble and about 5 parts of insoluble non-nitrogenous matter, and about 2 p.c. of ash. In dry clover or clover-hay there is about 15 p.c. of water, 13 p.c. of nitrogenous matter, and 7 p.c. of ash. This composition of grassy substances shows that they are capable of forming the same sort of charcoal as wood itself. It also shows the difference of nutritive properties existing between wood and the substances mentioned. These latter serve as food for animals, because they contain those substances which are capable of being dissolved (entering into the blood) and forming the body of animals; such substances are proteids, starch, &c. Let us remark here that with a good harvest an acre of land gives in the form of grass as much organic substance as it yields in the form of wood.

One hundred parts of dry wood are capable of giving, on dry distillation, besides 25 p.c. of charcoal and 10 p.c. or more of tar, 40 p.c. of watery liquid, containing acetic acid and wood spirit, and about 25 p.c. of gases, which may be used for heating or lighting purposes, because they do not differ from ordinary illuminating gas, which can indeed be obtained from wood. As wood-charcoal and tar are valuable products, in some cases the dry distillation of wood is carried on principally for producing them. For this purpose those kinds of woods are particularly advantageous which contain resinous substances, especially coniferous trees, such as fir, pine, &c.; birch, oak, and ash give much less tar, but on the other hand they yield more aqueous liquor. The latter is used for the manufacture of wood spirit, CH4O, and acetic acid, C2H4O2. In such cases, the dry distillation is carried on in stills. The stills are nothing more than horizontal or vertical cylindrical retorts, made of boiler plate, heated with fuel and having apertures at the top and sometimes also at the bottom for the exit of the light and heavy products of distillation. The dry distillation of wood in stoves is carried on in two ways, either by burning a portion of the wood inside the stove in order to submit the remainder to dry distillation by means of the heat obtained in this manner, or by placing the wood in a stove the thin sides of which are surrounded with a flue leading from the fuel, placed in a space below.

The first method does not give such a large amount of liquid products of the dry distillation as the latter. In the latter process there is generally an outlet below for emptying out the charcoal at the close of the operation. For the dry distillation of 100 parts of wood from forty to twenty parts of fuel are used.

In the north of Russia wood is so plentiful and cheap that this locality is admirably fitted to become the centre of a general trade in the products of its dry distillation. Coal (Note 6), sea-weed, turf, animal substances (Chapter VI.), &c., are also submitted to the process of dry distillation.

[2] The result of imperfect combustion is not only the loss of a part of the fuel and the production of smoke, which in some respects is inconvenient and injurious to health, but also a low flame temperature, which means that a less amount of heat is transmitted to the object heated. Imperfect combustion is not only always accompanied by the formation of soot or unburnt particles of charcoal, but also by that of carbonic oxide, CO, in the smoke (Chapter IX.) which burns, emitting much heat. In works and factories where large quantities of fuel are consumed, many appliances are adopted to ensure perfect combustion, and to combat against such a ruinous practice as the imperfect combustion of fuel. The most effective and radical means consists in employing combustible gases (producer and water gases), because by their aid perfect combustion can be easily realised without a loss of heat-producing power and the highest temperature can be reached. When solid fuel is used (such as coal, wood, and turf), imperfect combustion is most liable to occur when the furnace doors are opened for the introduction of fresh fuel. The step furnace may often prove a remedy for this defect. In the ordinary furnace fresh fuel is placed on the burning fuel, and the products of dry distillation of the fresh fuel have to burn at the expense of the oxygen remaining uncombined with the burnt fuel. Imperfect combustion is observed in this case also from the fact that the dry distillation and evaporation of the water of the fresh fuel lying on the top of that burnt, lowers the temperature of the flame, because part of the heat becomes latent. On this account a large amount of smoke (imperfect combustion) is observed when a fresh quantity of fuel is introduced into the furnace. This may be obviated by constructing the furnace (or managing the stoking) in such a way that the products of distillation pass through the red-hot charcoal remaining from the burnt fuel. It is only necessary in order to ensure this to allow a sufficient quantity of air for perfect combustion. All this may be easily attained by the use of step fire-bars. The fuel is fed into a hopper and falls on to the fire-bars, which are arranged in the form of a staircase. The burning charcoal is below, and hence the flame formed by the fresh fuel is heated by the contact of the red-hot burning charcoal. An air supply through the fire grate, an equal distribution of the fuel on the fire-bars (otherwise the air will blow through empty spaces and lower the temperature), a proper proportion between the supply of air and the chimney draught, and a perfect admixture of air with the flame (without an undue excess of air), are the means by which we can contend against the imperfect combustion of such kinds of fuel as wood, peat, and ordinary (smoky) coal. Coke, charcoal, anthracite, burn without smoke, because they do not contain hydrogenous substances which furnish the products of dry distillation, but imperfect combustion may occur with them also; in that case the smoke contains carbonic oxide.

[3] Under the action of air, organic substances are capable of oxidising to such an extent that all the carbon and all the hydrogen they contain will be transformed into carbonic anhydride and water. The refuse of plants and that of animals are subjected to such a change whether they slowly decompose and putrefy, or rapidly burn with direct access of air. But if the supply of air be limited, there can be no complete transformation into water and carbonic anhydride, there will be other volatile matters (rich in hydrogen), while charcoal must remain as a non-volatile substance. All organic substances are unstable, they do not resist heat, and change even at ordinary temperatures, particularly if water be present. It is therefore easy to understand that charcoal may in many cases be obtained through the transformation of substances entering into the composition of organisms, but that it is never found in a pure state.

However, water and carbonic anhydride are not the only products separated from organic substances. Carbon, hydrogen, and oxygen are capable of giving a multitude of compounds; some of these are volatile compounds, gaseous, soluble in water—they are carried off from organic matter, undergoing change without access of air. Others, on the contrary, are non-volatile, rich in carbon, unaffected by heat and other agents. The latter remain in admixture with charcoal in the place where the decomposition takes place; such, for example, are tarry substances. The quantity of those bodies which are found mixed with the charcoal is very varied, and depends on the energy and duration of the decomposing agent. The annexed table shows, according to the data of Violette, those changes which wood undergoes at various temperatures when submitted to dry distillation by means of superheated steam:—

Temperature Residue
from 100 parts
of alder wood
In 100 parts of the residual charcoal
C H O and N Ash
150° 100·0 47·5 6·1 46·3 0·1
350° 29·7 76·6 4·1 18·4 0·6
1032° 18·7 81·9 2·3 14·1 1·6
1500° 17·3 95·0 0·7 3·8 1·7

[4] The object of producing charcoal from wood has been explained in Note 1. Wood charcoal is obtained in so-called stacks by partially burning the wood, or by means of dry distillation (Note 1) without the access of air. It is principally manufactured for metallurgical processes, especially for smelting and forging iron. The preparation of charcoal in stacks has one advantage, and that is that it may be done on any spot in the forest. But in this way all the products of dry distillation are lost. For charcoal burning, a pile or stack is generally built, in which the logs are placed close together, either horizontally, vertically, or inclined, forming a stack of from six to fifty feet in diameter and even larger. Under the stack are several horizontal air passages, and an opening in the middle to let out the smoke. The surface of the stack is covered with earth and sods to a considerable thickness, especially the upper part, in order to hinder the free passage of air and to concentrate the heat inside. When the stack is kindled, the pile begins to settle down by degrees, and it is then necessary to look after the turf casing and keep it in repair. As the combustion spreads throughout the whole pile, the temperature rises and real dry distillation commences. It is then necessary to stop the air holes, in order as much as possible to prevent unnecessary combustion. The nature of the process is, that part of the fuel burns and develops the heat required for subjecting the remainder to dry distillation. The charring is stopped when the products of dry distillation, which are emitted, no longer burn with a brilliant flame, but the pale blue flame of carbonic oxide appears. Dry wood in stacks yields about one-fourth of its weight of charcoal.

[5] When dead vegetable matter undergoes transformation in air, in the presence of moisture and lower organisms, there remains a substance much richer in carbon—namely, humus, black earth or mould. 100 parts of humus in a dry state contain about 70 p.c. of carbon. The roots, leaves, and stems of plants which wither and fall to the ground form a soil rich in humus. The non-vital vegetable substances (ligneous tissue) first form brown matter (ulmic compounds), and then black matter (humic substances), which are both insoluble in water; after this a brown acid is produced, which is soluble in water (apocrenic acid), and lastly a colourless acid also soluble in water (crenic acid). Alkali dissolves a part of the original brown and black substances, forming solutions of a brown tint (ulmic and humic acids) which sometimes communicate their colour to springs and rivers. The proportion of humus in soil generally has a direct influence on its fertility; firstly, because putrefying plants develop carbonic anhydride and ammonia, and yield the substances forming the ashes of plants, which are necessary to vegetation; secondly, because humus is capable of attracting the moisture of the air and of absorbing water (twice its weight) and in this way keeps the soil in a damp condition, which is indispensable for nourishment; thirdly, humus renders the soil porous, and, fourthly, it renders it more capable of absorbing the heat of the sun's rays. On this account black earth is often most remarkable for its fertility. One object of manuring is to increase the quantity of humus in the soil, and any easily changeable vegetable or any animal matter (composts) may be used. The boundless tracts of black earth soil in Russia are capable of bestowing countless wealth on the country.

The origin and extent of black earth soil are treated in detail in Professor Dokouchaeff's works.

If those substances which produce humus undergo decomposition under water, less carbonic anhydride is formed, a quantity of marsh gas, CH4, is evolved, and the solid residue forms an acid humus found in great quantities in marshy places and called peat. Peat is especially abundant in the lowlands of Holland, North Germany, Ireland, and Bavaria. In Russia it is likewise found in large quantities, especially in the North-West districts. The old hard forms of peat resemble in composition and properties brown coal; the newest formations, as yet unhardened by pressure, form very porous masses which retain traces of the vegetable matter from which they have been formed. Dried (and sometimes pressed) peat is used as fuel. The composition of peat varies considerably with the locality in which it is found. When dried in air it does not contain less than 15 p.c. of water and 8 p.c. of ash; the remainder consists of 45 p.c. of carbon, 4 p.c. of hydrogen, 1 p.c. of nitrogen, and 28 p.c. of oxygen. Its heating power is about equivalent to that of wood. The brown earthy varieties of coal were probably formed from peat. In other cases they have a marked woody structure, and are then known as lignites. The composition of the brown sorts of coal resembles in a marked degree that of peat—namely, in a dried state brown coal contains on an average 60 p.c. of carbon, 5 p.c. of hydrogen, 26 p.c. of oxygen and nitrogen, and 9 p.c. of ash. In Russia brown coal is met with in many districts near Moscow, in the Governments of Toula and Tver and the neighbourhood; it is very usually used as fuel, particularly when found in thick seams. The brown coals usually burn with a flame like wood and peat, and are akin to them in heating power, which is half or a third that of the best coal.

[6] Grass and wood, the vegetation of primÆval seas and similar refuse of all geological periods, must have been in many cases subjected to the same changes they now undergo—that is, under water they formed peat and lignites. Such substances, preserved or a long time underground, subjected to the action of water, compressed by the new strata formed above them, transformed by the separation of their more volatile component parts (peat and lignites, even in their last condition, still continue to evolve nitrogen, carbonic anhydride, and marsh gases) form coal. Coal is a dense homogeneous mass, black, with an oily or glassy lustre, or more rarely dull without any evident vegetable structure; this distinguishes it in appearance from the majority of lignites. The density of coal (not counting the admixture of pyrites, &c.) varies from 1·25 (dry bituminous coal) to 1·6 (anthracite, flameless), and even reaches 1·9 in the very dense variety of coal found in the Olonetzky government (termed shungite), which according to the investigations of Professor Inostrantzeff may be regarded as the extreme member of the various forms of coal.

In order to explain the formation of coal from vegetable matter, Cagniard de la Tour enclosed pieces of dried wood in a tube and heated them to the boiling point of mercury, when the wood was changed into a semi-liquid black mass from which a substance exceedingly like coal separated. In this manner some kinds of wood formed coal which on being heated left caking coke, others non-caking; precisely as we find with the natural varieties of coal. Violette repeated these experiments with wood dried at 150°, and showed that when wood is decomposed in this way, a gas, an aqueous liquor, and a residue are formed. The latter at a temperature of 200° has the properties of wood charcoal incompletely burnt; at 300° and higher a homogeneous mass like coal is formed which at 340° is dense and without cavities. At 400° the residue resembles anthracite. In nature probably the decomposition was in rare cases effected by heat alone; more generally it was effected by means of water and heat, but in either case the result ought to be almost the same.

The average composition of coal compiled from many analyses, disregarding the ash, is as follows: 84 parts of carbon, 5 parts of hydrogen, 1 part of nitrogen, 8 parts of oxygen, 2 of sulphur. The quantity of ash is on an average 5 p.c., but there are coals which contain a larger quantity, and naturally they are not so advantageous for use as fuel. The amount of water does not usually exceed more than 10 p.c. The anthracites form a remarkable variety of coals, they do not give any volatile products, or but a very small amount, as they contain but little hydrogen compared to oxygen. In the average composition of coal we saw that for 5 parts of hydrogen there were 8 parts of oxygen; therefore 4 parts by weight of hydrogen are capable of forming hydrocarbons, because 1 part of hydrogen is necessary in order to form water with the 8 parts of oxygen. These 4 parts by weight of hydrogen can convert 48 parts of carbon into volatile products, because 1 part of hydrogen by weight in these substances combines with 12 parts of carbon. The anthracites differ essentially from this: neglecting the ash, their average composition is as follows: 94 parts of carbon, 3 of hydrogen, and 3 of oxygen and nitrogen. According to the analyses of A. A. Voskresensky, the Grousheffsky anthracite (Don district) contains: C = 93·8, H = 1·7, ash = 1·5. Therefore the anthracites contain but little hydrogen capable of combining with the carbon to form hydrocarbons which burn with a flame. Anthracites are the oldest forms of coal. The newest and least transformed coals, which resemble some of the brown varieties, are the dry coals. They burn with a flame like wood, and leave a coke having the appearance of lumps of coal, half their component parts being absorbed by the flame (they contain much hydrogen and oxygen). The remaining varieties of coal (gas coal, smithy coal, coking, and anthracite) according to GrÜner in all respects form connecting links between the dry coals and the anthracites. These coals burn with a very smoky flame, and on being heated leave coke, which bears the same relation to coal that charcoal does to wood. The quantity and quality of coke vary considerably with the different sorts of coal from which it is formed. In practice coals are most often distinguished by the properties and quantity of the coke which they give. In this particular the so-called bituminous coals are especially valuable, as even the slack of this kind gives on dry distillation large spongy masses of coke. If large pieces of these kinds of coal are subjected to dry distillation, they, as it were, melt, flow together, and form caking masses of coke. The best coking coals give 65 p.c. of dense caking coke. Such coal is very valuable for metallurgical purposes (see Note 8). Besides coke, the dry distillation of coal produces gas (see further, illuminating gas, p. 361), coal-tar (which gives benzene, carbolic acid, naphthalene, tar for artificial asphalt, &c.) and also an aqueous alkaline liquor (with wood and lignites the liquid is acid from acetic acid) which contains ammonium carbonate (see Note 6).

[7] In England in 1850 the output of coal was as much as 48 million tons, and in latter years it has risen to about 190 millions. Besides this other countries contribute 300 millions—Russia about 6 millions. The United States of America come next to England with an output of 160 million tons, then Germany 90 millions; France produces but little (25 millions), and takes about 5 million tons from England. Thus the world consumes about 500 million tons of coal yearly. Besides household purposes, coal is chiefly used as fuel for steam-engines. As every horse-power (= 75 kilogrammetres per second) of a steam-engine expends on the average more than 25 kilograms in 24 hours, or in a year (counting stoppages) not less than 5 tons per horse-power, and there are not less than 40 million horse-power at work in the world, the consumption of coal for motive-power is at least equal to half the whole production. For this reason coal serves as a criterion of the industrial development of a country. About 15 p.c. of coal is used for the manufacture of cast iron, wrought iron, steel, and articles made of them.

[8] The principal coal beds of Russia under exploitation are: The Don basin (150 million poods per annum, 62 poods = 1 ton), the Polish basin (Dombrovo and others 120 million poods per annum), the Toula and Riazan beds of the Moscow basin (up to 25 million poods), the Ural basin (10 million poods), the Caucasian (Kviboul, near Kutais), the Khirjhis steppes, the smithy coal basin (Gov. of Tomsk), the Sahaline, &c. The Polish and Moscow basins do not give any coking coals. The presence of every variety of coal (from the dry coal near Lisichansk on the Donetz to the anthracites of the entire south-east basin), the great abundance of excellent metallurgical coal (coking, see Note 6) in the western part of the basin, its vast extent (as much as 25,000 sq. versts), the proximity of the seams to the surface (the shafts are now from 20 to 100 fathoms deep, and in England and Belgium as deep as 500 fathoms), the fertility of the soil (black earth), the proximity of the sea (about 100 versts from the Sea of Azoff) and of the rivers Donetz, Don, and Dneiper, the most abundant seams of excellent iron ore (Korsan Mogila, Krivoy Rog, Soulin, &c., &c.), copper ore, mercury ore (near Nikitovka, in the Bakhmouth district of the Ekaterinoslav Gov.), and other ores, the richest probably in the whole world, the beds of rock-salt (near the stations of the Stoupka and Brianzovka) the excellent clay of all kinds (china, fire-clay), gypsum, slate, sandstone, and other wealth of the Don coal basin, give complete assurance of the fact that with the growth of industrial activity in Russia this bountiful land of the Cossacks and New Russia will become the centre of the most extensive productive enterprise, not for the requirements of Russia alone, but of the whole world, because in no other place can be found such a concentration of favourable conditions. The growth of enterprise and knowledge, together with the extinction of the forests which compels Russia to foster the production of coal, will help to bring about this desired result. England with a whole fleet of merchant vessels exports annually about 25 million tons of coal, the price of which is higher than on the Donetz (where a pood of worked coal costs less than 5 copecks on the average), where anthracites and semi-anthracites (like Cardiff or steam coal, which burns without smoke) and coking and metallurgical coals are able both in quantity and quality to satisfy the most fastidious requirements of the industry already existing and rapidly increasing everywhere. The coal mines of England and Belgium are approaching a state of exhaustion, whilst in those of the Don basin, only at a depth of 100 fathoms, 1,200,000 million poods of coal lie waiting to be worked.

[9] As it is difficult to separate from the charcoal the admixture of ash—that is, the earthy matter contained in the vegetable substance used for producing charcoal—in order to obtain it in its purest condition it is necessary to use such organic substances as do not contain any ash, for example completely refined or purified crystallised sugar, crystallised tartaric acid, &c.

[10] The cavities in charcoal are the passages through which those volatile products formed at the same time as the charcoal have passed. The degree of porosity of charcoal varies considerably, and has a technical significance, in different kinds of charcoal. The most porous charcoal is very light; a cubic metre of wood charcoal weighs about 200 kilograms. Many of the properties of charcoal which depend exclusively on its porosity are shared by many other porous substances, and vary with the density of the charcoal and depend on the way it was prepared. The property which charcoal has of absorbing gases, liquids, and many substances in solution, is a case in point. The densest kind of charcoal is formed by the action of great heat on sugar and other fusible substances. The lustrous grey dense coke formed in gas retorts is also of this character. This dense coke collects on the internal walls of the retorts subjected to great heat, and is produced by the vapours and gases separated from the heated coal in the retorts. In virtue of its density such coke becomes a good conductor of the galvanic current and approaches graphite. It is principally used in galvanic batteries. Coke, or the charcoal remaining from the imperfect combustion of coal and tarry substances, is also but slightly porous, brilliant, does not soil or mark paper, is dense, almost devoid of the faculty of retaining liquids and solids, and does not absorb gases. The light sorts of charcoal produced from charred wood, on the other hand, show this absorptive power in a most marked degree. This property is particularly developed in that very fine and friable charcoal prepared by heating animal substances such as hides and bones. The absorptive power of charcoal for gases is similar to the condensation of gases in spongy platinum. Here evidently there is a case of the adherence of gases to a solid, precisely as liquids have the property of adhering to various solids. One volume of charcoal will absorb the following volumes of gases (charcoal is capable of absorbing an immense amount of chlorine, almost equal to its own weight):—

Saussure.
Boxwood Charcoal
Favre.
Cocoanut Charcoal
Heat emitted
per gram of gas
NH3 90 172 vols. 494 units
CO2 35 97 158
N2O 40 99 169
HCl 85 165 274

The quantity of gas absorbed by the charcoal increases with the pressure, and is approximately proportional to it. The quantity of heat given out by the absorption nearly approaches that set free on dissolving, or passing into a liquid condition.

Charcoal absorbs not only gases, but a number of other substances. For instance, alcohol which contains disagreeably smelling fusel oil, on being mixed with charcoal or filtered through it, loses most of the fusel oil. The practice of filtering substances through charcoal in order to get rid of foreign matters is often applied in chemical and manufacturing processes. Oils, spirits, various extracts, and vegetable and other solutions are filtered through charcoal in order to purify them. The bleaching power of charcoal may be tested by using various coloured solutions—such as aniline dyes, litmus, &c. Charcoal, which has absorbed one substance to saturation is still capable of absorbing certain other substances. Animal charcoal, produced in a very finely-divided state, especially by heating bones, makes the best sort for the purposes of absorption. Bone charcoal is used in large quantities in sugar works for filtering syrups and all saccharine solutions, in order to purify them, not only from colouring and odorous matter, but also from the lime which is mixed with the syrups in order to render them less unstable during boiling. The absorption of lime by animal charcoal depends, in all probability, in a great degree on the mineral component parts of bone charcoal.

[11] Charcoal is a very bad conductor of heat, and therefore forms an excellent insulator or packing to prevent the transmission of heat. A charcoal lining is often used in crucibles for heating many substances, as it does not melt and resists a far greater heat than many other substances.

[12] The unalterability of charcoal under the action of atmospheric agencies, which produce changes in the majority of stony and metallic substances, is often made use of in practice. For example, charcoal is frequently strewn in boundary ditches. The surface of wood is often charred to render it durable in those places where the soil is damp and wood itself would soon rot. The chambers (or in some works towers) through which acids pass (for example, sulphuric and hydrochloric) in order to bring them into contact with gases or liquids, are filled with charcoal or coke, because at ordinary temperatures it resists the action of even the strongest acids.

[12 bis] Maquenne (1892) discovered that carbon is capable of combining with the alkali metals. A 20 p.c. amalgam of the metals was heated to a red heat with charcoal powder in a stream of hydrogen. The compounds so obtained possessed, after the mercury had been driven off, the compositions BaC2, SrC2, CaC2. All these compounds react with water forming acetylene, for example:

BaC2 + 2H2O = C2H2 + Ba(OH)2

Maquenne proposes the barium carbide as a source of acetylene. He obtained this compound by heating carbonate of barium, magnesium powder, and retort carbon in a Perreau furnace (BaCO3 + 3Mg + C = 3MgO + BaC2). One hundred grams of BaC2 evolve 5,200 to 5,400 c.c. of acetylene, mixed with about 2–3 p.c. of hydrogen.

The relation of acetylene, C2H2, to these metallic carbides is evident from the fact that these metals (Ca, Sr, Ba) replace 2 atoms of hydrogen, and therefore C2Ba corresponds to C2H2, so that they may be regarded as metallic derivatives of acetylene. Moissan (1893) obtained similar carbides directly from the oxides by subjecting them to the action of the voltaic arc, in the presence of carbon, for instance, BaO + 3C = CO + C2Ba, although at a furnace heat carbon has no action on the oxides CaO, BaO, SrO. Concerning Al4C5, see Chapter XVII. Note 38.

[13] When subjected to pressure, charcoal loses heat, hence the densest form stands to the less dense as a solid to a liquid, or as a compound to an element. From this the conclusion may be drawn that the molecules of graphite are more complex than those of charcoal, and those of the diamond still more so. The specific heat shows the same variation, and as we shall see further on, the increased complexity of a molecule leads to a diminution of the specific heat. At ordinary temperatures the specific heat of charcoal is 0·24, graphite 0·20, the diamond 0·147. For retort carbon Le Chatelier (1893) found that the product of the sp. heat and atomic weight varies, between 0° and 250°, according to the formula: = 1·92 + 0·0077t, and between 250° and 1000°, = 3·54 + 0·00246t (see Chapter XIV. Note 4).

[14] There are places where anthracite gradually changes into graphite as the strata sink. I myself had the opportunity of observing this gradual transformation in the valley of Aosta.

[15] Pencils are made of graphite worked up into a homogeneous mass by disintegrating, powdering, and cleansing it from earthy impurities; the best kinds are made of completely homogeneous graphite sawn up into the requisite sticks. Graphite is found in many places. In Russia the so-called Aliberoffsky graphite is particularly renowned; it is found in the Altai mountains near the Chinese frontier; in many places in Finland and likewise on the banks of the Little Tungouska, Sidoroff also found a considerable quantity of graphite. When mixed with clay, graphite is used for making crucibles and pots for melting metals.

Graphite, like most forms of charcoal, still contains a certain quantity of hydrogen, oxygen, and ash, so that in its natural state it does not contain more than 98 p.c. of carbon.

In practice, graphite is purified simply by washing it when in a finely-ground state, by which means the bulk of the earthy matter may be separated. The following process, proposed by Brodie, consists in mixing the powdered graphite with 1/14 part of its weight of potassium chlorate. The mixture is then heated with twice its weight of strong sulphuric acid until no more odoriferous gases are emitted; on cooling, the mixture is thrown into water and washed; the graphite is then dried and heated to a red heat; after this it shrinks considerably in volume and forms a very fine powder, which is then washed. By acting on graphite several times with a mixture of potassium chlorate and nitric acid heated up to 60°, Brodie transformed it into a yellow insoluble acid substance which he called graphitic acid, C11H4O5. The diamond remains unchanged when subjected to this treatment, whilst amorphous charcoal is completely oxidised. Availing himself of this possibility of distinguishing graphite from the diamond or amorphous charcoal, Berthelot showed that when compounds of carbon and hydrogen are decomposed by heat, amorphous charcoal is mainly formed, whilst when compounds of carbon with chlorine, sulphur, and boron are decomposed, graphite is principally deposited.

[15 bis] Diamonds are found in a particular dense rock, known by the name of itacolumite, and are dug out of the dÉbris produced by the destruction of the itacolumite by water. When the dÉbris is washed the diamonds remain behind; they are principally found in Brazil, in the provinces of Rio and Bahia, and at the Cape of Good Hope. The dÉbris gives the black or amorphous diamond, carbonado, and the ordinary colourless or yellow translucent diamond. As the diamond possesses a very marked cleavage, the first operation consists in splitting it, and then roughly and finely polishing it with diamond powder. It is very remarkable that Professors P. A. Latchinoff and ErofÉeff found (1887) diamond powder in a meteoric stone which fell in the Government of Penza, in the district of Krasnoslobodsk, near the settlement of Novo Urei (Sept. 10, 1886). Up to that time charcoal and graphite (a special variety, cliftonite) had been found in meteorites and the diamond only conjectured to occur therein. The Novo Urei meteorite was composed of siliceous matter and metallic iron (with nickel) like many other meteorites.

[16] Diamonds are sometimes found in the shape of small balls, and in that case it is impossible to cut them because directly the surface is ground or broken they fall into minute pieces. Sometimes minute diamond crystals form a dense mass like sugar, and this is generally reduced to diamond powder and used for grinding. Some known varieties of the diamond are almost opaque and of a black colour. Such diamonds are as hard as the ordinary ones, and are used for polishing diamonds and other precious stones, and also for rock boring and tunnelling.

[16 bis] Hannay, in 1880, obtained diamonds by heating a mixture of heavy liquid hydrocarbons (paraffin oils) with magnesium in a thick iron tube. This investigation, however, was not repeated.

[17] The electrical furnace is an invention of recent times, and gives the possibility of obtaining a temperature of 3,500°, which is not only not obtainable in ordinary furnaces, but even in the oxyhydrogen flame, whose temperature does not exceed 2,000°. The electrical furnace consists of two pieces of lime, laid one on the other. A cavity is made in the lower piece for the reception of the substance to be melted between two thick electrodes of dense carbon. On passing a current of 70 volts and 450 ampÈres a temperature of 3,000° is easily obtained. At a temperature of 2,500° (100 ampÈres and 40 volts) not only do all metals melt, but even lime and magnesia (when placed in the space between the carbon electrodes, i.e. in the voltaic arc) become soft and crystallise on cooling. At 3,000° lime becomes very fluid, metallic calcium partially separates out and a carbon compound, which remains liquid for a long time. At this temperature oxide of uranium is reduced to the suboxide and metal, zirconia and rock crystal fuse and partially volatilise, as also does alumina; platinum, gold, and even carbon distinctly volatilise; the majority of the metals form carbides. At such a temperature also cast iron and carbon give graphite, while according to Rousseau, between 2,000° and 3,000° the diamond passes into graphite and conversely graphite into the diamond, so that this is a kind of reversible reaction.

[17 bis] Moissan first investigated the solution of carbon in molten metals (and the formation of the carbides) such as magnesium, aluminium, iron, manganese, chromium, uranium, silver, platinum, and silicon. At the same time Friedel, owing to the discovery of the diamond in meteoric iron, admitted that the formation of the diamond is dependent upon the influence of iron and sulphur. With this object, that is to obtain the diamond, Friedel caused sulphur to react upon samples of cast iron rich in carbon, in a closed vessel at a maximum temperature of 500°, and after dissolving the sulphide of iron formed, he obtained a small quantity of a black powder which scratched corundum, i.e. diamond. Moissan's experiments (1893) were more successful, probably owing to his having employed the electrical furnace. If iron be saturated with carbon at a temperature between 1,100° and 3,000°, then at 1,100°–1,200° a mixture of amorphous carbon and graphite is formed, while at 3,000° graphite alone is obtained in very beautiful crystals. Thus under these conditions the diamond is not formed, and it can only be obtained if the high temperature be aided by powerful pressures. For this purpose Moissan took advantage of the pressure produced in the passage of a mass of molten cast iron from a liquid into a solid state. He first melted 150–200 grams of iron in the electrical furnace, and quickly introduced a cylinder of carbon into the molten iron. He then removed the crucible with the molten iron from the furnace and plunged it into a reservoir containing water. After treating with boiling hydrochloric acid, three varieties of carbon were obtained: (1) a small amount of graphite (if the cooling be rapid); (2) carbon of a chestnut colour in very fine twisted threads, showing that it had been subjected to a very high pressure (a similar variety was met with in various samples of the Canon Diabolo), and lastly (3) an inconsiderable quantity of an exceeding dense mass which was freed from the admixture of the lighter modifications by treatment with aqua regia, sulphuric and hydrofluoric acids, and from which Moissan, by means of liquid bromoform (sp. gr. 2·900), succeeded in separating some small pieces, having a greater density than bromoform, which scratched the ruby and had the properties of the diamond. Some of these pieces were black, others were transparent and refracted light strongly. The dark grey tint of the former resembled that of the black diamonds (carbonado). Their density was between 3 and 3·5. The transparent specimens had a greasy appearance and seemed to be, as it were, surrounded by an envelope of carbon. At 1,050° they did not burn entirely in a current of air, so that the imperfectly burnt particles, and a peculiar form of grains of a light ochre colour, which retained their crystalline form, could be examined under the microscope. Similar grains also remain after the imperfect combustion of the ordinary diamond. Moissan obtained the same results by rapidly cooling in a stream of coal gas a piece of cast iron, saturated with carbon obtained from sugar and first heated to 2,000°. In this instance he obtained small crystals of diamonds. K. Chroustchoff showed that at its boiling point silver dissolves 6 p.c. of carbon. This silver was rapidly cooled, so that a crust formed on the surface and prevented the metal expanding, and so produced a powerful pressure. A portion of the carbon which separates out under these conditions exhibits the properties of the diamond.

[18] The existence of a molecule S6 is known (up to 600°), and it must be held that this accounts for the formation of hydrogen persulphide, H2S5. Phosphorus appears in the molecule P4 and gives P4H2. When expounding the data on specific heat we shall have occasion to return to the question of the complexity of the carbon molecule.

[19] The hydrocarbons poor in hydrogen and containing many atoms of carbon, like chrysene and carbopetrocene, &c., CnH2(n-m), are solids, and less fusible as n and m increase. They present a marked approach to the properties of the diamond. And in proportion to the diminution of the water in the carbohydrates CnH2mOm—for example in the humic compounds (Note 5)—the transition of complex organic substances to charcoal is very evident. That residue resembling charcoal and graphite which is obtained by the separation (by means of copper sulphate and sodium chloride) of iron from white cast-iron containing carbon chemically combined with the iron, also seems, especially after the researches of G. A. Zaboudsky, to be a complex substance containing C12H6O3. The endeavours which have been directed towards determining the measure of complexity of the molecules of charcoal, graphite, and the diamond will probably at some period lead to the solution of this problem and will most likely prove that the various forms of charcoal, graphite, and the diamond contain molecules of different and very considerable complexity. The constancy of the grouping of benzene, C6H6, and the wide diffusion and facility of formation of the carbohydrates containing C6 (for example, cellulose, C6H10O5, glucose, C6H12O6) give reason for thinking that the group C6 is the first and simplest of those possible to free carbon, and it may be hoped that some time or other it may be possible to get carbon in this form. Perhaps in the diamond there may be found such a relation between the atoms as in the benzene group, and in charcoal such as in carbohydrates.

[20] When charcoal burns, the complex molecule Cn is resolved into the simple molecules nCO2, and therefore part of the heat—probably no small amount—is expended in the destruction of the complex molecule Cn. Perhaps by burning the most complex substances, which are the poorest as regards hydrogen, it may be possible to form an idea of the work required to split up Cn into separate atoms.

[21] The viscosity, or degree of mobility, of liquids is determined by their internal friction. It is estimated by passing the liquids through narrow (capillary) tubes, the mobile liquids passing through with greater facility and speed than the viscid ones. The viscosity varies with the temperature and nature of the liquids, and in the case of solutions changes with the amount of the substance dissolved, but is not proportional to it. So that, for example, with alcohol at 20° the viscosity will be 69, and for a 50 p.c. solution 160, the viscosity of water being taken as 100. The volume of the liquid which passes through by experiment (Poiseuille) and theory (Stokes) is proportional to the time, the pressure, and the fourth power of the diameter of the (capillary) tube, and inversely proportional to the length of the tube; this renders it possible to form comparative estimates of the coefficients of internal friction and viscosity.

As the complexity of the molecules of hydrocarbons and their derivatives increases by the addition of carbon (or CH2), so does the degree of viscosity also rise. The extensive series of investigations referring to this subject still await the necessary generalisation. That connection which (already partly observed) ought to exist between the viscosity and the other physical and chemical properties, forces us to conclude that the magnitude of internal friction plays an important part in molecular mechanics. In investigating organic compounds and solutions, similar researches ought to stand foremost. Many observations have already been made, but not much has yet been done with them; the bare facts and some mechanical data exist, but their relation to molecular mechanics has not been cleared up in the requisite degree. It has already been seen from existing data that the viscosity at the temperature of the absolute boiling point becomes as small as in gases.

[22] In a number of hydrocarbons and their derivatives such a substitution of metals for the hydrogen may be attained by indirect means. The property shown by acetylene, C2H2, and its analogues, of forming metallic derivatives is in this respect particularly characteristic. Judging from the fact that carbon is an acid element (that is, gives an acid anhydride with oxygen), though comparatively slightly acid (for carbonic acid is not at all a strong acid and compounds of chlorine and carbon, even CCl4, are not decomposed by water as is the case with phosphorus chloride and even silicic chloride and boric chloride, although they correspond with acids of but little energy), one might expect to find in the hydrogen of hydrocarbons this faculty for being substituted by metals. The metallic compounds which correspond with hydrocarbons are known under the name of organo-metallic compounds. Such, for instance, is zinc ethyl, Zn(C2H5)2, which corresponds with ethyl hydride or ethane, C2H6, in which two atoms of hydrogen have been exchanged for one of zinc.

[23] Gaseous and volatile hydrocarbons decompose when passed through a heated tube. When hydrocarbons are decomposed by heating, the primary products are generally other more stable hydrocarbons, among which are acetylene, C2H2, benzene, C6H6, naphthalene, C10H8, &c.

[24] Wagner (1888) showed that when unsaturated hydrocarbons are shaken with a weak (1 p.c.) solution of potassium permanganate, KMnO4, at ordinary temperatures, they form glycols—for example, C2H4 yields C2H6O2.

[25] My article on this subject appeared in the Journal of the St. Petersburg Academy of Sciences in 1861. Up to that time, although many additive combinations with hydrocarbons and their derivatives were known, they had not been generalised, and were even continually quoted as cases of substitution. Thus the combination of ethylene, C2H4, with chlorine, Cl2, was often regarded as a formation of the products of the substitution of C2H5Cl and HCl, which it was supposed were held together as the water of crystallisation is in salts. Even earlier than this (1857, Journal of the Petroffsky Academy) I considered similar cases as true compounds. In general, according to the law of limits, an unsaturated hydrocarbon, or its derivative, on combining with rX2, gives a substance which is saturated or else approaching the limit. The investigations of Frankland with many organo-metallic compounds clearly showed the limit in the case of metallic compounds, which we shall constantly refer to later on.

[26] The conception of homology has been applied by Gerhardt to all organic compounds in his classical work, ‘TraitÉ de Chimie Organique,’ finished in 1855 (4 vols.), in which he divided all organic compounds into fatty and aromatic, which is in principle still adhered to at the present time, although the latter are more often called benzene derivatives, on account of the fact that KekulÉ, in his beautiful investigations on the structure of aromatic compounds, showed the presence in them all of the ‘benzene nucleus,’ C6H6.

[27] This is always true for hydrocarbons, but for derivatives of the lower homologues the law is sometimes different; for instance, in the series of saturated alcohols, CnH2n+1(OH), when n = 0, we obtain water, H(OH), which boils at 100°, and whose specific gravity at 15° = 0·9992; when n = 1, wood spirit CH3(OH), which boils at 66°, and at 15° has a specific gravity = 0·7964; when n = 2, ordinary alcohol, C2H5(OH), boiling at 78°, specific gravity at 15° = 0·7936, and with further increase of CH2 the specific gravity increases. For the glycols CnH2n(OH)2 the phenomenon of a similar kind is still more striking; at first the temperature of the boiling point and the density increase, and then for higher (more complex) members of the series diminish. The reason for this phenomenon, it is evident, must be sought for in the influence and properties of water, and that strong affinity which, acting between hydrogen and oxygen, determines many of the exceptional properties of water (Chapter I.).

[28] As, for example, in the saturated series of hydrocarbons CnH2n+2, the lowest member (n = 0) must be taken as hydrogen H2, a gas which (t.c. below -190°) is liquefied with great difficulty, and when in a liquid state has doubtless a very small density. Where n = 1, 2, 3, the hydrocarbons CH4, C2H6, C3H8 are gases, more and more readily liquefiable. The temperature of the absolute boiling point for CH4 = -100°, and for ethane C2H6, and in the higher members it rises. The hydrocarbon C4H10, liquefies at about 0°. C5H12 (there are several isomers) boils at from +9° (Lvoff) to 37°, C6H14 from 58° to 78°, &c. The specific gravities in a liquid state at 15° are:—

C5H12 C6H14 C7H16 C10H22 C16H34
0·63 0·66 0·70 0·75 0·85

[29] If, at the ordinary temperature (assuming therefore that the water formed will be in a liquid state) a gram molecule (26 grams) of acetylene, C2H2, be burnt, 310 thousand calories will be emitted (Thomsen), and as 12 grams of charcoal produce 97 thousand calories, and 2 grams of hydrogen 69 thousand calories, it follows that, if the hydrogen and carbon of the acetylene were burnt there would be only 2 × 97 + 69, or 263 thousand calories produced. It is evident, then, that acetylene in its formation absorbs 310–263, or 47 thousand calories.

For considerations relative to the combustion of carbon compounds, we will first enumerate the quantity of heat separated by the combustion of definite chemical carbon compounds, and then give a few figures bearing on the kinds of fuel used in practice.

For molecular quantities in perfect combustion the following amounts of heat are given out (when gaseous carbonic anhydride and liquid water are formed), according to Thomsen's data (1) for gaseous CnH2n+2: 52·8 + 158·8n thousand calories; (2) for CnH2n: 17·7 + 158·1n thousand calories; (3) according to Stohmann (1888) for liquid saturated alcohols, CnH2n+2O: 11·8 + 156·3n, and as the latent heat of evaporation = about 8·2 + 0·6n, in a gaseous state, 20·0 + 156·9n; (4) for monobasic saturated liquid acids, CnH2nO2:—95·3 + 154·3n, and as their latent heat of evaporation is about 5·0 + 1·2n, in a gaseous form, about—90 + 155n; (5) for solid saturated bibasic acids, CnH2n-2O4:—253·8 + 152·6n, if they are expressed as CnH2nC2H2O4, then 51·4 + 152·6n; (6) for benzene and its liquid homologues (still according to Stohmann) CnH2n-6:—158·6 + 156·3n, and in a gaseous form about—155 + 157n; (7) for the gaseous homologues of acetylene, CnH2n-2 (according to Thomsen)—5 + 157n. It is evident from the preceding figures that the group CH2, or CH3 substituted for H, on burning gives out from 152 to 159 thousand calories. This is less than that given out by C + H2, which is 97 + 69 or 166 thousand; the reason for this difference (it would be still greater if carbon were gaseous) is the amount of heat separated during the formation of CH2. According to Stohmann, for dextroglucose, C6H12O6, it is 673·7; for common sugar, C12H22O11, 1325·7; for cellulose, C6H10O5, 678·0; starch, 677·5; dextrin, 666·2; glycol, C2H6O2, 281·7; glycerine, 397·2, &c. The heat of combustion of the following solids (determined by Stohmann) is expressed per unit of weight: naphthalene, C10H8, 9,621; urea, CN2H4O, 2,465; white of egg, 5,579; dry rye bread, 4,421; wheaten bread, 4,302; tallow, 9,365; butter, 9,192; linseed oil, 9,323. The most complete collection of arithmetical data for the heats of combustion will be found in V. F. Longinin's work, ‘Description of the Various Methods of Determining the Heats of Combustion of Organic Compounds’ (Moscow, 1894).

The number of units of heat given out by unit weight during the complete combustion and cooling of the following ordinary kinds of fuel in their usual state of dryness and purity are:—(1) for wood charcoal, anthracite, semi-anthracite, bituminous coal and coke, from 7,200 to 8,200; (2) dry, long flaming coals, and the best brown coals, from 6,200 to 6,800; (3) perfectly dry wood, 3,500; hardly dry, 2,500; (4) perfectly dry peat, best kind, 4,500; compressed and dried, 3,000; (5) petroleum refuse and similar liquid hydrocarbons, about 11,000; (6) illuminating gas of the ordinary composition (about 45 vols. H, 40 vols. CH4, 5 vols. CO, and 5 vols. N), about 12,000; (7) producer gas (see next Chapter), containing 2 vols. carbonic anhydride, 30 vols. carbonic oxide, and 68 vols. nitrogen for one part by weight of the whole carbon burnt, 5,300, and for one part by weight of the gas, 910, units of heat; and (8) water gas (see next chapter) containing 4 vols. carbonic anhydride, 8 vols. N2, 24 vols. carbonic oxide, and 46 vols. H2, for one part by weight of the carbon consumed in the generator 10,900, and for one part by weight of the gas, 3,600 units of heat. In these figures, as in all calorimetric observations, the water produced by the combustion of the fuel is supposed to be liquid. As regards the temperature reached by the fuel, it is important to remark that for solid fuel it is indispensable to admit (to ensure complete combustion) twice the amount of air required, but liquid, or pulverised fuel, and especially gaseous fuel, does not require an excess of air; therefore, a kilogram of charcoal, giving 8,000 units of heat, requires about 24 kilograms of air (3 kilograms of air per thousand calories) and a kilogram of producer gas requires only 0·77 kilogram of air (0·85 kilo. of air per 1,000 calories), 1 kilogram of water gas about 4·5 of air (1·25 kilo. of air per 1,000 calories).

[29 bis] Manure which decomposes under the action of bacteria gives off CO2 and CH4.

[30] It is easy to collect the gas which is evolved in marshy places if a glass bottle be inverted in the water and a funnel put into it (both filled with water); if the mud of the bottom be now agitated, the bubbles which rise may be easily caught by the inverted funnel.

[31]

see caption

Fig. 58.—General view of gas works. B, retorts; f, hydraulic main; H and I, tar well; i, condensers; L, purifiers; P, gasholder.

see caption

Fig. 59.—Blowpipe. Air is blown in at the trumpet-shaped mouthpiece, and escapes in a fine stream from the platinum jet placed at the extremity of the side tube.

see caption

Fig. 60.—Davy safety-lamp. [Modern form.]

Illuminating gas is generally prepared by heating gas coal (see Note 6) in oval cylindrical horizontal cast-iron or clay retorts. Several such retorts BB (fig. 58) are disposed in the furnace A, and heated together. When the retorts are heated to a red heat, lumps of coal are thrown into them, and they are then closed with a closely fitting cover. The illustration shows the furnace, with five retorts. Coke (see Note 1, dry distillation) remains in the retorts, and the volatile products in the form of vapours and gases travel along the pipe d, rising from each retort. These pipes branch above the stove, and communicate with the receiver f (hydraulic main) placed above the furnace. Those products of the dry distillation which most easily pass from the gaseous into the liquid and solid states collect in the hydraulic main. From the hydraulic main the vapours and gases travel along the pipe g and the series of vertical pipes j (which are sometimes cooled by water trickling over the surface), where the vapours and gases cool from the contact of the colder surface, and a fresh quantity of vapour condenses. The condensed liquids pass from the pipes g and j and into the troughs H. These troughs always contain liquid at a constant level (the excess flowing away) so that the gas cannot escape, and thus they form, as it is termed, a hydraulic joint. In the state in which it leaves the condensers the gas consists principally of the following vapours and gases: (1) vapour of water, (2) ammonium carbonate, (3) liquid hydrocarbons, (4) hydrogen sulphide, H2S, (5) carbonic anhydride, CO2, (6) carbonic oxide, CO, (7) sulphurous anhydride, SO2, but a great part of the illuminating gas consists of (8) hydrogen, (9) marsh gas, (10) olefiant gas, C2H4, and other gaseous hydrocarbons. The hydrocarbons (3, 9, and 10), the hydrogen, and carbonic oxide are capable of combustion, and are useful component parts, but the carbonic anhydride, the hydrogen sulphide, and sulphurous anhydride, as well as the vapours of ammonium carbonate, form an injurious admixture, because they do not burn (CO2, SO2) and lower the temperature and brilliancy of the flame, or else, although capable of burning (for example, H2S, CS2, and others), they give out during combustion sulphurous anhydride which has a disagreeable smell, is injurious when inhaled, and spoils many surrounding objects. In order to separate the injurious products, the gas is washed with water, a cylinder (not shown in the illustration) filled with coke continually moistened with water serving for this purpose. The water coming into contact with the gas dissolves the ammonium carbonate; hydrogen sulphide, carbonic anhydride, and sulphurous anhydride, being only partly soluble in water, have to be got rid of by a special means. For this purpose the gas is passed through moist lime or other alkaline liquid, as the above-mentioned gases have acid properties and are therefore retained by the alkali. In the case of lime, calcium carbonate, sulphite and sulphide, all solid substances, are formed. It is necessary to renew the purifying material as its absorbing power decreases. A mixture of lime and sulphate of iron, FeSO4, acts still better, because the latter, with lime, Ca(HO)2, forms ferrous hydroxide, Fe(HO)2 and gypsum, CaSO4. The suboxide (partly turning into oxide) of iron absorbs H2S, forming FeS and H2O, and the gypsum retains the remainder of the ammonia, the excess of lime absorbing carbonic anhydride and sulphuric anhydride. [In English works a native hydrated ferric hydroxide is used for removing hydrogen sulphide.] This purification of the gas takes place in the apparatus L, where the gas passes through perforated trays m, covered with sawdust mixed with lime and sulphate of iron. It is necessary to remark that in the manufacture of gas it is indispensable to draw off the vapours from the retorts, so that they should not remain there long (otherwise the hydrocarbons would in a considerable degree be resolved into charcoal and hydrogen), and also to avoid a great pressure of gas in the apparatus, otherwise a quantity of gas would escape at all cracks such as must inevitably exist in such a complicated arrangement. For this purpose there are special pumps (exhausters) so regulated that they only pump off the quantity of gas formed (the pump is not shown in the illustration). The purified gas passes through the pipe n into the gasometer (gasholder) P, a dome made of iron plate. The edges of the dome dip into water poured into a ring-shaped channel g, in which the sides of the dome rise and fall. The gas is collected in this holder, and distributed to its destination by pipes communicating with the pipe o, issuing from the dome. The pressure of the dome on the gas enables it, on issuing from a long pipe, to penetrate through the small aperture of the burner. A hundred kilograms of coal give about 20 to 30 cubic metres of gas, having a density from four to nine times greater than that of hydrogen. A cubic metre (1,000 litres) of hydrogen weighs about 87 grams; therefore 100 kilograms of coal give about 18 kilograms of gas, or about one-sixth of its weight. Illuminating gas is generally lighter than marsh gas, as it contains a considerable amount of hydrogen, and is only heavier than marsh gas when it contains much of the heavier hydrocarbons. Thus olefiant gas, C2H4, is fourteen times, and the vapours of benzene thirty-nine times, heavier than hydrogen, and illuminating gas sometimes contains 15 p.c. of its volume of them. The brilliancy of the flame of the gas increases with the quantity of olefiant gas and similar heavy hydrocarbons, as it then contains more carbon for a given volume and a greater number of carbon particles are separated. Gas usually contains from 35 to 60 p.c. of its volume of marsh gas, from 30 to 50 p.c. of hydrogen, from 3 to 5 p.c. of carbonic oxide, from 2 to 10 p.c. heavy hydrocarbons, and from 3 to 10 p.c. of nitrogen. Wood gives almost the same sort of gas as coal and almost the same quantity, but the wood gas contains a great deal of carbonic anhydride, although on the other hand there is an almost complete absence of sulphur compounds. Tar, oils, naphtha, and such materials furnish a large quantity of good illuminating gas. An ordinary burner of 8 to 12 candle-power burns 5 to 6 cubic feet of coal gas per hour, but only 1 cubic foot of naphtha gas. One pood (36 lbs. Eng.) of naphtha gives 500 cubic feet of gas—that is, one kilogram of naphtha produces about one cubic metre of gas. The formation of combustible gas by heating coal was discovered in the beginning of the last century, but only put into practice towards the end by Le-Bon in France and Murdoch in England. In England, Murdoch, together with the renowned Watt, built the first gas works in 1805.

In practice illuminating gas is not only used for lighting (electricity and kerosene are cheaper in Russia), but also as the motive power for gas engines (see p. 175), which consume about half a cubic metre per horse-power per hour; gas is also used in laboratories for heating purposes. When it is necessary to concentrate the heat, either the ordinary blowpipe (fig. 59) is applied, placing the end in the flame and blowing through the mouthpiece; or, in other forms, gas is passed through the blowpipe; when a large, hot, smokeless flame is required for heating crucibles or glass-blowing, a foot-blower is used. High temperatures, which are often required for laboratory and manufacturing purposes, are most easily attained by the use of gaseous fuel (illuminating gas, producer gas, and water gas, which will be treated of in the following chapter), because complete combustion may be effected without an access of air. It is evident that in order to obtain high temperatures means must be taken to diminish the loss of heat by radiation, and to ensure perfect combustion.

[32] The gas which is set free in coal mines contains a good deal of nitrogen, some carbonic anhydride, and a large quantity of marsh gas. The best means of avoiding an explosion consists in efficient ventilation. It is best to light coal mines with electric lamps.

[33] The Davy lamp, of which an improved form is represented in the accompanying figure, is used for lighting coal and other mines where combustible gas is found. The wick of the lamp is enclosed in a thick glass cylinder which is firmly held in a metallic holder. Over this a metallic cylinder and the wire gauze are placed. The products of combustion pass through the gauze, and the air enters through the space between the cylinder and the wire gauze. To ensure greater safety the lamp cannot be opened without extinguishing the flame.

[34] In Pennsylvania (beyond the Alleghany mountains) many of the shafts sunk for petroleum only emitted gas, but many useful applications for it were found and it was conducted in metallic pipes to works hundreds of miles distant, principally for metallurgical purposes.

[35] The purest gas is prepared by mixing the liquid substance called zinc methyl, Zn(CH3)2, with water, when the following reaction occurs:

Zn(CH3)2 + 2HOH = Zn(HO)2 + 2CH3H.

[36] Methylene, CH2, does not exist. When attempts are made to obtain it (for example, by removing X2 from CH2X2), C2H4 or C3H6 are produced—that is to say, it undergoes polymerisation.

[37] Although the methods of formation and the reactions connected with hydrocarbons are not described in this work, because they are dealt with in organic chemistry, yet in order to clearly show the mechanism of those transformations by which the carbon atoms are built up into the molecules of the carbon compounds, we here give a general example of reactions of this kind. From marsh gas, CH4, on the one hand the substitution of chlorine or iodine, CH3Cl, CH3I, for the hydrogen may be effected, and on the other hand such metals as sodium may be substituted for the hydrogen, e.g. CH3Na. These and similar products of substitution serve as a means of obtaining other more complex substances from given carbon compounds. If we place the two above-named products of substitution of marsh gas (metallic and haloid) in mutual contact, the metal combines with the halogen, forming a very stable compound—namely, common salt, NaCl, and the carbon groups which were in combination with them separate in mutual combination, as shown by the equation:

CH3Cl + CH3Na = NaCl + C2H6.

This is the most simple example of the formation of a complex hydrocarbon from these radicles. The cause of the reaction must be sought for in the property which the haloid (chlorine) and sodium have of entering into mutual combination.

[38] When m = n - 1, we have the series CnH2. The lowest member is acetylene, C2H2. These are hydrocarbons containing a minimum amount of hydrogen.

[39] For instance, ethylene, C2H4, combines with Br2, HI, H2SO4, as a whole molecule, as also does amylene, C5H10, and, in general, CnH2n.

[40] For instance, ethylene is obtained by removing the water from ethyl alcohol, C2H5(OH), and amylene, C5H10, from amyl alcohol, C5H11(OH), or in general CnH2n, from CnH2n+1(OH).

[41] Acetylene and its polymerides have an empirical composition CH, ethylene and its homologues (and polymerides) CH2, ethane CH3, methane CH4. This series presents a good example of the law of multiple proportions, but such diverse proportions are met with between the number of atoms of the carbon and hydrogen in the hydrocarbons already known that the accuracy of Dalton's law might be doubted. Thus the substances C30H62 and C30H60 differ so slightly in their composition by weight as to be within the limits of experimental error, but their reactions and properties are so distinct that they can be distinguished beyond a doubt. Without Dalton's law chemistry could not have been brought to its present condition, but it cannot alone express all those gradations which are quite clearly understood and predicted by the law of Avogadro-Gerhardt.

[42] The conception of the structure of carbon compounds—that is, the expression of those unions and correlations which their atoms have in the molecules—was for a long time limited to the representation that organic substances contained complex radicles (for instance, ethyl C2H5, methyl CH3, phenyl C6H5, &c.); then about the year 1840 the phenomena of substitution and the correspondence of the products of substitution with the primary bodies (nuclei and types) were observed, but it was not until about the year 1860 and later when on the one hand the teaching of Gerhardt about molecules was spreading, and on the other hand the materials had accumulated for discussing the transformations of the simplest hydrocarbon compounds, that conjectures began to appear as to the mutual connection of the atoms of carbon in the molecules of the complex hydrocarbon compounds. Then KekulÉ and A. M. Butleroff began to formulate the connection between the separate atoms of carbon, regarding it as a quadrivalent element. Although in their methods of expression and in some of their views they differ from each other and also from the way in which the subject is treated in this work, yet the essence of the matter—namely, the comprehension of the causes of isomerism and of the union between the separate atoms of carbon—remains the same. In addition to this, starting from the year 1870, there appears a tendency which from year to year increases to discover the actual spacial distribution of the atoms in the molecules. Thanks to the endeavours of Le-Bel (1874), Van't Hoff (1874), and Wislicenus (1887) in observing cases of isomerism—such as the effect of different isomerides on the direction of the rotation of the plane of polarisation of light—this tendency promises much for chemical mechanics, but the details of the still imperfect knowledge in relation to this matter must be sought for in special works devoted to organic chemistry.

[43] Direct experiment shows that however CH3X is prepared (where X = for instance Cl, &c.) it is always one and the same substance. If, for example, in CX4, X is gradually replaced by hydrogen until CH3X is produced, or in CH4, the hydrogen by various means is replaced by X, or else, for instance, if CH3X be obtained by the decomposition of more complex compounds, the same product is always obtained.

This was shown in the year 1860, or thereabout, by many methods, and is the fundamental conception of the structure of hydrocarbon compounds. If the atoms of hydrogen in methyl were not absolutely identical in value and position (as they are not, for instance, in CH3CH2CH3 or CH3CH2X), then there would be as many different forms of CH3X as there were diversities in the atoms of hydrogen in CH4. The scope of this work does not permit of a more detailed account of this matter. It is given in works on organic chemistry.

[44] The union of carbon atoms in closed chains or rings was first suggested by KekulÉ as an explanation of the structure and isomerism of the derivatives of benzene, C6H6, forming aromatic compounds (Note 26).

[45] The following are the most generally known of the oxygenised but non-nitrogenous hydrocarbon derivatives. (1) the alcohols. These are hydrocarbons in which hydrogen is exchanged for hydroxyl (OH). The simplest of these is methyl alcohol, CH3(OH), or wood spirit obtained by the dry distillation of wood. The common spirits of wine or ethyl alcohol, C2H3(OH), and glycol, C2H4(OH)2, correspond with ethane. Normal propyl alcohol, CH3CH2CH2(OH), and isopropyl alcohol, CH3CH(OH)CH3, propylene-glycol, C3H6(OH)2, and glycerol, C3H3(OH)3 (which, with stearic and other acids, forms fatty substances), correspond with propane, C3H8. All alcohols are capable of forming water and ethereal salts with acids, just as alkalis form ordinary salts. (2) Aldehydes are alcohols minus hydrogen; for instance, acetaldehyde, C2H4O, corresponds with ethyl alcohol. (3) It is simplest to regard organic acids as hydrocarbons in which hydrogen has been exchanged for carboxyl (CO2H), as will be explained in the following chapter. There are a number of intermediate compounds; for example, the aldehyde-alcohols, alcohol-acids (or hydroxy-acids), &c. Thus the hydroxy-acids are hydrocarbons in which some of the hydrogen has been replaced by hydroxyl, and some by carboxyl; for instance, lactic acid corresponds with C2H6, and has the constitution C2H4(OH)(CO2H). If to these products we add the haloid salts (where H is replaced by Cl, Br, I), the nitro-compounds containing NO2 in place of H, the amides, cyanides, ketones, and other compounds, it will be readily seen what an immense number of organic compounds there are and what a variety of properties these substances have; this we see also from the composition of plants and animals.

[46] Ethylene bromide, C2H4Br2, when gently heated in alcoholic solution with finely divided zinc, yields pure ethylene, the zinc merely taking up the bromine (Sabaneyeff).

[47] Ethylene decomposes somewhat easily under the influence of the electric spark, or a high temperature. In this case the volume of the gas formed may remain the same when olefiant gas is decomposed into carbon and marsh gas, or may increase to double its volume when hydrogen and carbon are formed, C2H4 = CH4 + C = 2C + 2H2. A mixture of olefiant gas and oxygen is highly explosive; two volumes of this gas require six volumes of oxygen for its perfect combustion. The eight volumes thus taken then resolve themselves into eight volumes of the products of combustion, a mixture of water and carbonic anhydride, C2H4 + 3O2 = 2CO2 + 2H2O. On cooling after the explosion diminution of volume occurs because the water becomes liquid. For two volumes of the olefiant gas taken, the diminution will be equal to four volumes, and the same for marsh gas. The quantity of carbonic anhydride formed by both gases is not the same. Two volumes of marsh gas give only two volumes of carbonic anhydride, and two volumes of ethylene give four volumes of carbonic anhydride.

[48] The homologues of ethylene, CnH2n, are also capable of direct combination with halogens, &c., but with various degrees of facility. The composition of these homologues can be expressed thus: (CH3)_x(CH2)y(CH)zCr, where the sum of x + z is always an even number, and the sum of x + z + r is equal to half the sum of 3x + z, whence z + 2r = x; by this means the possible isomerides are determined. For example, for butylenes, C4H8, (CH3)2(CH)2, (CH3)2(CH2)C, (CH2)(CH2)2CH, and (CH2)4 are possible.

[48 bis] See also method of preparing C2H2 in Note 12 bis.

[49] This is easily accomplished with those gas burners which are used in laboratories and mentioned in the Introduction. In these burners the gas is first mixed with air in a long tube, above which it is kindled. But if it be lighted inside the pipe it does not burn completely, but forms acetylene, on account of the cooling effect of the walls of the metallic tube; this is detected by the smell, and may be shown by passing the issuing gas (by aid of an aspirator) into an ammoniacal solution of cuprous chloride.

[50] Amongst the homologues of acetylene CnH2n-2, the lowest is C3H4; allylene, CH3CCH, and allene, CH2CCH2, are known, but the closed structure, CH2(CH)2, is little investigated.

[51] The saturated hydrocarbons predominate in American petroleum, especially in its more volatile parts; in Baku naphtha the hydrocarbons of the composition CnH2n form the main part (Lisenko, Markovnikoff, Beilstein) but doubtless (MendelÉeff) it also contains saturated ones, CnH2n+2. The structure of the naphtha hydrocarbons is only known for the lower homologues, but doubtless the distinction between the hydrocarbons of the Pennsylvanian and Baku naphthas, boiling at the same temperature (after the requisite refining by repeated fractional distillation, which can be very conveniently done by means of steam rectification—that is, by passing the steam through the dense mass), depends not only on the predominance of saturated hydrocarbons in the former, and naphthenes, CnH2n, in the latter, but also on the diversity of composition and structure of the corresponding portions of the distillation. The products of the Baku naphtha are richer in carbon (therefore in a suitably constructed lamp they ought to give a brighter light), they are of greater specific gravity, and have greater internal friction (and are therefore more suitable for lubricating machinery) than the American products collected at the same temperature.

[52] The formation of naphtha fountains (which burst forth after the higher clay strata covering the layers of sands impregnated with naphtha have been bored through) is without doubt caused by the pressure or tension of the combustible hydrocarbon gases which accompany the naphtha, and are soluble in it under pressure. Sometimes these naphtha fountains reach a height of 100 metres—for instance, the fountain of 1887 near Baku. Naphtha fountains generally act periodically and their force diminishes with the lapse of time, which might be expected, because the gases which cause the fountains find an outlet, as the naphtha issuing from the bore-hole carries away the sand which was partially choking it up.

[53] This is a so-called intermediate oil (between kerosene and lubricating oils), solar oil, or pyronaphtha. Lamps are already being manufactured for burning it but still require improvement. Above all, however, it requires a more extended market, and this at present is wanting, owing to the two following reasons: (1) Those products of the American petroleum which are the most widely spread and almost universally consumed contain but little of this intermediate oil, and what there is is divided between the kerosene and the lubricating oils; (2) the Baku naphtha, which is capable of yielding a great deal (up to 30 p.c.) of intermediate oil, is produced in enormous quantities, about 300 million poods, but has no regular markets abroad, and for the consumption in Russia (about 25 million poods of kerosene per annum) and for the limited export (60 million poods per annum) into Western Europe (by the Trans-Caucasian Railway) those volatile and more dangerous parts of the naphtha which enter into the composition of the American petroleum are sufficient, although Baku naphtha yields about 25 p.c. of such kerosene. For this reason pyronaphtha is not manufactured in sufficient quantities, and the whole world is consuming the unsafe kerosene. When a pipe line has been laid from Baku to the Black Sea (in America there are many which carry the raw naphtha to the sea-shore, where it is made into kerosene and other products) then the whole mass of the Baku naphtha will furnish safe illuminating oils, which without doubt will find an immense application. A mixture of the intermediate oil with kerosene or Baku oil (specific gravity 0·84 to 0·85) may be considered (on removing the benzoline) to be the best illuminating oil, because it is safe (flashing point from 40° to 60°), cheaper (Baku naphtha gives as much as 60 p.c. of Baku oil), and burns perfectly well in lamps differing but little from those made for burning American kerosene (unsafe, flashing point 20° to 30°).

[54] The substitution of Baku pyronaphtha, or intermediate oil, or Baku oil (see Note 53), would not only be a great advantage as regards safety from fire, but would also be highly economical. A ton (62 poods) of American crude petroleum costs at the coast considerably more than 24s. (12 roubles), and yields two-thirds of a ton of kerosene suitable for ordinary lamps. A ton of raw naphtha in Baku costs less than 4s. (1 rouble 80 copecks), and with a pipe line to the shore of the Black Sea would not cost more than 8 roubles, or 16s. Moreover, a ton of Baku naphtha will yield as much as two-thirds of a ton of kerosene, Baku oil, and pyronaphtha suitable for illuminating purposes.

[55] Naphtha has been applied for heating purposes on a large scale in Russia, not only on account of the low cost of naphtha itself and of the residue from the preparation of kerosene, but also because the products of all the Baku naphtha do not find an outlet for general consumption. Naphtha itself and its various residues form excellent fuel, burning without smoke and giving a high temperature (steel and iron may be easily melted in the flame). A hundred poods of good coal (for instance, Don coal) used as fuel for heating boilers are equivalent to 36 cubic feet (about 250 poods) of dry wood, while only 70 poods of naphtha will be required; and moreover there is no need for stoking, as the liquid can be readily and evenly supplied in the required quantity. The economic and other questions relating to American and Baku petroleums have been discussed more in detail in some separate works of mine (D. MendelÉeff): (1) ‘The Naphtha Industry of Pennsylvania and the Caucasus,’ 1870; (2) ‘Where to Build Naphtha Works,’ 1880; (3) ‘On the Naphtha Question,’ 1883; (4) ‘The Baku Naphtha Question,’ 1886; (5) the article on the naphtha industry in the account of the Russian industries printed for the Chicago Exhibition.

[56] As during the process of the dry distillation of wood, sea-weed, and similar vegetable dÉbris, and also when fats are decomposed by the action of heat (in closed vessels), hydrocarbons similar to those of naphtha are formed, it was natural that this fact should have been turned to account to explain the formation of the latter. But the hypothesis of the formation of naphtha from vegetable dÉbris inevitably assumes coal to be the chief element of decomposition, and naphtha is met with in Pennsylvania and Canada, in the Silurian and Devonian strata, which do not contain coal, and correspond to an epoch not abounding in organic matter. Coal was formed from the vegetable dÉbris of the Carboniferous, Jurassic, and other recent strata, but judging more from its composition and structure, it has been subjected to the same kind of decomposition as peat; nor could liquid hydrocarbons have been thus formed to such an extent as we see in naphtha. If we ascribe the derivation of naphtha to the decomposition of fat (adipose, animal fat) we encounter three almost insuperable difficulties: (1) Animal remains would furnish a great deal of nitrogenous matter, whilst there is but very little in naphtha; (2) the enormous quantity of naphtha already discovered as compared with the insignificant amount of fat in the animal carcase; (3) the sources of naphtha always running parallel to mountain chains is completely inexplicable. Being struck with this last-mentioned circumstance in Pennsylvania, and finding that the sources in the Caucasus surround the whole Caucasian range (Baku, Tiflis, Gouria, Kouban, Tamman, Groznoe, Dagestan), I developed in 1876 the hypothesis of the mineral origin of naphtha expounded further on.

[57] During the upheaval of mountain ranges crevasses would be formed at the peaks with openings upwards, and at the foot of the mountains with openings downwards. These cracks in course of time fill up, but the younger the mountains the fresher the cracks (the Alleghany mountains are, without doubt, more ancient than the Caucasian, which were formed during the tertiary epoch); through them water must gain access deep into the recesses of the earth to an extent that could not occur on the level (on plains). The situation of naphtha at the foot of mountain chains is the principal argument in my hypothesis.

Another fundamental reason is the consideration of the mean density of the earth. Cavendish, Airy, Cornu, Boys, and many others who have investigated the subject by various methods, found that, taking water = 1, the mean density of the earth is nearly 5·5. As at the surface water and all rocks (sand, clay, limestone, granite, &c.) have a density less than 3, it is evident (as solid substances are but slightly compressible even under the greatest pressure) that inside the earth there are substances of a greater density—indeed, not less than 7 or 8. What conclusion, then, can be arrived at? Anything heavy contained in the bosom of the earth must be distributed not only on its surface, but throughout the whole solar system, for everything tends to show that the sun and planets are formed from the same material, and according to the hypothesis of Laplace and Kant it is most probable, and indeed must necessarily be held, that the earth and planets are but fragments of the solar atmosphere, which have had time to cool considerably and become masses semi-liquid inside and solid outside, forming both planets and satellites. The sun amongst other heavy elements contains a great deal of iron, as shown by spectrum analysis. There is also much of it in an oxidised condition on the surface of the earth. Meteoric stones, carried as fragmentary planets in the solar system and sometimes falling upon the earth, consisting of siliceous rocks similar to terrestrial ones, often contain either dense masses of iron (for example, the Pallosovo iron preserved in the St. Petersburg Academy of Sciences) or granular masses (for instance, the Okhansk meteorite of 1886). It is therefore possible that the interior of the earth contains much iron in a metallic state. This might be anticipated from the hypothesis of Laplace, for the iron must have been compressed into a liquid at that period when the other component parts of the earth were still strongly heated, and oxides of iron could not then have been formed. The iron was covered with slags (mixtures of silicates like glass fused with rocky matter) which did not allow it to burn at the expense of the oxygen of the atmosphere or of water, just at that time when the temperature of the earth was very high. Carbon was in the same state; its oxides were also capable of dissociation (Deville); it is also but slightly volatile, and has an affinity for iron, and iron carbide is found in meteoric stones (as well as carbon and even the diamond). Thus the supposition of the existence of iron carbides in the interior of the earth was derived by me from many indications, which are to some extent confirmed by the fact that granular pieces of iron have been found in some basalts (ancient lava) as well as in meteoric stones. The occurrence of iron in contact with carbon during the formation of the earth is all the more probable because those elements predominate in nature which have small atomic weights, and among them the most widely diffused, the most difficultly fusible, and therefore the most easily condensed (Chapter XV.) are carbon and iron. They passed into the liquid state when all compounds were at a temperature of dissociation.

[58] The following is the typical equation for this formation:

3FemCn + 4mH2O = mFe3O4 (magnetic oxide) + C5nH8m (see Chapter XVII., Note 38).

[59] Cloez investigated the hydrocarbons formed when cast-iron is dissolved in hydrochloric acid, and found CnH2n and others. I treated crystalline manganiferous cast-iron with the same acid, and obtained a liquid mixture of hydrocarbons exactly similar to natural naphtha in taste, smell, and reaction.

[60] Probably naphtha was produced during the upheaval of all mountain chains, but only in some cases were the conditions favourable to its being preserved underground. The water penetrating below formed there a mixture of naphtha and watery vapours, and this mixture issued through fissures to the cold parts of the earth's crust. The naphtha vapours, on condensing, formed naphtha, which, if there were no obstacles, appeared on the surface of land and water. Here part of it soaked through formations (possibly the bituminous slates, schists, dolomites, &c., were thus formed), another part was carried away on the water, became oxidised, evaporated, and was driven to the shores (the Caucasian naphtha probably in this way, during the existence of the Aralo-Caspian sea, was carried as far as the Sisran banks of the Volga, where many strata are impregnated with naphtha and products of its oxidation resembling asphalt and pitch); a great part of it was burnt in one way or another—that is, gave carbonic anhydride and water. If the mixture of vapours, water, and naphtha formed inside the earth had no free outlet to the surface, it nevertheless would find its way through fissures to the superior and colder strata, and there become condensed. Some of the formations (clays) which do not absorb naphtha were only washed away by the warm water, and formed mud, which we also now observe issuing from the earth in the form of mud volcanoes. The neighbourhood of Baku and the whole of the Caucasus near the naphtha districts are full of such volcanoes, which from time to time are in a state of eruption. In old naphtha beds (such as the Pennsylvanian) even these blow-holes are closed, and the mud volcanoes have had time to be washed away. The naphtha and the gaseous hydrocarbons formed with it under the pressure of the overlying earth and water impregnated the layers of sand, which are capable of absorbing a great quantity of such liquid, and if above this there were strata impermeable to naphtha (dense, clayey, damp strata) the naphtha would accumulate in them. It is thus preserved from remote geological periods up to the present day, compressed and dissolved under the pressure of the gases which burst out in places forming naphtha fountains. If this be granted, it may be thought that in the comparatively new (geologically speaking) mountain chains, such as the Caucasian, naphtha is even now being formed. Such a supposition may explain the remarkable fact that, in Pennsylvania, localities where naphtha had been rapidly worked for five years have become exhausted, and it becomes necessary to constantly have recourse to sinking new wells in fresh places. Thus, from the year 1859, the workings were gradually transferred along a line running parallel to the Alleghany mountains for a distance of more than 200 miles, whilst in Baku the industry dates from time immemorial (the Persians worked near the village of Ballaghana) and up to the present time keeps to one and the same place. The amounts of the Pennsylvanian and Baku annual outputs are at present equal—namely, about 250 million poods (4 million tons). It may be that the Baku beds, as being of more recent geological formation, are not so exhausted by nature as those of Pennsylvania, and perhaps in the neighbourhood of Baku naphtha is still being formed, which is partially indicated by the continued activity of the mud volcanoes. As many varieties of naphtha contain in solution solid slightly volatile hydrocarbons like paraffin and mineral wax, the production of ozocerite, or mountain wax, is accounted for in conjunction with the formation of naphtha. Ozocerite is found in Galicia, also in the neighbourhood of Novorossisk, in the Caucasus, and on the islands of the Caspian Sea (particularly in the Chileken and Holy Islands); it is met with in large masses, and is used for the production of paraffin and ceresene, for the manufacture of candles, and similar purposes.

As the naphtha treasures of the Caucasus have hardly been exploited (near Baku and near Kouban and Grosnyi), and as naphtha finds numerous uses, the subject presents most interesting features to chemists and geologists, and is worthy of the close attention of practical men.

                                                                                                                                                                                                                                                                                                           

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