CHAPTER II. EXPLOSIVE AGENTS USED IN BLASTING ROCKS. Section I.--Phenomena accompanying an Explosion. Nature of an Explosion.

—The combination of oxygen with other substances for which it has affinity is called generally “oxidation.” The result of this combination is a new substance, and the process of change is accompanied by the liberation of heat. The quantity of heat set free when two substances combine chemically is constant, that is, it is the same under all conditions. If the change takes place within a short space of time, the heat becomes sensible; but if the change proceeds very slowly, the heat cannot be felt. The same quantity, however, is liberated in both cases. Thus, though the quantity of heat set free by a chemical combination is under all conditions the same, the degree or intensity of the heat is determined by the rapidity with which the change is effected.

When oxidation is sufficiently rapid to cause a sensible degree of heat, the process is described as “combustion.” The oxidation of a lump of coke in the furnace, for example, is effected within a short space of time, and, as the quantity of heat liberated by the oxidation of that weight of carbon is great, a high degree results. And it is well known and obvious that as combustion is quickened, or, in other words, as the time of change is shortened, the intensity of the heat is proportionally increased. So in the case of common illuminating gas, the oxidation of the hydrogen is rapidly effected, and, consequently, a high degree of heat ensues.

When oxidation takes place within a space of time so short as to be inappreciable to the senses, the process is described as “explosion.” The combustion of a charge of gunpowder, for example, proceeds with such rapidity that no interval can be perceived to intervene between the commencement and the termination of the process. Oxidation is in this case, therefore, correctly described as an explosion; but the combustion of a train of gunpowder, or of a piece of quick-match, though exceedingly rapid, yet, as it extends over an appreciable space of time, is not to be so described. By analogy, the sudden change of state which takes place when water is “flashed” into steam, is called an explosion. It may be remarked here that the application of this expression to the bursting of a steam boiler is an abuse of language; as well may we speak of an “explosion” of rock.

From a consideration of the facts stated in the foregoing paragraphs, it will be observed that oxidation by explosion gives the maximum intensity of heat.

Measure of Heat, and specific Heat.

—It is known that if a certain quantity of heat will raise the temperature of a body one degree, twice that quantity will raise its temperature two degrees, three times the quantity, three degrees, and so on. Thus we may obtain a measure of heat by which to determine, either the temperature to which a given quantity of heat is capable of raising a given body, or the quantity of heat which is contained in a given body at a given temperature. The quantity of heat requisite to produce a change of one degree in temperature is different for different bodies, but is practically constant for the same body, and this quantity is called the “specific heat” of the body. The standard which has been adopted whereby to measure the specific heat of bodies is that of water, the unit being the quantity of heat required to raise the temperature of 1 lb. of water through 1° Fahr., say from 32° to 33°. The quantity of heat required to produce this change of temperature in 1 lb. of water is called the “unit of heat,” or the “thermal unit.” Having determined the specific heat of water, that of air may in like manner be ascertained, and expressed in terms of the former. It has been proved by experiments that if air be heated at constant pressure through 1° Fahr., the quantity of heat absorbed is 0·2375 thermal units, whatever the pressure or the temperature of the air may be. Similarly it has been shown that the specific heat of air at constant volume is, in thermal units, 0·1687; that is, if the air be confined so that no expansion can take place, 0·1687 of a thermal unit will be required to increase its temperature one degree.

Heat liberated by an Explosion.

—In the oxidation of carbon, one atom of oxygen may enter into combination with one atom of that substance; the resulting body is a gas known as “carbonic oxide.” As the weight of carbon is to that of oxygen as 12 is to 16, 1 lb. of the former substance will require for its oxidation 1¹/₃ lb. of the latter; and since the two enter into combination, the product, carbonic oxide, will weigh 1 + 1¹/₃ = 2¹/₃ lb. The combining of one atom of oxygen with one of carbon throughout this quantity, that is, 1¹/₃ lb. of oxygen, with 1 lb. of carbon, generates 10,100 units of heat. Of this quantity, 5700 units are absorbed in changing the carbon from the solid into the gaseous state, and 4400 are set free. The quantity of heat liberated, namely, the 4400 units, will be expended in raising the temperature of the gas from 32° Fahr., which we will assume to be that of the carbon and the oxygen previous to combustion, to a much higher degree, the value of which may be easily determined. The 4400 units would raise 1 lb. of water from 32° to 32 + 4400 = 4432°; and as the specific heat of carbonic oxide is 0·17 when there is no increase of volume, the same quantity of heat will raise 1 lb. of that gas from 32° to 32 + 4400 0·17 = 25,914°. But in the case under consideration, we have 2¹/₃ lb. of the gas, the resulting temperature of which will be 25,9142¹/₃ = 9718°.

In the oxidation of carbonic oxide, one atom of oxygen combines with one atom of the gaseous carbon; the resulting body is a gas known as “carbonic acid.” Since 2¹/₃ lb. of carbonic oxide contains 1 lb. of carbon, that quantity of the oxide will require 1¹/₃ lb. of oxygen to convert it into the acid, that is, to completely oxidize the original pound of solid carbon. By this combination, 10,100 units of heat are generated, as already stated, and since the carbon is now in the gaseous state, the whole of that quantity will be set free. Hence the temperature of the resulting 3²/₃ lb. of carbonic acid will be

32 + 4400 + 10,100 0·17 × 3·667 = 23,516°.

It will be seen from the foregoing considerations that if 1 lb. of pure carbon be burned in 2²/₃ lb. of pure oxygen, 3²/₃ lb. of carbonic acid is produced, and 14,500 units of heat are liberated; and further, that if the gas be confined within the space occupied by the carbon and the oxygen previously to their combination, the temperature of the product may reach 23,516° Fahr.

In the oxidation of hydrogen, one atom of oxygen combines with two atoms of the former substance; the resulting body is water. As the weight of hydrogen is to that of oxygen as 1 is to 16, 1 lb. of the former gas will require for its oxidation 8 lb. of the latter; and since the two substances enter into combination, the product, water, will weigh 1 + 8 = 9 lb. By this union, 62,032 units of heat are generated. Of this quantity, 8694 are absorbed in converting the water into steam, and 53,338 are set free. The specific heat of steam at constant volume being 0·37, the temperature of the product of combustion, estimated as before, will be

32 + 53,338 0·37 × 9 = 16,049°.

Hence it will be observed that if 1 lb. of hydrogen be burned in 8 lb. of oxygen, 9 lb. of steam will be produced, and 53,338 units of heat will be liberated; and further, that the temperature of the product may reach 16,049°.

Gases generated by an Explosion.

—It was shown in the preceding paragraph that in the combustion of carbon, one atom of oxygen may unite with one atom of carbon to form carbonic oxide, or two atoms of oxygen may unite with one atom of carbon to form carbonic acid. When the combination takes place according to the former proportions, the reaction is described as “imperfect combustion,” because the carbon is not fully oxidized; but when the combination is effected in the latter proportions, the combustion is said to be “perfect,” because no more oxygen can be taken up. The products of combustion are in both cases gaseous. Carbonic oxide, the product of imperfect combustion, is an extremely poisonous gas; it is this gas which is so noisome in close headings, and in all ill-ventilated places, after a blast has been fired. A cubic foot of carbonic oxide, the specific gravity of which is 0·975, weighs, at the mean atmospheric pressure, 0·075 lb., so that 1 lb. will occupy a space of 13·5 cubic feet. Thus 1 lb. of carbon imperfectly oxidized will give 2¹/₃ lb. of carbonic oxide, which, at the mean atmospheric pressure of 30 inches and the mean temperature of 62° Fahr., will occupy a space of 13·5 × 2¹/₃ = 31·5 cubic feet. The product of perfect combustion, carbonic acid, is a far less noxious gas than the oxide, and it is much more easily expelled from confined places, because water possesses the property of absorbing large quantities of it. In an ill-ventilated but wet heading, the gas from a blast is soon taken up. Carbonic acid is a comparatively heavy gas, its specific gravity relatively to that of common air being 1·524. Hence a cubic foot at the ordinary pressure and temperature will weigh 0·116 lb., and 1 lb. of the gas under the same conditions will occupy a space of 8·6 cubic feet. Thus if 1 lb. of carbon be completely oxidized, there will result 3²/₃ lb. of carbonic acid, which will fill a space of 8·6 × 3²/₃ = 31·5 cubic feet. It will be observed that, though an additional pound of oxygen has been taken up during this reaction, the product occupies the same volume as the oxide. In complete combustion, therefore, a contraction takes place.

In the oxidation of hydrogen, as already pointed out, one atom of oxygen combines with two atoms of the former substance to form water. In this case, the product is liquid. But the heat generated by the combustion converts the water into steam, so that we have to deal with this product also in the gaseous state, in all considerations relating to the effects of an explosion. A cubic foot of steam, at atmospheric pressure and a temperature of 212° Fahr., weighs 0·047 lb.; 1 lb. of steam under these conditions will, therefore, occupy a space of 21·14 cubic feet. Thus the combustion of 1 lb. of hydrogen will produce 9 lb. of steam, which, under the conditions mentioned, will fill a space of 21·14 × 9 = 190·26 cubic feet.

Usually in an explosion a large quantity of nitrogen gas is liberated. This gas, which is not in itself noxious, has a specific gravity of 0·971, so that practically a cubic foot will weigh 0·075 lb., and 1 lb. will occupy a space of 13·5 cubic feet, which are the weight and the volume of carbonic oxide. Other gases are often formed as products of combustion; but the foregoing are the chief, viewed as the results of an explosion, since upon these the force developed almost wholly depends.

Force developed by an Explosion.

—A consideration of the facts enunciated in the foregoing paragraphs will show to what the tremendous energy developed by an explosion is due. It was pointed out that the combustion of 1 lb. of carbon gives rise to 31·5 cubic feet of gas. If this volume of gas be compressed within the space of 1 cubic foot it will obviously have a tension of 31·5 atmospheres; that is, it will exert upon the walls of the containing vessel a pressure of 472 lb. to the square inch. If the same volume be compressed into a space one-eighth of a cubic foot in extent, say a vessel of cubical form and 6 inches side, the tension will be 31·5 × 8 = 252 atmospheres, and the pressure 472 × 8 = 3776 lb. to the square inch. Assuming now the oxygen to exist in the solid state, and the two bodies carbon and oxygen to occupy together a space of one-eighth of a cubic foot, the combustion of the carbon will develop upon the walls of an unyielding containing vessel of that capacity a pressure of 252 atmospheres. Also the combustion of 1 lb. of hydrogen gives rise, as already remarked, to 190·26 cubic feet of steam; and if combustion take place under similar conditions with respect to space, the pressure exerted upon the containing vessel will be 22,830 lb., or nearly 10·5 tons, to the square inch, the tension being 190·26 × 8 = 1522 atmospheres.

The force thus developed is due wholly to the volume of the gas generated, and by no means represents the total amount developed by the explosion. The volume of the gases evolved by an explosion is estimated for a temperature of 62°; but it was shown in a former paragraph that the temperature of the products of combustion at the moment of their generation is far above this. Now it is a well-known law of thermo-dynamics that, the volume remaining the same, the pressure of a gas will vary directly as the temperature; that is, when the temperature is doubled, the pressure is also doubled. By temperature is understood the number of degrees measured by Fahrenheit’s scale on a perfect gas thermometer, from a zero 461°·2 below the zero of Fahrenheit’s scale, that is, 493°·2 below the freezing point of water. Thus the temperature of 62° for which the volume has been estimated is equal to 461·2 + 62 = 523°·2 absolute.

It was shown that the temperature of the product of combustion when carbon is burned to carbonic oxide is 9718° Fahr., which is equivalent to 10179°·2 absolute. Hence it will be observed that the temperature has been increased 10179°·2 523°·2 = 19·45 times. According to the law above enunciated, therefore, the pressure will be increased in a like ratio, that is, it will be, for the volume and the space already given, 3776 × 19·45 = 73,443 lb. = 32·8 tons to the square inch.

When carbon is burned to carbonic acid, the temperature of the product was shown to be 23,516° Fahr., which is equivalent to 23977·2 absolute. In this case, it will be observed that the temperature has been increased 23977·2 523·2 = 45·83 times. Hence the resulting pressure will be 3776 × 45·83 = 173,154 lb. = 77·3 tons to the square inch. It will be seen from these pressures that when combustion is complete, the force developed is 2·36 greater than when combustion is incomplete; and also that the increase of force is due to the larger quantity of heat liberated, since the volume of the gases is the same in both cases. If we suppose the carbon burned to carbonic oxide in the presence of a sufficient quantity of oxygen to make carbonic acid, we shall have 31·5 cubic feet of the oxide + 15·7 cubic feet of free oxygen, or a total volume of 42·7 cubic feet of gases. If this volume be compressed within the space of one-eighth of a cubic foot, it will have a tension of 42·7 × 8 = 341·6 atmospheres, and will exert upon the walls of the containing vessel a pressure of 5124 lb. to the square inch. The temperature of the gases will be 32 + 4400 0·190 × 3·667 = 6347° Fahr. = 6808°·2 absolute, the mean specific heat of the gases being 0·190; whence it will be seen that the temperature has been increased 6808°·2 523·2 = 13·01 times. According to the law of thermo-dynamics, therefore, the pressure under the foregoing conditions will be 5124 × 13·01 = 66,663 lb. = 29·8 tons to square inch. So that, under the conditions assumed in this case, the pressures developed by incomplete and by complete combustion are as 29·8 to 77·3, or as 1 to 2·59.

Similarly, when hydrogen is burned to water, the temperature of the product will be, as shown in a former paragraph, 16,049 Fahr. = 16510·2 absolute; and the pressure will be 22,830 × 16510·2 523·2 = 720,286 lb. = 321·1 tons to the square inch.

It will be observed, from a consideration of the foregoing facts, that a very large proportion of the force developed by an explosion is due to the heat liberated by the chemical reactions which take place. And hence it will plainly appear that, in the practical application of explosive agents to rock blasting, care should be taken to avoid a loss of the heat upon which the effects of the explosion manifestly so largely depend.

Section II.—Nature of Explosive Agents.

Mechanical Mixtures.

—In the preceding section, it was shown that an explosion is simply the rapid oxidation of carbon and hydrogen. To form an explosive agent, the problem is, how to bring together in a convenient form the combustible, carbon or hydrogen, and the oxygen required to oxidize it. Carbon may be obtained pure, or nearly pure, in the solid form. As wood charcoal, for example, that substance may be readily procured in any needful abundance; but pure oxygen does not exist in that state, and it is hardly necessary to point out that only the solid form is available in the composition of an explosive agent. In nature, however, oxygen exists in the solid state in very great abundance in combination with other substances. Silica, for example, which is the chief rock constituent, is a compound of silicon and oxygen, and the common ores of iron are made up chiefly of that metal and oxygen. The elementary constituents of cellulose, or wood fibre, are carbon, hydrogen, and oxygen; and the body known as saltpetre, or nitrate of potash, is compounded of potassium, nitrogen, and oxygen. But though oxygen is thus found in combination with many different substances, it has not the same affinity for all. When it is combined with a substance for which its affinity is strong, as in the silica and the iron oxide, it cannot be separated from that substance without difficulty; but if the affinity be weak, dissociation may be more easily effected. The former combination is said to be “stable,” and the latter is, in contradistinction, described as “unstable.” It will be evident on reflection that only those compounds in which the oxygen exists in unstable combination can be made use of as a constituent part of an explosive agent, since it is necessary that, when required, the oxygen shall be readily given up. Moreover, it will also appear that when one of these unstable oxygen compounds and carbon are brought together the mixture will constitute an explosive agent, since the oxygen which is liberated by the dissociation of the unstable compound will be taken up by the carbon for which it has a stronger affinity. Saltpetre is one of those compounds, and a mixture of this body with charcoal constitutes gunpowder. The means employed to dissociate the elements of saltpetre is heat. It is obvious that other compounds of oxygen might be substituted for the saltpetre, but this body being easily procurable is always employed. The chlorate of potash, for example, is less stable than the nitrate, and therefore an explosive mixture containing the former substance will be more violent than another containing the latter. For the violence of an explosion is in a great measure determined by the readiness with which the oxygen is given up to the combustible. But the chlorate is much more costly than the nitrate. As, however, the force developed is greater, the extra cost would perhaps be compensated by the increased effect of the explosion. But the instability of the chlorate is such that friction or a moderately light blow will produce explosion in a mixture containing that substance, a circumstance that renders it unfit to be the oxidizer in an explosive agent in common use. The nitrate is therefore preferred on the ground of safety. Saltpetre, or nitrate of potash, consists, as already pointed out, of the metal potassium in combination with the substances nitrogen and oxygen. Of these, the last only is directly concerned in the explosion; but the two former, and especially the nitrogen, act indirectly to intensify its effects in a manner that will be explained hereafter.

The chemical formula for nitrate of potash is KNO₃, which signifies that three atoms of oxygen exist in this body in combination with one atom of nitrogen and one atom of kalium or potassium. As the atomic weights of these substances are 16, 14, and 39 respectively, the weight of the molecule is 101, that is, in 101 lb. of nitrate of potash there are 39 lb. of potassium, 14 lb. of nitrogen, and (16 × 3) = 48 lb. of oxygen. Hence the proportion of oxygen in nitrate of potash is by weight 47·5 per cent. It will be seen from this proportion that to obtain 1 lb. of oxygen, 2·1 lb. of the nitrate must be decomposed.

The carbon of gunpowder is obtained from wood charcoal, the light woods, such as alder, being preferred for that purpose. The composition of the charcoal varies somewhat according to the degree to which the burning has been carried, the effect of the burning being to drive out the hydrogen and the oxygen. But, generally, the composition of gunpowder charcoal is about 80 per cent. carbon, 3·25 per cent. hydrogen, 15 per cent. oxygen, and 1·75 per cent. ash. Knowing the composition of the charcoal, it is easy to calculate the proportion of saltpetre required in the explosive mixture.

Thus far we have considered gunpowder as composed of charcoal and saltpetre only. But in this compound, combustion proceeds too slowly to give explosive effects. Were the chlorate of potash used instead of the nitrate, the binary compound would be sufficient. The slowness of combustion in the nitrate mixture is due to the comparatively stable character of that body. To accelerate the breaking up of the nitrate, a quantity of sulphur is mixed up with it in the compound. This substance possesses the property of burning at a low temperature. The proportion of sulphur added varies from 10 per cent. in powder used in fire-arms, to 20 per cent. in that employed for blasting purposes. The larger the proportion of sulphur, the more rapid, within certain limits, is the combustion. Thus ordinary gunpowder is a ternary compound, consisting of charcoal, saltpetre, and sulphur.

As the composition of charcoal varies, it is not practicable to determine with rigorous accuracy the proportion of saltpetre required in every case; a mean value is therefore assumed, the proportions adopted being about—

Charcoal	15
Saltpetre	75
Sulphur	10
	100

With these proportions, the carbon should be burned to carbonic acid, and the sulphur should be all taken up by the potassium. Powder of this composition is used for fire-arms. For blasting purposes, as before remarked, the proportion of sulphur is increased at the expense of the saltpetre, in order to quicken combustion and to lessen the cost, to 20 per cent. as a maximum. With such proportions, some of the carbon is burned to carbonic oxide only, and some of the sulphur goes to form sulphurous acid, gases that are particularly noisome to the miner.

It is essential to the regular burning of the mixture that the ingredients be finely pulverized and intimately mixed. The manufacture of gunpowder consists of operations for bringing about these results. The several substances are broken up by mechanical means, and reduced to an impalpable powder. These are then mixed in a revolving drum, and afterwards kneaded into a paste by the addition of a small quantity of water. This paste is subjected to pressure, dried, broken up, and granulated; thus, the mixing being effected by mechanical means, the compound is called a mechanical mixture. It will be observed that in a mechanical mixture the several ingredients are merely in contact, and are not chemically united. They may therefore be separated if need be, or the proportions may be altered in any degree. Mechanical mixtures, provided the bodies in contact have no chemical action one upon another, are stable, that is, they are not liable, being made up of simple bodies, to decompose spontaneously.

Chemical Compounds.

—In a mechanical mixture, as we have seen, the elements which are to react one upon another are brought together in separate bodies. In gunpowder, for example, the carbon is contained in the charcoal, and the oxygen in the saltpetre. But in a chemical compound, these elements are brought together in the same body. In a mechanical mixture, we may put what proportion of oxygen we please. But elements combine chemically only in certain definite proportions, so that in the chemical compound we can introduce only a certain definite proportion of oxygen. The oxygen in saltpetre is in chemical combination with the potassium and the nitrogen, and, as we have already seen, these three substances hold certain definite proportions one to another. That is, to every atom of potassium, there are one atom of nitrogen and three atoms of oxygen. Or, which amounts to the same thing, in 1 lb. of saltpetre, there are 0·386 lb. of potassium, 0·139 lb. of nitrogen, and 0·475 lb. of oxygen. Moreover, these elements occupy definite relative positions in the molecule of saltpetre. But in the mechanical mixture, the molecules of which it is made up have no definite relative positions. Even if the three substances—charcoal, saltpetre, and sulphur—of which gunpowder is composed, could be so finely divided as to be reduced to their constituent molecules, the relative position of these would be determined by the mixing, and it would be impossible so to distribute them that each should find itself in immediate proximity to those with which it was to combine. But so far are we from being able to divide substances into their constituent molecules, that when we have reduced them to an impalpable powder, each particle of that powder contains a large number of molecules. Thus, in a mechanical mixture, we have groups of molecules of one substance mingled irregularly with groups of molecules of another substance, so that the atoms which are to combine are not in close proximity one to another, but, on the contrary, are, many of them, separated by wide intervals. In the chemical compound, however, the atoms are regularly distributed throughout the whole mass of the substance, and are, relatively to one another, in the most favourable position for combining. Viewed from this point, the chemical compound may be regarded as a perfect mixture, the mechanical mixture being a very imperfect one. This difference has an important influence on the effect of an explosion. All the atoms in a chemical compound enter at once into their proper combinations, and these combinations take place in an inconceivably short space of time, while, in a mechanical mixture, the combinations are less direct, and are much less rapidly effected. This is the reason why the former is more violent in its action than the latter. The one is crushing and shattering in its effects, the other rending and projecting. The compound gives a sudden blow; the mixture applies a gradually increasing pressure. It is this sudden action of the compound that allows it to be used effectively without tamping. The air, which rests upon the charge, and which offers an enormous resistance to motion at such inconceivably high velocities, serves as a sufficient tamping.

Gun-cotton may be taken as an example of a chemical compound. The woody or fibrous part of plants is called “cellulose.” Its chemical formula is C₆H₁₀O₅, that is, the molecule of cellulose consists of six atoms of carbon in combination with ten atoms of hydrogen and five atoms of oxygen. If this substance be dipped into concentrated nitric acid, some of the hydrogen is displaced and peroxide of nitrogen is substituted for it. The product is nitro-cellulose, the formula of which is C₆H₇(NO₂)₃O₅. If this formula be compared with the last, it will be seen that three atoms of hydrogen have been eliminated and their place taken by three molecules of the peroxide of nitrogen NO₂; so that we now have a compound molecule, which is naturally unstable. The molecules of the peroxide of nitrogen are introduced into the molecule of cellulose for the purpose of supplying the oxygen needed for the combustion of the carbon and the hydrogen, just as the groups of molecules of saltpetre were introduced into the charcoal of the gunpowder for the combustion of the carbon and the hydrogen of that substance. Only, in the former case, the molecules of the peroxide are in chemical combination, not merely mixed by mechanical means as in the latter. The compound molecule of nitro-cellulose may be written C₆H₇N₃O₁₁, that is, in 297 lb. of the substance, there are (6 × 12) 72 lb. of carbon, (7 × 1) 7 lb. of hydrogen, (3 × 14) 42 lb. of nitrogen, and (11 × 16) 176 lb. of oxygen; or 24·2 per cent. carbon, 2·3 per cent. hydrogen, 14·1 per cent. nitrogen, and 59·4 per cent. oxygen. When the molecule is broken up by the action of heat, the oxygen combines with the carbon and the hydrogen, and sets the nitrogen free. But it will be observed that the quantity of oxygen present is insufficient to completely oxidize the carbon and the hydrogen. This defect, though it does not much affect the volume of gas generated, renders the heat developed, as shown in a former section, considerably less than it would be were the combustion complete, and gives rise to the noxious gas carbonic oxide.

Cotton is one of the purest forms of cellulose, and, as it may be obtained at a cheap rate, it has been adopted for the manufacture of explosives. This variety of nitro-cellulose is known as “gun-cotton.” The raw cotton made use of is waste from the cotton mills, which waste, after being used for cleaning the machinery, is swept from the floors and sent to the bleachers to be cleaned. This is done by boiling in strong alkali and lime. After being picked over by hand to remove all foreign substances, it is torn to pieces in a “teasing” machine, cut up into short lengths, and dried in an atmosphere of 190° F. It is then dipped into a mixture of one part of strong nitric acid and three parts of strong sulphuric acid. The use of the sulphuric acid is, first, to abstract water from the nitric acid, and so to make it stronger; and, second, to take up the water which is formed during the reaction. After the dipping, it is placed in earthenware pots to digest for twenty-four hours, in order to ensure the conversion of the whole of the cotton into gun-cotton. To remove the acid, the gun-cotton is passed through a centrifugal machine, and subsequently washed and boiled. It is then pulped, and again washed with water containing ammonia to neutralize any remaining trace of acid. When rendered perfectly pure, it is compressed into discs and slabs of convenient dimensions for use.

Another important chemical compound is nitro-glycerine. Glycerine is a well-known, sweet, viscous liquid that is separated from oils and fats in the processes of candle-making. Its chemical formula is: C₃H₈O₃; that is, the molecule is composed of three atoms of carbon, in combination with eight atoms of hydrogen, and three atoms of oxygen. In other words, glycerine consists of carbon 39·1 per cent., hydrogen 8·7 per cent., and oxygen 52·2 per cent. When this substance is treated, like cellulose, with strong nitric acid, a portion of the hydrogen is displaced, and peroxide of nitrogen is substituted for it; thus the product is: C₃H₅(NO₂)₃O₃, similar, it will be observed, to nitro-cellulose. This product is known as nitro-glycerine. The formula may be written C₃H₅N₃O₉. Hence, in 227 lb. of nitro-glycerine, there are (3 × 12) 36 lb. of carbon; (5 × 1) 5 lb. of hydrogen; (3 × 14) 42 lb. of nitrogen; and (9 × 16) 144 lb. of oxygen; or 15·8 per cent. is carbon, 2·2 per cent. hydrogen, 18·5 per cent. nitrogen, and 63·5 per cent. oxygen. When the molecule is broken up by the action of heat, the oxygen combines with the carbon and the hydrogen, and sets the nitrogen free. And it will be seen that the quantity of oxygen present is more than sufficient to completely oxidize the carbon and the hydrogen. In this, the nitro-glycerine is superior to the nitro-cotton. In both of these compounds, the products of combustion are wholly gaseous, that is, they give off no smoke, and leave no solid residue.

In the manufacture of nitro-glycerine, the acids, consisting of one part of strong nitric acid and two parts of strong sulphuric acid, are mixed together in an earthenware vessel. When quite cold, the glycerine is run slowly into this mixture, which, during the process, is kept in a state of agitation, as heat is developed in the process; and, as the temperature must not rise above 48° F., the vessels are surrounded with iced water, which is kept in circulation. When a sufficient quantity of glycerine has been run into the mixture, the latter is poured into a tub of water. The nitro-glycerine being much heavier than the dilute acid mixture, sinks to the bottom; the acid liquid is then poured off, and more water added, this process being repeated until the nitro-glycerine is quite free from acid.

Nitro-glycerine is, at ordinary temperatures, a clear, nearly colourless, oily liquid, having a specific gravity of about 1·6. It has a sweet, pungent taste, and if placed upon the tongue, or even if allowed to touch the skin in any part, it causes a violent headache. Below 40° F. it solidifies in crystals.

Dynamite is nitro-glycerine absorbed in a silicious earth called kieselguhr. Usually it consists of about 75 per cent. nitro-glycerine and 25 per cent. kieselguhr. The use of the absorbent is to remove the difficulties and dangers attending the handling of a liquid. Dynamite is a pasty substance of the consistence of putty, and is, for that reason, very safe to handle. It is made up into cartridges, and supplied for use always in that form.

Section III.—Relative Strength of the Common Explosive Agents.

Force developed by Gunpowder.

—In the combustion of gunpowder, the elements of which it is composed, which elements, as we have seen, are carbon, hydrogen, nitrogen, oxygen, potassium, and sulphur, combine to form, as gaseous products, carbonic acid, carbonic oxide, nitrogen, sulphuretted hydrogen, and marsh gas or carburetted hydrogen, and, as solid products, sulphate, hyposulphite, sulphide, and carbonate of potassium. Theoretically, some of these compounds should not be produced; but experiment has shown that they are. It has also been ascertained that the greater the pressure, the higher is the proportion of carbonic acid produced, so that the more work the powder has to do, the more perfect will be the combustion, and, consequently, the greater will be the force developed. This fact shows that overcharging is not only very wasteful of the explosive, but that the atmosphere is more noxiously fouled thereby. The same remark applies even more strongly to gun-cotton and the nitro-glycerine compounds.

The careful experiments of Messrs. Noble and Abel have shown that the explosion of gunpowder produces about 57 per cent. by weight of solid matters, and 43 per cent. of permanent gases. The solid matters are, at the moment of explosion, in a fluid state. When in this state, they occupy 0·6 of the space originally filled by the gunpowder, consequently the gases occupy only 0·4 of that space. These gases would, at atmospheric pressure and 32° F. temperature, occupy a space 280 times that filled by the powder. Hence, as they are compressed into 0·4 of that space, they would give a pressure of 2800·4 × 15 = 10,500 lb., or about 4·68 tons to the square inch. But a great quantity of heat is liberated in the reaction, and, as it was shown in a former section, this heat will enormously increase the tension of the gases. The experiments of Noble and Abel showed that the temperature of the gases at the instant of explosion is about 4000° F. Thus the temperature of 32° + 461°·2 = 493°·2 absolute, has been raised 4000493°·2 = 8·11 times, so that the total pressure of the gases will be 4·68 × 8·11 = 42·6 tons to the square inch. And this pressure was, in the experiments referred to, indicated by the crusher-gauge. When, therefore, gunpowder is exploded in a space which it completely fills, the force developed may be estimated as giving a pressure of about 42 tons to the square inch.

Relative Force developed by Gunpowder, Gun-cotton, and Nitro-glycerine.

—Unfortunately no complete experiments have hitherto been made to determine the absolute force developed by gun-cotton and nitro-glycerine. We are, therefore, unable to estimate the pressure produced by the explosion of those substances, or to make an accurate evaluation of their strength relatively to that of gunpowder. It should, however, be borne in mind that a correct estimate of the pressure produced to the square inch would not enable us to make a full comparison of the effects they were capable of causing. For though, by ascertaining that one explosive gives twice the pressure of another, we learn that one will produce twice the effect of another; yet it by no means follows from that fact that the stronger will produce no more than twice the effect of the weaker. The rending effect of an explosive depends, in a great measure, on the rapidity with which combustion takes place. The force suddenly developed by the decomposition of the chemical compounds acts like a blow, and it is a well-known fact that the same force, when applied in this way, will produce a greater effect than when it is applied as a gradually increasing pressure. But some calculations have been made, and some experiments carried out, which enable us to form an approximate estimate of the relative strength of these explosive substances.

Messrs. Roux and Sarrau give the following as the result of their investigations, derived from a consideration of the weight of the gases generated and of the heat liberated. The substances are simply exploded, and the strength of gunpowder is taken as unity.

Substance.	Relative Weight of Gases.	Heat in Units liberated from 1 lb.	Relative Strength.
Gunpowder	0·414	1316	1·00
Gun-cotton	0·850	1902	3·00
Nitro-glycerine	0·800	3097	4·80

The relative strength is that due to the volume of the gases and the heat, no account being taken of the increased effect due to the rapidity of the explosion.

Alfred Noble has essayed to appreciate the effects of these different explosives by means of a mortar loaded with a 32-lb. shot and set at an angle of 10°, the distances traversed by the shot being taken as the results to be compared. Considered, weight for weight, he estimates as follows the relative strengths of the substances compared, gunpowder being again taken as unity:—

Gunpowder	1·00
Gun-cotton	2·84
Dynamite	2·89
Nitro-glycerine	4·00

The relative strength, bulk for bulk, is, however, of greater importance in rock blasting. This is easily computed from the foregoing table and the specific gravity of the substances, which is 1·00 for gunpowder and compressed gun-cotton, 1·60 for nitro-glycerine, and 1·65 for dynamite. Compared in this way, bulk for bulk, these explosives range as follows:—

Gunpowder	1·00
Gun-cotton	2·57
Dynamite	4·23
Nitro-glycerine	5·71

Hence, for a given height of charge in a bore-hole, gun-cotton exerts about 2¹/₂ times the force of gunpowder, and dynamite about 4¹/₄ times that force.

Section IV.—Means of Firing the Common Explosive Agents.

Action of Heat.

—We have seen that the oxygen required for the combustion of the carbon in gunpowder is stored up in the saltpetre. So long as the saltpetre remains below a certain temperature, it will retain its oxygen; but when that temperature is reached, it will part with that element. To fire gunpowder, heat is therefore made use of to liberate the oxygen, which at once seizes upon the carbon with which it is in presence. The means employed to convey heat to an explosive have been described in the preceding chapter. It is necessary to apply heat to one point only of the explosive; it is sufficient if it be applied to only one grain. That portion of the grain which is thus raised in temperature begins to “burn,” as it is commonly expressed, that is, this portion enters at once into a state of combustion, the saltpetre giving up its oxygen, and the liberated oxygen entering into combination with the carbon. The setting up of this action is called “ignition.” The hot gases generated by the combustion set up ignite other grains surrounding the one first ignited; the gases resulting from the combustion of these ignite other grains; and, in this way, ignition is conveyed throughout the mass. Thus the progress of ignition is gradual. But though it takes place, in every case, gradually, if the gases are confined within the space occupied by the powder, it may be extremely rapid. It is easy to see that the gases evolved from a very small number of grains are sufficient to fill all the interstices, and to surround every individual grain of which the charge is composed. But besides this ignition from grain to grain, the same thing goes on from the outside to the inside of each individual grain, the grain burning gradually from the outside to the inside in concentric layers. The successive ignitions in this direction, however, of layer after layer, is usually described as the progress of combustion. Thus the time of an explosion is made up of that necessary for the ignition of all the grains, and of that required for their complete combustion.

The time of ignition is determined in a great measure by the proportion which the interstices, or empty spaces between the grains, bear to the whole space occupied by the powder. If the latter be in the form of an impalpable dust, ignition cannot extend throughout the mass in the manner we have described; but we shall have merely combustion proceeding from grain to grain. If, on the contrary, the powder be in large spherical grains or pellets, the interstices will be large, and the first gases formed will flash through these, and ignite all the grains one after another with such rapidity that ignition may be regarded as simultaneous. Thus the time of ignition is shortened by increasing the size of the grains and approximating the latter to the spherical form.

But the time of combustion is determined by conditions contrary to these. As combustion proceeds gradually from the outside to the inside of a grain, it is obvious that the larger the grain is, the longer will be the time required to burn it in. Also it is evident that if the grain be in the form of a thin flake, it will be burned in a much shorter time than if it be in the spherical form. Thus the conditions of rapid ignition and rapid combustion are antagonistic. The minimum time of explosion is obtained when the grains are irregular in shape and only sufficiently large to allow a fairly free passage to the hot gases. There are other conditions which influence the time of combustion; among them is the density of the grain. This is obvious, since the denser the grain, the greater is the quantity of material to be consumed. But besides this, combustion proceeds more slowly through a dense grain than through an open one. The presence of moisture also tends to retard combustion.

The progress both of ignition and of combustion is accelerated, not uniform. In proportion as the grains are ignited, the gases evolved increase in volume, and as the progress of combustion continues to generate gases, the tension of these increases, until, as we have seen, the pressure rises as high as 42 tons to the square inch. As the pressure increases, the hot gases are forced more and more deeply into the grains, and combustion, consequently, proceeds more and more rapidly.

Detonation.

—By detonation is meant the simultaneous breaking up of all the molecules of which the explosive substance is composed. Properly the term is applicable to the chemical compounds only. But it is applied to gunpowder to denote the simultaneous ignition of all the grains. The mode of firing by detonation is obviously very favourable to the rending effect required of blasting powder, since it reduces to a minimum the time of explosion. It is brought about, in all cases, by means of an initial explosion. The detonator, which produces this initial explosion, consists of an explosive compound, preferably one that is quick in its action, contained within a case sufficiently strong to retain the gases until they have acquired a considerable tension. When the case bursts, this tension forces them instantaneously through the interstices of the powder, and so produces simultaneous ignition. A pellet of gun-cotton, or a cartridge of dynamite, the latter especially, makes a good detonator for gunpowder. Fired in this way, very much better effects may be obtained from gunpowder than when fired in the usual manner. Indeed, in many kinds of rock, more work may be done with it than with gun-cotton or with dynamite.

The action of a detonator upon a chemical compound is different. In this case, the explosion seems to be due more to the vibration caused by the blow than by the heat of the gases from the detonator. Probably both of these causes operate in producing the effect. However this may be, the fact is certain that under the influence of the explosion of the detonator, the molecules of a chemical compound, like nitro-glycerine, are broken up simultaneously, or at least, so nearly simultaneously, that no tamping is needed to obtain the full effect of the explosion. Dynamite is always, and gun-cotton is usually, fired by means of a detonator. A much larger quantity of explosive is needed to detonate gunpowder than is required for dynamite, or gun-cotton, since, for the former explosive, a large volume of gases is requisite. Dynamite detonators usually consist of from six to nine grains of fulminate of mercury contained in a copper cap, as described in the preceding chapter. Gun-cotton detonators are similar, but have a charge of from ten to fifteen grains of the fulminate. An insufficient charge will only scatter the explosive instead of firing it, if it be unconfined, and only explode it without detonation, if it be in a confined space.

Section V.—Some Properties of the Common Explosive Agents.

Gunpowder.

—The combustion of gunpowder, as we have seen, is gradual and comparatively slow. Hence its action is rending and projecting rather than shattering. This constitutes one of its chief merits for certain purposes. In many quarrying operations, for instance, the shattering action of the chemical compounds would be very destructive to the produce. In freeing blocks of slate, or of building stone, a comparatively gentle lifting action is required, and such an action is exerted by gunpowder. Moreover, this action may be modified by using light tamping, or by using no tamping, a mode of employing gunpowder often adopted in slate quarries. The effect of the violent explosives cannot be modified in this way.

Gunpowder is injured by moisture. A high degree of moisture will destroy its explosive properties altogether, so that it cannot be used in water without some protective covering. Even a slight degree of moisture, as little as one per cent. of its weight, materially diminishes its strength. For this reason, it should be used, in damp ground, only in cartridges. This is, indeed, the most convenient and the most economical way of using gunpowder in all circumstances. It is true that there is a slight loss of force occasioned by the empty space around the cartridge, in holes that are far from circular in shape. But at least as much will be lost without the cartridge from the moisture derived from the rock, even if the hole be not wet. But in all downward holes, the empty spaces may be more or less completely filled up with dry loose sand.

The products of the explosion of gunpowder are partly gaseous, partly solid. Of the former, the most important are carbonic acid, carbonic oxide, and nitrogen. The sulphuretted and the carburetted hydrogen are formed in only small quantities. The carbonic oxide is a very noxious gas; but it is not formed in any considerable quantity, except in cases of overcharging. The solid products are compounds of potassium and sulphur, and potassium and carbon. These constitute the smoke, the dense volumes of which characterize the explosion of gunpowder. This smoke prevents the immediate return of the miner to the working face after the blast has taken place.

Gun-cotton.

—The combustion of gun-cotton takes place with extreme rapidity, in consequence of which its action is very violent. Its effect is rather to shatter the rock than to lift it out in large blocks. This quality renders it unsuitable to many quarrying operations. In certain kinds of weak rock, its disruptive effects are inferior to those produced by gunpowder. But in ordinary mining operations, where strong tough rock has to be dealt with, its superior strength and quickness of action, particularly the latter quality, produce much greater disruptive effect than can be obtained from gunpowder. Moreover, its shattering action tends to break up into small pieces the rock dislodged, whereby its removal is greatly facilitated.

Gun-cotton may be detonated when in a wet state by means of a small quantity of the dry material. This is a very important quality, inasmuch as it allows the substance to be used in a wet hole without protection, and conduces greatly to the security of those who handle it. When in the wet state, it is uninflammable, and cannot be exploded by the heaviest blows. Only a powerful detonation will bring about an explosion in it when in the wet state. It is, therefore, for safety, kept and used in that state. Since it is insensible to blows, it may be rammed tightly into the bore-hole, so as to fill up all empty spaces. The primer of dry gun-cotton, however, which is to detonate it, must be kept perfectly dry, and handled with caution, as it readily detonates from a blow. Gun-cotton, when ignited in small quantities in an unconfined space, burns fiercely, but does not explode.

The products of the combustion of gun-cotton are:—carbonic acid, carbonic oxide, water, and a little carburetted hydrogen or marsh-gas. On account of the insufficiency of oxygen, already pointed out, a considerable proportion of carbonic oxide is formed, which vitiates the atmosphere into which it is discharged. Overcharging, as in the case of gunpowder, causes an abnormal quantity of the oxide to be formed.

Dynamite.

—As combustion takes place more rapidly in nitro-glycerine than in gun-cotton, the effects of dynamite are more shattering than those of the latter substance. Gun-cotton holds, indeed, a mean position in this respect between dynamite, on the one hand, and gunpowder on the other. Dynamite is, therefore, even less suitable than gun-cotton for those uses which are required to give the produce in large blocks. But in very hard and tough rock, it is considerably more effective than gun-cotton, and, under some conditions, it will bring out rock which gun-cotton fails to loosen.

Dynamite is unaffected by water, so that it may be used in wet holes; indeed, water is commonly used as tamping, with this explosive. In upward holes, where water cannot, of course, be used, dynamite is generally fired without tamping, its quick action rendering tamping unnecessary.

The pasty form of dynamite constitutes a great practical advantage, inasmuch as it allows the explosive to be rammed tightly into the bore-hole so as to fill up all empty spaces and crevices. This is important, for it is obvious that the more compactly the charge is placed in the hole, the greater will be the effect of the explosion. Moreover, this plastic character renders it very safe to handle, as blows can hardly produce sufficient heat in it to cause explosion. If a small quantity of dynamite be placed upon an anvil and struck with a hammer, it explodes readily; but a larger quantity so struck does not explode, because the blow is cushioned by the kieselguhr. If ignited in small quantities in an unconfined space, it burns quietly without explosion.

If dynamite be much handled out of the cartridges, it causes violent headaches; and the same effect is produced by being in a close room in which there is dynamite in the unfrozen state.

Dynamite possesses one quality which places it at a disadvantage with respect to other explosives, namely, that of freezing at a comparatively high temperature. At about 40° F. the nitro-glycerine solidifies, and the dynamite becomes chalky in appearance. In this state, it is exploded with difficulty, and, consequently, it has to be thawed before being used. This may be safely done with hot water; performed in any other way the operation is dangerous.

The products of the combustion of dynamite are carbonic acid, carbonic oxide, water, and nitrogen. As, however, there is more than a sufficiency of oxygen in the compound, but little of the oxide is formed when the charge is not excessive. If, therefore, dynamite be properly detonated, and overcharging be avoided, its explosion will not greatly vitiate the atmosphere. But if it be only partially detonated hypo-nitric fumes are given off, which have a very deleterious effect upon the health. It is, thus, of the highest importance that complete detonation should be effected, not merely to obtain the full effect of the explosive, but to avoid the formation of this noxious gas. This may be done by using a detonator of sufficient strength, and placing it well into the primer.

Firing Points of the Common Explosive Compounds.

—The following table shows the temperatures at which the commonly used compounds explode:—

	When slowly Heated.	When suddenly Heated.
Gunpowder	..	from 500° to 540°
Gun-cotton	360°	482°
Kieselguhr dynamite	356°	446°
Cellulose dynamite	342°	446°

Cotton powder explodes at the same temperatures as gun-cotton, and lithofracteur at the same temperature as kieselguhr dynamite.

Section VI.—Some Varieties of the Nitro-cellulose and the Nitro-glycerine Compounds.

Nitrated Gun-cotton.

—It has been shown that gun-cotton contains an insufficient quantity of oxygen for its complete combustion. To furnish that which is wanting, gun-cotton has sometimes incorporated with it a certain proportion of nitrate of potash, or of nitrate of baryta. This compound, which, it will be observed, is at once a chemical compound and a mechanical mixture, is known as “nitrated gun-cotton.”

Cotton Powder, or Tonite.

—The explosive which is now well known as “tonite” or “cotton powder,” is essentially nitrated gun-cotton. It is produced in a granulated form, and is compressed into cartridges of various dimensions to suit the requirements of practice. The convenient form in which tonite is made up, ready to the miner’s hand, has greatly contributed towards bringing it into favour. But irrespective of this, the fact of its being so highly compressed as to give it a density equal, or nearly equal, to dynamite gives it a decided advantage over the other nitro-cotton compounds as they are at present used.

Schultze’s Powder.

—In Schultze’s powder, the cellulose is obtained from wood. The wood is first sawn into sheets, about ¹/₁₆ inch thick, and then passed through a machine, which punches it up into grains of a uniform size. These are deprived of their resinous matters by a process of boiling in carbonate of soda, and are further cleansed by washing in water, steaming, and bleaching by chloride of lime. The grains, which are then pure cellulose, are converted into nitro-cellulose in the same way as cotton, namely, by being treated with a mixture of nitric and sulphuric acids. The nitro-cellulose thus produced is subsequently steeped in a solution of nitrate of potash. Thus the finished compound is similar in character to nitrated gun-cotton.

Lithofracteur.

—Lithofracteur is a nitro-glycerine compound in which a portion of the base is made explosive. In dynamite, the base, or absorbent material, is, as we have said, a silicious earth, called “kieselguhr.” In lithofracteur, the same substance is used; but in addition, a mixture of nitrate of baryta and charcoal, a kind of gunpowder, is introduced. The object of employing this explosive mixture is to increase the force of the explosion, the kieselguhr being an inert substance. Obviously this object would be attained if the explosive mixture possessed the same absorbent power as the kieselguhr. But unfortunately it does not, and, as a consequence, less nitro-glycerine is used. Thus what is gained in the absorbent is lost in the substance absorbed. The composition of lithofracteur varies somewhat; but its average proportion of ingredients are the following:—

Nitro-glycerine	52·50
Nitrate of baryta	16·40
Charcoal	2·85
Sulphur	25·75
Kieselguhr	22·50
	100·00

Brain’s Powder.

—Brain’s powder is a nitro-glycerine compound, similar in character to lithofracteur. The exact composition of the base has never been published, so far as relates to the proportions of the ingredients. But it is composed of chlorate of potash, charcoal, and nitrated sawdust. The proportion of nitro-glycerine never exceeds 40 per cent. Horseley’s powder contains about the same proportion of nitro-glycerine in a base of chlorate of potash and nut-galls.

Cellulose Dynamite.

—In Germany, gun-cotton is used as an absorbent for nitro-glycerine, the compound being known as “Cellulose dynamite.” It is chiefly used for primers to explode frozen dynamite. It is more sensitive to blows than the kieselguhr dynamite.

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