CHEMICAL PHENOMENA.

Water—Its Constituents—Oxygen—Hydrogen—Peroxide of Hydrogen—Physical Property of Water—Ice—Sea Water—Chlorine—Muriatic Acid—Iodine—Bromine—Compounds of Hydrogen with Carbon—Combustion—Flame—Safety Lamp—Respiration—Animal Heat—The Atmosphere—Carbonic Acid—Influence of Plants on the Air—Chemical Phenomena of Vegetation—Compounds of Nitrogen—Mineral Kingdom, &c. &c.

Without attempting anything which shall approach even to the character of a sketch of chemical science, we may be allowed, in our search after exalting truths, to select such examples of the results of combination as may serve to elucidate any of the facts connected with natural phenomena. In doing this, by associating our examination with well-known natural objects or conditions, the interpretation afforded by analysis will be more evident, and the operation of the creative forces rendered more striking and familiar, particularly if at the same time we examine such physical conditions as are allied in action, and are sufficiently explanatory of important features.

A large portion of this planet is covered by the waters of the ocean, of lakes and rivers. Water forms the best means of communication between remote parts of the earth. It is in every respect of the utmost importance to the animal and vegetable kingdom; and, indeed, it is indispensable in all the great phenomena of the inorganic world. The peculiarities of saltness or freshness in water are dependent upon its solvent powers. The waters of the ocean are saline from holding dissolved various saline compounds, which are received in part from, and imparted also to, the marine plants. Perfectly pure water is without taste: even the pleasant character of freshly-drawn spring-water is due to the admixture of atmospheric air and carbonic acid. The manner in which water absorbs air is evidently due to a peculiar physical attractive force, the value of which we do not at present clearly perceive or correctly estimate. It is chemically composed of two volumes of hydrogen gas—the lightest body known, and at the same time a highly inflammable one—united with one volume of oxygen, which excites combustion, and continues that action,—producing heat and light,—with great energy. By weight, one part of hydrogen is united with eight of oxygen, or in 100 parts of water we find 88·9 oxygen, and 11·1 of hydrogen gas. That two such bodies should unite to furnish the most refreshing beverage, and indeed the only natural drink for man and animals, is one of the extraordinary facts of science. Hydrogen will not support life—we cannot breathe it and live; and oxygen would over-stimulate the organic system, and, producing a kind of combustion, give rise to fever in the animal frame; but, united, they form that drink, for a drop of which the fevered monarch would yield his diadem, and the deprivation of which is one of the most horrid calamities that can be inflicted upon any living thing. Water appears as the antagonist principle to fire, and the ravages of the latter are quenched by the assuaging powers of the former; yet a mixture of oxygen and hydrogen gases, in the exact proportion in which they form water, explodes with the utmost violence on the contact of flame, and, when judiciously arranged, produces the most intense degree of heat known;—such is the remarkable difference between a merely mechanical mixture and a chemical combination. Beyond this, we have already noticed the remarkable fact that water deprived of air is explosive at a comparatively low temperature, less than 300°; gunpowder requiring a temperature of nearly 1000° F.

If we place in a globe, oxygen and hydrogen gases, in the exact proportions in which they combine to form water, they remain without change of state. They appear to mix intimately; and, notwithstanding the difference in the specific gravities of the two gases, the lighter one is diffused through the heavier in a curious manner, agreeably to a law which has an important bearing on the conditions of atmospheric phenomena.[212] The moment, however, that an incandescent body, or the spark from an electric machine, is brought into contact with the mixed gases, they ignite, explode violently, and combine to form water. The discovery of the composition of water was thus synthetically made by Cavendish—its constitution having been previously theoretically announced by Watt.[213]

If, instead of combining oxygen and hydrogen in the proportions in which they form water, we compel the hydrogen to combine with an additional equivalent of oxygen, we have a compound possessing many properties strikingly different from water. This—peroxide of hydrogen, as it is called—is a colourless liquid, less volatile than water, having a metallic taste. It is decomposed at a low temperature, and, at the boiling point, the oxygen escapes from it with such violence, that something like an explosion ensues. All metals, except iron, tin, antimony, and tellurium, have a tendency to decompose this compound, and separate it into oxygen and water. Some metals are oxidized during the decomposition, but gold, silver, platinum, and a few others, still retain their metallic state. If either silver, lead, mercury, gold, platinum, manganese, or cobalt, in their highest states of oxidation, are put into a tube, containing this peroxide of hydrogen, its oxygen is liberated with the rapidity of an explosion, and so much heat is excited that the tube becomes red hot. These phenomena, to which we have already referred in noticing catalysis, are by no means satisfactorily explained, and the peculiar bleaching property possessed by the peroxide of hydrogen sufficiently distinguishes it from water. There are few combinations which show more strikingly than this the difference arising from the chemical union of an additional atom of one element. Similar instances are numerous in the range of chemical science; but scarcely any two exhibit such dissimilar properties. During the ordinary processes of combustion, it has been long known that water is formed by the combination of the hydrogen of the burning body with the oxygen of the air. The recent researches of SchÖnbein have shown that a peculiar body, which has been regarded as a peroxide of hydrogen, to which he has given the name of Ozone, is produced at the same time, and that it is developed in a great many ways, particularly during electrical changes of the atmosphere. Thus we obtain evidence that this remarkable compound, which was considered as a chemical curiosity merely, is diffused very generally through nature, and produced under a great variety of circumstances. During the excitation of an electrical machine, or the passage of a galvanic current through water by the oxidation of phosphorus, and probably in many similar processes—in particular those of combustion, and we may therefore infer also of respiration—this body is formed. From observations which have been made, it would appear that, during the night, when the activity of plants is not excited by light, they act upon the atmosphere in such a way as to produce this ozone; and its presence is said to be indicated by its peculiar odour during the early hours of morning. We are not yet acquainted with this body sufficiently to speculate on its uses in nature: without doubt, they are important, perhaps second to those of water only. It is probable, as we have already had occasion to remark, that ozone may be the active agent in removing from the atmosphere those organic poisons to which many forms of pestilence are traceable; and it is a curious fact, that a low electrical intensity, and a consequent deficiency of atmospheric ozone, marks the prevalence of cholera, and an excess distinguishes the reign of influenza.[214]

Some interesting researches appear to show the probability that ozone is simply oxygen in a state of high activity. It has been found, indeed, that perfectly dry oxygen, which will not bleach vegetable colours in the dark, acquires, by exposure to sunshine, the power of destroying them. Becquerel has proved that this ozonous state may be produced in dry oxygen by passing a succession of electric sparks through it. Fremy passed the electric sparks on the outside of a tube which contained perfectly dry oxygen, and it was found to have acquired the properties of ozone. In this case, and probably in the experiments of Becquerel, the light of the spark, rather than the electricity, appears to have been the active agent in producing this change. SchÖnbein himself does not appear disposed to regard ozone as being either peroxide of hydrogen, or an allotropic oxygen. He leans to his first view of its being an entirely new chemical element. The energy of this ozone is so great, that it has been found to destroy almost instantaneously the Indian-rubber union joints of the apparatus in which it is formed.[215]

Water, from the consideration of which a digression has been indulged in, to consider the curious character of one of its elements,—water is one of the most powerful chemical agents, having a most extensive range of affinities, entering directly into the composition of a great many crystallizable bodies and organic compounds. In those cases where it is not combined as water, its elements often exist in the proportions in which water is formed. Gum, starch, and sugar, only differ from each other in the proportions in which the elements of water are combined with the carbon.

In saline combinations, and also in many organic forms, we must regard the water as condensed to the solid form; that is, to exist as ice. We well know that, by the abstraction of heat, this condition is produced; but, in chemical combinations, this change must be the result of the mechanical force exerted by the power of the agency directing affinity.

In the case of water passing from a liquid to a solid state, we have a most beautiful exemplification of the perfection of natural operations. Water conducts heat downwards but very slowly; a mass of ice will remain undissolved but a few inches under water, on the surface of which, ether, or any other inflammable body, is burning. If ice (solid water) swam beneath the surface, the summer sun would scarcely have power to thaw it; and thus our lakes and seas would be gradually converted into solid masses at our ordinary winter temperatures.

All similar bodies contract equally during the process of cooling, from the highest to the lowest points to which the experiments have been carried. It has been thought that if this applied to water, the result would be the sudden consolidation of the whole mass. A modification of the law has been supposed to take place to suit the peculiar circumstances of water. Nature never modifies a law for a particular purpose; we must, therefore, seek to explain the action of the formation of ice, as we know it, by some more rational view.

Water expands by heat, and contracts by cold; consequently, the coldest portions of this body occupy the lower portions of the fluid; but it must be remembered that these parts are warmed by the earth. Ross, however, states that at the depth of 1,000 fathoms the sea has a constant temperature of 39°. Water is said to be at its point of greatest density at 40° of Fahrenheit’s thermometer; in cooling further, this fluid appears to expand, in the same way as if heated: and, consequently, water colder than this point, instead of being heavier, is lighter, and floats on the surface of the warmer fluid. It does not seem that any modification of the law is required to account for this phenomenon. Water cooled to 40° still retains its peculiar corpuscular arrangement; but immediately it passes below that temperature, it begins to dispose itself in such a manner that visible crystals may form the moment it reaches 32°. Now, if we conceive the particles of water, at 39°, to arrange themselves in the manner necessary for the assumption of the solid form, by the particular grouping of molecules in an angular instead of a spheroidal shape, it will be clear, from what we know of the arrangement of crystals of water—ice—that they must occupy a larger space than when the particles are disposed, side by side, in minute spheres. Even the escape of air from the water in which it is dissolved is sufficient to give an apparent lightness to the colder water. This expansion still goes on increasing, from the same cause, during the formation of ice, so that the specific gravity of a mass of frozen water is less than that of water at any temperature below 40°. It must not be forgotten that ice always contains a large quantity of air, by which it is rendered buoyant.

Water, at rest, may be cooled many degrees below the freezing point without becoming solid. This is easily effected in a thin glass flask; but the moment it is agitated, it becomes a firm mass. Here we have the indication of another cause aiding in producing crystals of ice on the surface of water, under the influence of the disturbance produced by the wind, which does not extend to any depth.

As oxygen and hydrogen gases enter largely into other chemical compounds besides water, it is important to consider some of the forms of matter into the composition of which these elements enter. To examine this thoroughly, a complete essay on chemical philosophy would be necessary; we must, therefore, be content with referring to a few of the more remarkable instances.

The waters of the ocean are salt: this arises from their holding, in solution chloride of sodium (muriate of soda—common culinary salt) and other saline bodies. Water being present, this becomes muriate of soda,—that is, a compound of muriatic acid and soda: muriatic acid is hydrogen, combined with a most remarkable gaseous body, called, from its yellow colour, chlorine; and soda, oxygen in union with the metal sodium,—therefore, when anhydrous, culinary salt is truly a chloride of sodium. Chlorine in some respects resembles oxygen; it attacks metallic bodies with great energy; and, in many cases, produces the most vivid incandescence, during the process of combination. It is a powerful bleaching agent, is destructive to animal life, and rapidly changes all organic tissues. There are two other bodies in many respects so similar to chlorine, although one is at the ordinary temperatures solid, and the other fluid, and which are also discovered in sea-water, or in the plants growing in it, that it is difficult to consider them otherwise than as different forms of the same principle. These are iodine and bromine, and they both unite with hydrogen to form acids. The part which chlorine performs in nature is a great and important one. Combined in muriate of soda, we may trace it in large quantities through the three kingdoms of nature, and the universal employment of salt as a condiment indicates the importance to the animal economy of the elements composing it. Iodine has been traced through the greater number of marine plants, existing, apparently as an essential element of their constitution; in some land plants it has also been found, particularly in the Armeria maritima, when this plant grows near the sea:[216] it has been detected in some mineral springs, and in small quantities in the mineral kingdom[217] combined as iodide of silver, and in the aluminous slate of Latorp in Sweden.[218] Bromine is found in sea-water, although in extremely minute quantities, in a few saline springs, and in combination with silver; but we have no evidence to show that its uses are important in nature.

Hydrogen, again, unites with carbon in various proportions, producing the most dissimilar compounds. The air evolved from stagnant water, and the fire-damp of the coal mine, are both carburetted hydrogen; and the gas which we employ so advantageously for illumination, is the same, holding an additional quantity of carbon in suspension. Naphtha, and a long list of organic bodies, are composed of these two chemical elements.

These combinations lead us, naturally, to the consideration of the great chemical phenomena of combustion, which involve, indeed, the influences of all the physical powers. By the application of heat, we produce an intense action in a body said to be combustible; it burns,—a chemical action of the most energetic character is in progress, the elements which constitute the combustible body are decomposed, they unite with some other elementary principles, and new compounds are formed. A body burns—it is entirely dissipated, or it leaves a very small quantity of ashes behind unconsumed, but nothing is lost. Its volatile parts have entered into new arrangements, the form of the body is changed, but its constituents are still playing an important purpose in creation.

The ancient notion that fire was an empyreal element, and the Stahlian hypothesis of a phlogistic principle on which all the effects of combustion depended,[219] have both given way to the philosophy of the unfortunate Lavoisier—which has, indeed, been modified in our own times—who showed that combustion is but the development of heat and light under the influence of chemical combination.

Combustion was, at one period, thought to be always due to the combination of oxygen with the body burning, but research has shown that vivid combustion may be produced where there is no oxygen. The oxidizable metals burn most energetically in chlorine, and some of them in the vapour of iodine and bromine, and many other unions take place with manifestations of incandescence. Supporters of combustion were, until lately, regarded as bodies distinct from those undergoing combustion. For example, hydrogen was regarded as a combustible body, and oxygen as a supporter of combustion. Such an arrangement is a most illogical one, since we may burn oxygen in an atmosphere of hydrogen, in the same manner as we burn hydrogen in one of oxygen; and so, in all the other cases, the supporter of combustion may be burnt in an atmosphere formed of the, so called, combustible. The ordinary phenomena of combustion are, however, due to the combination of oxygen with the body burning; therefore every instance of oxidization may be regarded as a condition of combustion, the difference being only one of degree.

Common iron, exposed to air and moisture, rusts; it combines with oxygen. Pure iron, in a state of fine division, unites with oxygen so eagerly, that it becomes incandescent, and in both cases oxide of iron is formed. This last instance is certainly a case of combustion; but in what does it differ from the first one, except in the intensity of the action? The cases of spontaneous combustion which are continually occurring are examples of an analogous character to the above. Oxygen is absorbed, it enters more or less quickly, according to atmospheric conditions, into chemical combination; heat is evolved, and eventually,—the action continually increasing,—true combustion takes place. In this way our cotton-ships, storehouses of flax, piles of oiled-cloth, sawdust, &c., frequently ignite; and to such an influence is to be attributed the destruction of two of our ships of war, a few years since, in Devonport naval arsenal.[220]

In the economic production of heat and light, we have the combination of hydrogen and carbon with the oxygen of common air, forming water and carbonic acid. In our domestic fires we employ coal, which is essentially a compound of carbon and hydrogen containing a little oxygen and some nitrogen, with some earthy matters which must be regarded as impurities; the taper, whether of wax or tallow, is made up of the same bodies, differing only in their combining proportions, and, like coal gas, these burn as carburetted hydrogen. All these bodies are very inflammable, having a tendency to combine energetically with oxygen at a certain elevation of temperature.

We are at a loss to know how heat can cause the combination of those bodies. Sir Humphry Davy has shown that hydrogen will not burn, nor a mixture of it with oxygen explode, unless directly influenced by a body heated so as to emit light.[221] May we not, therefore, conclude that the chemical action exhibited in a burning body is a development of some latent force, with which we are unacquainted, produced by the absorption of light;—that a repulsive action at first takes place, by which the hydrogen and carbon are separated from each other;—and that in the nascent state they are seized by the oxygen, and again compelled, though in the new forms of water and carbonic acid, to resume their chains of combining affinity?

Every equivalent of carbon and of hydrogen in the burning body unites with two equivalents of oxygen, in strict conformity with the laws of combination. The flame of hydrogen, if pure, gives scarcely any light, but combined with the solid particles of carbon, it increases in brightness. The most brilliant of the illuminating gases is the olefiant gas, produced by the decomposition of alcohol, and it is only hydrogen charged with carbon to the point of saturation. Flame is a cone of heated vapour, becoming incandescent at the points of contact with the air; a mere superficial film only being luminous. It is evident that all the particles of the gas are in a state of very active repulsion over the surface, since flame will not pass through wire gauze of moderate fineness. Upon this discovery is founded the inimitable safety-lamp of Davy, by means of which the explosive gases of a mine are harmlessly ignited within a cage of wire gauze. This effect has been attributed to a cooling influence of the metal; but, since the wires may be brought to a degree of heat but little below redness without igniting the fire-damp, this does not appear to be the cause. The conditions of the safety lamp may be regarded as presenting examples exactly the converse of those already stated with reference to the spheroidal state of water; and it affords additional evidence that the condition of bodies at high temperatures is subject to important physical changes.

The principle upon which the safety lamp is constructed is, that a mixture of the fire-damp and atmospheric air in certain proportions explodes upon coming in contact with a flame.

This mixture passes readily through a wire gauze, under all circumstances, and it, of course, thus approaches the flame of the lamp enclosed within such a material, and it explodes. But, notwithstanding the mechanical force with which the exploding gas is thrown back against the bars of its cage, it cannot pass them. Consequently, the element of destruction is caught and caged; and notwithstanding its fierceness and energy, it cannot impart to the explosive atmosphere without, any of its force. No combustion can be communicated through the wire gauze.

The researches which led to the safety-lamp may be regarded as among the most complete examples of correct inductive experiment in the range of English science, and the result is certainly one of the proudest achievements of physico-chemical research. By merely enveloping the flame of a lamp with a metallic gauze, the labourer in the recesses of the gloomy mine may feel himself secure from that outpouring current of inflammable gas, which has been so often the minister of death; he may walk unharmed through the explosive atmosphere, and examine the intensity of its power, as it is wasted in trifling efforts within the little cage he carries. Accidents have been attributed to the “Davy,” as the lamp is called among the colliers; but they may in most cases be traced to carelessness on the part of those whose duty it has been to examine the lamps, or to the recklessness of the miners themselves.

That curious metal, platinum, and also palladium, possesses a property of maintaining a slow combustion, which the discoverer of the safety-lamp proposed to render available to a very important purpose. If we take a coil of platinum wire, and, having made it red-hot, plunge it into an explosive atmosphere of carburetted hydrogen and common air, it continues to glow with considerable brightness, producing, by this very peculiar influence, a combination of the gases, which is discovered by the escape of pungent acid vapours. Over the little flame of the safety-lamp, it was proposed by Davy to suspend a coil of platinum which would be thus kept constantly at a red heat. If the miner became accidentally enveloped in an atmosphere of fire-damp, although the flame of his lamp might be extinguished, the wire would continue to glow with sufficient brightness to light him from his danger, through the dark winding passages which have been worked in the bed of fossil fuel. This very beautiful arrangement has not, however, been adopted by our miners.

It is thus that the discoveries of science, although they may appear of an abstract character, constantly, sooner or later, are applied to uses by which some branch of human labour is assisted, the necessities of man’s condition relieved, and the amenities of life advanced.

The respiration of animals is an instance of the same kind of chemical phenomena as we discover in ordinary combustion. In the lungs the blood becomes charged with oxygen, derived from the atmospheric air, with which it passes through the system, performing its important offices, and the blood is returned to the lungs with the carbonic acid formed by the separation of carbon from the body which is thrown off at every expiration. It will be quite evident that this process is similar to that of ordinary combustion. In man or animals, as in the burning taper,—which is aptly enough employed by poets as the symbol of life,—we have hydrogen and carbon, with some nitrogen superadded; the hydrogen and oxygen form water under the action of the vital forces; the carbon with oxygen produces carbonic acid, and, by a curious process, the nitrogen and hydrogen also combine, to form ammonia.[222]

All the carbon which is taken into the animal economy passes, in the process of time, again into the atmosphere, in combination with oxygen, this being effected in the body, under the catalytic power of tissues, immediately influenced by the excitation of nervous forces, which are the direct manifestations of vital energy. The quantity of carbonic acid thus given out to the air is capable of calculation, with only a small amount of error. It appears that upwards of fifty ounces of carbonic acid must be given off from the body of a healthy man in twenty-four hours. On the lowest calculation, the population of London must add to the atmosphere daily 4,500,000 pounds of carbonic acid. It must also be remembered that in every process for artificial illumination, and in all the operations of the manufactures in which fire is used, and also in our arrangements to secure domestic comfort, immense quantities of this gas are formed. We may, indeed, fairly estimate the amount, if we ascertain the quantity of wood and coal consumed, of all the carbon which combines with oxygen while burning, and escapes into the air, either as carbonic acid or carbonic oxide. The former gas, the same as that which accumulates in deep wells and in brewers’ vats, is highly destructive to life, producing very distressing symptoms, even when mixed with atmospheric air, in but slight excess over that proportion which it commonly contains. The oppressive atmosphere of crowded rooms is in a great measure due to the increased proportion of carbonic acid given off from the lungs of those assembled, and collected in the almost stagnant air of badly ventilated apartments. It will be evident to every one, that unless some provision was made for removing this deleterious gas from the atmosphere as speedily as it formed, consequences of the most injurious character to the animal races would ensue. It is found, however, that the quantity in the atmosphere is almost constantly about one per cent. The peculiar properties of carbonic acid in part ensure its speedy removal. It is among the heaviest of gaseous bodies, and it is readily absorbed by water; consequently, floating within a short distance from the surface of the earth, a large quantity is dissolved by the waters spread over it. A considerable portion is removed by the vegetable kingdom; indeed, the whole of that produced by animals, and by the processes of combustion, eventually becomes part of the vegetable world, being absorbed with water by the roots, and separated from the air by the peculiar functions of the leaves. However, this heavy gas unites with the lighter atmospheric fluid in obedience to that law which determines the diffusion of different specific gravities through each other.

The leaves of plants may be regarded as performing similar offices to the lungs of animals. They are the breathing organs. In the animal economy a certain quantity of carbon is necessarily retained, in combination with nitrogen and other elements, to form muscle; but this is constantly undergoing change; the entire system being renewed within a comparatively limited period. The conditions with plants are somewhat different. For instance, the carbon is fixed in a tree, and remains as woody fibre until it decays, even though the life of the plant may extend over centuries.

Animals, then, are constantly supplying carbonic acid; plants are as constantly feeding on it; thus is the balance for ever maintained between the two kingdoms. Another condition is, however, required to maintain for the uses of men and animals the necessary supply of oxygen gas. This is effected by one of those wonderful operations of nature’s chemistry which must strike every reflecting mind with admiration. During the night plants absorb carbonic acid; but there is a condition of repose prevailing then in their functions, and consequently their powers of effecting the decomposition of this gas are reduced to their minimum. The plant sleeps, and vital power reposes; its repose being as necessary to the plant as to the animal. With the first gleam of the morning sun the dormant energies of the plant are awakened into full action; it decomposes this carbonic acid, secretes the carbon, to form the rings of wood which constitute so large a part of its structure, and pour out oxygen gas to the air. The plant is, therefore, an essential element in the conditions necessary for the support of animal life.

The animal produces carbonic acid in an exact proportion to the quantity of carbonaceous matter which it consumes. Fruit and herbage contain a small quantity of carbon in comparison with muscle and fat. But let us confine our attention to the human race. Man within the Tropics, where the natural temperature is high, does not require so great an amount of chemical action to go on within him for the purpose of maintaining the requisite animal heat; consequently his Maker has surrounded him with fruits and grains which constitute his food.

As we advance to the colder regions of the earth man becomes a flesh-eater, and his carnivorous appetite increases as the external temperature diminishes. Eventually we reach the coldest zones, and the human being there devours enormous quantities of fat to supply the necessities of his condition.

It must necessarily follow, that the inhabitants of the tropics do not produce so much carbonic acid as those who dwell in colder regions. In the first place, their habits of life are different, and they are not under the necessity of maintaining animal heat by the use of artificial combustion, as are the people of colder climes. The vegetation of the regions of the tropics is much more luxuriant than that of the temperate and arctic zones. Hence an additional supply of carbonic acid is required between the torrid zones, and a less quantity is produced by its animals. These cases are all met by the great aËrial movements. A current of warmed air, rich in oxygen, moves from the equator towards the poles, whilst the cooler air, charged with the excess of carbonic acid, sets in a constant stream towards the equator. By this means the most perfect equalization of the atmospheric conditions is preserved.

The carbonic acid poured out from the thousand mouths of our fiery furnaces,—produced during the laborious toil of the hard-working artizan,—and exhaled from every populous town of this our island home,—is borne away by this our aËrial currents to find its place in the pines of the Pacific Islands, the spice-trees of the Eastern Archipelago, and the cinchonas of Southern America. The plants of the valley of the Caucasus, and those which flourish amongst the Himalayas, equally with the less luxuriant vegetation of our temperate climes, are directly dependent upon man and the lower animals for their supply of food.

If all plants were removed from the earth, animals could not exist. How would it be if the animal kingdom was annihilated?—would it be possible for vegetation to continue? This question is not quite so easily answered; but, if we suppose all the carbon-producing machines—the animals—to be extinct, from whence would the plants draw their supply? It has been supposed that during the epoch of the coal formation a luxuriant vegetation must have gone on over the earth’s surface, when the existence of animal life was regarded as problematical. It is supposed that the air was then charged with carbonic acid, and that the calamites, lepidodendra, and sigilaria, were employed to remove it, and fit the earth for the oxygen-breathing races. The evidence upon these points is by no means satisfactory; and although at one time quite disposed to acquiesce in a conjecture which appears to account so beautifully for the observed geological phenomena of carboniferous periods, we do not regard the necessities for such a condition of the atmosphere as clearly made out.[223] Geological research, too, has shown that the immense forests from which our coal is formed teemed with life. A frog as large as an ox existed in the swamps, and the existence of insects proves the high order of organic creation at this epoch.

In all probability the same mutual dependence which now exists between the animal and vegetable kingdoms existed from the beginning of time, and will continue to do so under varying circumstances through the countless ages of the earth’s duration.

There is yet another very important chain of circumstances which binds these two great kingdoms together. This is the chain of the animal necessities. A large number of races feed directly upon vegetables; herbs and fruits are the only things from which they gain those elements required to restore the waste of their systems.

These herbivorous animals, which must necessarily form fat and muscle from the elements of their vegetable diet, are preyed on by the carnivorous races; and from these the carbon is again restored to the vegetable world. Sweep off from the earth the food of the herbivora, they must necessarily very soon perish, and with their dissolution, the destruction of the carnivora is certainly ensured. To illustrate this on a small scale, it may be mentioned that around the coasts of Cornwall, pilchards were formerly caught in very great abundance, in the shallow water within coves, where these fish are now but rarely seen. From the investigations of the Messrs. Couch, whose very accurate observations on the Cornish fauna have placed both father and son amongst the most eminent of British naturalists,[224] it appears that the absence of these fish is to be attributed entirely to the practice of the farmers, who cut the sea-weed from the rocks for the purpose of manuring their lands. By this they destroy all the small crustacea inhabiting these immature marine forests feeding on the algÆ, and as these, the principal food of the pilchards, have perished they seek for a substitute in more favourable situations. Mr. Darwin remarks, that if the immense sea-weeds of the Southern Ocean were removed by any cause, the whole fauna of these seas would be changed.

We have seen that animals and vegetables are composed principally of four elementary principles,—oxygen, hydrogen, nitrogen, and carbon. We have examined the remarkable manner in which they pass from one condition—from one kingdom of nature—into another. The animal, perishing and dwindling by decomposition into the most simple forms of matter, mingling with the atmosphere as mere gas, gradually becomes part of the growing plant, and by like changes vegetable organism progresses onward to form a portion of the animal structure.

A plant exposed to the action of natural or artificial decomposition passes into air, leaving but a few grains of solid matter behind it. An animal, in like manner, is gradually resolved into “thin air.” Muscle, and blood, and bones, having undergone the change, are found to have escaped as gases, leaving only “a pinch of dust,” which belongs to the more stable mineral world. Our dependency on the atmosphere is therefore evident. We derive our substance from it—we are, after death, resolved again into it. We are really but fleeting shadows. Animal and vegetable forms are little more than consolidated masses of the atmosphere. The sublime creations of the most gifted bard cannot rival the beauty of this, the highest and the truest poetry of science. Man has divined such changes by the unaided powers of reason, arguing from the phenomena which science reveals in unceasing action around him. The Grecian sage’s doubts of his own identity, were only an extension of a great truth beyond the limits of our reason. Romance and superstition resolve the spiritual man into a visible form of extreme ethereality in the spectral creations, “clothed in their own horror,” by which their reigns have been perpetuated.

When Shakespeare made his charming Ariel sing—

he painted, with considerable correctness, the chemical changes by which decomposing animal matter is replaced by a siliceous or calcareous formation.

But the gifted have the power of looking through the veil of nature, and they have revelations more wonderful than even those of the philosopher, who evokes them by perpetual toil and brain-racking struggle with the ever-changing elements around him.

The mysteries of flowers have ever been the charm of the poet’s song. Imagination has invested them with a magic influence, and fancy has almost regarded them as spiritual things. In contemplating their surpassing loveliness, the mind of every observer is improved, and the sentiments which they inspire, by their mere external elegance, are great and good. But in examining the real mysteries of their conditions, their physical phenomena, the relations in which they stand to the animal world, “stealing and giving odours” in the marvellous interchange of carbonic acid and ammonia for the soul-inspiring oxygen—all speaking of the powers of some unseen, in-dwelling principle, directed by a supreme ruler—the philosopher finds subjects for deep and soul-trying contemplation. Such studies lift the mind into the truly sublime of nature. The poet’s dream is the dim reflection of a distant star: the philosopher’s revelation is a strong telescopic examination of its features. One is the mere echo of the remote whisper of nature’s voice in the dim twilight; the other is the swelling music of the harp of Memnon, awakened by the sun of truth, newly risen from the night of ignorance.

To return from our long, but somewhat natural digression, to a consideration of the chemical phenomena connected with the atmosphere, and its curious and important element, nitrogen, we must first examine the evidence we have of the condition of the air itself.

The mean pressure exerted upon the surface of the earth, as indicated by the barometer, is equal to a column of mercury thirty inches high; that is, the column of air from the surface of the ocean to its highest limits exactly balances that quantity of mercury. If our tube of mercury had the area of one square inch, the columns would weigh fifteen pounds, which represents a pressure of fifteen pounds upon every square inch of the earth’s surface. This pressure, it must be remembered, is the compound weight of the gaseous envelope, and the elastic force of the aqueous vapour contained in it.[225] If the atmosphere were of uniform condition, its height, as inferred from the barometer, would be about five miles and a half. The density of the air, however, diminishes with the pressure upon it, so that at the height of 11,556 feet, the atmosphere is of half density; or one volume of air, as taken at the surface of the earth, is expanded into two at that height. Thus the weight is continually diminishing; but this is regularly opposed by the decreasing temperature, which diminishes the rate of about one degree for every 352 feet of ascent, although in all probability it is less rapid at great distances from the earth.

It has been calculated from certain phenomena of refraction, that our atmosphere must extend to about forty miles from the surface of the earth. It may, in a state of extreme tenuity, extend still further; but it is probable that the intense cold produced by rarefaction sets limits to any extension much beyond this elevation.

The uses of the atmosphere are many. It is the medium for regulating the dispersion of watery vapours over the earth. If there were no atmosphere, and that, as now, the equatorial climes were hot and the poles cold, evaporation would be continually going on at the equator, and condensation in the colder regions. The sky of the tropical climes would be perpetually cloudless, whilst in the temperate and arctic zones we should have constant rain and snow. By having a gaseous atmosphere, a more uniform state of things is produced; the vapours arising from the earth become intimately mixed with the air, and are borne by it over large tracts of country, and only precipitated when they enter some stratum much colder than that which involves them. There are opposite tendencies in an atmosphere of air and one of vapour. The air circulates from the colder to the warmer parts, and the vapour from the warmer to the colder regions; and as the currents of the air, from the distribution of land and sea—the land, from its low conducting power, being more quickly heated than the sea—are very complicated, and as some force is employed in keeping the vapour suspended in the air, water is less suddenly deposited on the earth than it would have been, had not these tendencies of the air and its hygrometric peculiarities been such as we find them.

The blue colour of the sky, which is so much more agreeable to the eye than either red or yellow, is due to a tendency of the mixed gas and vapour to reflect the blue rays rather than red or yellow. The white light which falls upon the surface of the earth, without absorption or decomposition in its passage from the sun, is partially absorbed by, and in part reflected back from, the earth. The reflected rays pass with tolerable freedom through this transparent medium, but a portion of the blue rays are interrupted and rendered visible to us. That it is reflected light, is proved by the fact of its being in a polarized state.[226] Clouds of vapour reflect to us again, not isolated rays, but the undecomposed beam, and consequently they appear white as snow to our vision.

The golden glories of sunset,—when, “like a dying dolphin,” heaven puts on the most gorgeous hues, which are continually changing,—depend entirely upon the quantity of watery vapour which is mixed with air, and its state of condensation. It has been observed, that steam at night, issuing into the atmosphere under a pressure of twenty or thirty pounds to the square inch, transmits and reflects orange-red light. This we may, therefore, conclude to be the property of such a condition of mixed vapour and air, as prevails when the rising or the setting sun is shedding over the eastern or the western horizon the glory of its coloured rays.[227]

Thus science points out to us the important uses of the air. We learn that life and combustion are entirely dependent on it, and that it is made the means for securing greater constancy in the climates of the earth than could otherwise be obtained. The facts already dwelt upon are sufficient to convince every thinking mind that the beautiful system of order which is displayed in the composition of the atmosphere, in which the all-exciting element, oxygen, is subdued to a tranquil state by another element, nitrogen, (which, we shall have presently to show, is itself, under certain conditions, one of the most energetic agents with which we are acquainted,) indicates a supreme power, omniscient in the adaptation of things to an especial end. Oxygen and nitrogen are here mixed for the benefit of man; man unites them by the aid of powers with which he is gifted, and the consequences are of a fatal kind. The principles which the great Chemist of Nature renders mild are transformed into sources of evil by the chemist of art.

Beyond all this, the atmosphere produces effects on light which add infinitely to the beauty of the world. Were there no atmosphere, we should only see those objects upon which the sun’s rays directly fell, or from which they were reflected. A ray falling through a small hole into a dark room, illuminating one object, which reflects some light upon another, is an apt illustration of the effect of light upon the earth, if it existed without its enveloping atmosphere. By the dispersive powers of this medium, sunlight is converted into daylight; and instead of unbearable, parallel rays illuminating brilliantly, and scorching up with heat those parts upon which they directly fall, leaving all other parts in the darkness of night, we enjoy the blessings of a diffusion of its rays, and experience the beauties of soft shades and slowly-deepening shadows. Without an atmosphere, the sun of the morning would burst upon us with unbearable brilliancy, and leave us suddenly, at the close of day, at once in utter darkness. With an atmosphere we have the twilight with all its tempered loveliness,—a “time for poets made.”

In chemical character, atmospheric air is composed of twenty-one volumes of oxygen, and seventy-nine volumes of nitrogen: or one hundred grains of air consist of 23·1 grains of the former, and 76·9 grains of the latter. Whether the air is taken from the greatest depths or the most exalted heights to which man has ever reached, an invariable proportion of the gases is maintained. The air of Chimborazo, of the arid plains of Egypt, of the pestilential delta of the Niger, or even of the infected atmosphere of an hospital, all give the same proportions of these two gases as we find existing on the healthful hills of Devonshire, or in the air of the city of London. This constancy in constitution leads to the supposition that the oxygen and nitrogen are chemically combined; but many eminent philosophers have contended that they are merely mechanically mixed; and they have shown that some peculiar properties prevail amongst gaseous bodies, which very fully explain the equal admixture of two gases the specific gravities of which are different. This is particularly exemplified in the case of carbonic acid, of which gas one per cent. can be detected in all regions of the air to which the investigations of man have reached. This gas, although so heavy, is, by the law of diffusion, mixed with great uniformity throughout the mass.[228] Every exhalation from the earth, of course, passes into the air; but these are generally either so light that they are carried into the upper regions, and there perform their parts in the meteorological phenomena, or they are otherwise very readily absorbed by water or growing plants, and thus is the atmosphere preserved in a state of purity for the uses of animals. Again, the quantity of oxygen contained in the air, and its very peculiar character, ensures the oxidation of all the volatile organic matters which are constantly passing off,—as the odoriferous principles of plants, the miasmata of swamps, and the products of animal putrefaction; these are rapidly converted into water, carbonic acid, or nitric acid, and quickly enter into new and harmless combinations. The elements of contagion we are unacquainted with; but since the attention of inquirers has been of late directed to this important and delicate subject, some light may possibly be thrown upon it before long.

Nothing, shows more strikingly the admirable adaptation of all things for their intended uses than the atmosphere. In it we find the source of life and health; and chemistry teaches us, most indisputably, that it is composed of certain proportions of oxygen and nitrogen gases; and experience informs us that it is on the oxygen that we are dependent for all that we enjoy. So beautifully is the atomic or molecular constitution ordered, that it is impossible to produce any change in the air without rendering it injurious to the vegetable and animal economy. It might be thought, from the well-known exhilirating character of oxygen gas, that, if a larger quantity existed in the atmosphere than that which we find there, the enjoyments of life would be of a more exciting kind; but the consequences of any increase would be exceedingly injurious; and, by quickening all the processes of life to an unnatural extent, the animal fabric would soon decay: excited into fever, it would be destroyed by its own fires. Chemistry has made us acquainted with six other compounds of oxygen and nitrogen, neither of them fitted for the purposes of vitality, of which the following are the most remarkable:—

Nitrous oxide, or the, so called, laughing gas, which contains two volumes of nitrogen to one of oxygen, would prove more destructive than even pure oxygen, from the delirious intoxication which it produces.

Nitric oxide is composed, according to Davy, of two volumes of nitrogen and two of oxygen. It is of so irritating a nature, that the glottis contracts spasmodically when any attempt is made to breathe it; and the moment it escapes into the air it combines with more oxygen, and forms the deep red fumes of nitrous acid.

Nitrous acid and the peroxide of nitrogen each contains an additional proportion of oxygen, and they are still more destructive to all organization.

Nitric acid contains five volumes of oxygen united to two of nitrogen; and the well-known destructive properties of aqua fortis it is unnecessary to describe.

The atmosphere, and these chemically active compounds, contain the same elements, but their mode of combining is different; and what is, in the one case, poisonous to the highest degree, is, in the other, rendered salubrious, and essential to all organized beings.

Nitrogen gas may be regarded in the light of a diluent to the oxygen. In its pure state it is only characterised by its negative properties. It will not burn, or act as a supporter of combustion. Animals speedily perish if confined in it; but they die rather through the absence of oxygen than from any poisonous property of this gas. Yet, in combination, we find nitrogen exhibiting powers of a most energetic character. In addition to the fulminating compounds and the explosive substances already named, which are among the most remarkable instances of unstable affinity with which we are acquainted, we have also the well-known pungent body, ammonia. From the analogous nature of this volatile compound, and the fixed alkalies soda and potash, it was inferred that it must, like them, be an oxide of a metallic base. Davy exposed ammonia to the action of potassium, and to the influence of the voltaic arc produced from 2,000 double plates, without at all changing its character. From its slight tendency to combination, and from its being found abundantly in the organs of animals feeding on substances that do not contain it, it is, however, probably a compound body. A phenomenon of an obscure and mysterious character is presented in the formation of the “ammoniacal amalgam,” as it is called.

Mercury, being mixed with an ammoniacal salt, is exposed to powerful galvanic action; and a compound, maintaining its metallic appearance, but of considerable lightness and very porous, presents itself.[229] This preparation has been carefully examined by Davy, Berzelius, and others. It is always resolved into ammonia and mercury; and, although the latter chemist is strongly inclined to regard it as affording evidence of the compound nature of nitrogen,—and he has, indeed, proposed the name nitricum for its hypothetical base,—yet, to the present time, we have no satisfactory explanation of this apparent metallization of ammonia.

No attempt will be made to describe the various elementary substances which come under the class of metallic bodies, much less to enumerate their combinations. Many of the metals, as silver and copper, are found sometimes in a native state, or nearly pure; but, for the most part, they exist, in nature, in combination with oxygen or sulphur; gold furnishing a remarkable exception. They are occasionally found combined with other bodies,—as oxidized carbon, phosphorus, chlorine, &c.; but these cases are by no means so common. Those substances called metals are generally found embedded in the rocks, or deposited in fissures formed through them; but it is one of the great discoveries of modern science, that those rocks themselves are metallic oxides. With metals we generally associate the idea of great density; but potassium and sodium, the metallic bases of potash and soda, are lighter than water, and they consequently float upon that fluid. We learn, therefore, from the researches of science, that the crust of this earth is composed entirely of metals, combined with gaseous elements; and there is reason for believing that one, or perhaps two, of the gases we have already named are also of a metallic character. Strange as it may appear, there is nothing, as will be seen on attentive consideration, irrational in this idea. Many of the metals proper, under the influence of such heat as we can, by artificial means, command, are dissipated in vapour, and may be maintained in this state perfectly invisible. Indeed, the transparent space above the surface of the mercury in the tube of a barometer, known as the Torricellian vacuum, is filled with the vapour of mercury. There is, therefore, no reason why nitrogen, or even hydrogen, should not be metallic molecules kept by the force of the repulsive powers of heat, or some other influence, at a great distance from each other. The peculiar manner in which nitrogen unites with mercury, and the property which hydrogen possesses of combining with antimony, zinc, arsenic, potassium, sodium, and possibly other metals, besides its union with sulphur and carbon—in all which cases there is no such change of character as occurs when they combine with oxygen—appear to indicate bodies which, chemically, are not very dissimilar to those metals themselves, although, physically, they have not the most remote resemblance.

“We know nothing,” says Davy, “of the true elements belonging to nature; but, so far as we can reason from the relations of the properties of matter, hydrogen is the substance which approaches nearest to what the elements may be supposed to be. It has energetic powers of combination, its parts are highly repulsive as to each other, and attractive of the particles of other matter; it enters into combination in a quantity very much smaller than any other substance, and in this respect it is approached by no known body.”[230]

Many of the elements are common to the three kingdoms of nature: most of those found in one condition of organization are discovered in another. The carbonates are an abundant mineral class. In the vegetable kingdom we find carbon combining with oxygen, hydrogen, and nitrogen: these elements, also, constitute the substance of animals, the proportion of nitrogen being, however, much larger. If one element, more than another, belongs especially to the animal economy, it is phosphorus, although this is not wanting in the vegetable world; and it is not uncommon in the mineral. Sulphur is common to the three kingdoms: it is abundant in the mineral, being one of the products of volcanic action; it is united with the metals, forming sulphurets; and is found in our rocks in the state of sulphuric acid or oxidized vapour, combined with the metallic bases of lime and other earths. In the vegetable kingdom we discover sulphur in all plants of the onion kind, in the mustard, and some others; it enters into the composition of vegetable albumen, and appears always combined with albumen, fibrine, and caseine, in the animal economy.

Chlorine is found most abundantly in combination with sodium, as common salt: in this state, in particular, we may trace it from the depths of the earth, its waters, and its rocks, to the plants and animals of the surface. Iodine is most abundant in marine plants; but it has been found in the mineral world, traced to plants, and it is indicated in the flesh of some animals. Bromine is known to us as a product of certain saline waters, and a few specimens of natural bromide of silver have been examined. Fluorine, the base of the acid which, combining with lime, forms fluor-spar, is found to exist to some considerable extent in bones; it has been discovered in milk and blood; and investigations have proved its existence in the vegetable world. It must not be forgotten that the earths, lime and magnesia, enter into the composition of the more solid parts of plants and animals. Lime is one of the principal constituents of animal bone and shells, and it is found in nearly all vegetables.

Silica, or the earth of flints, is met with in beautiful transparent crystals, in the depths of the mine; in all rock and soils we find it. In the bark of many plants, particularly the grasses, it is discovered, forming the hard supporting cuticle of the stalk, in wheat, the Dutch rush, the sugar-cane, the bamboo, and many other plants.

It is thus that we find the same elementary principle presenting itself in every form of matter, under the most Protean shapes. Numerous phenomena of even a more striking character than those selected, are exhibited in every department of chemistry; but within the limits of this essay it is impracticable to speak of any beyond those which directly explain natural phenomena.

The chemical elements, which actually exist in nature as simple bodies, are probably but few. Most of the gases are in all probability compounds of some ethereal ultimate principles; and with the advance of science we may fairly hope to discover the means of reducing some of them to a yet more simple state.

Curious relations, which can be traced through certain bodies, lead us to believe that they may be only modified conditions of one element. Flint and charcoal do not at first appear allied; but carbon in some of its states approaches very near to the condition of silicon, the metallic base of flint. When we remember the differences which are evident in three forms of one body—coke, graphite, and diamond—the dissimilitude between flint, a quartz crystal, and carbon, will cease to be a strong objection to the speculation.

Phosphorus, sulphur, and selenium, have many properties in common. Iodine, bromine, chlorine, and fluorine, appear to belong to the same group. Iron and nickel, and cobalt, have a close relation. Silver and lead are usually combined, and exhibit a strong relationship. Gold, platinum, and the rarer metals, have so many properties in common, that they may form a separate group from all the others.

Indeed, a philosophical examination of the elements now supposed to constitute the material world, enables us to divide them into about six well-defined groups. Wide differences exist within these groups; but still we find a sufficient number of common properties to warrant our classing them in one family.

The dream of the alchemists, in the vain endeavour to realise which they exhausted their lives and dissipated their wealth, had its foundation in a natural truth. The transmutation of one form of matter into another may be beyond the power of man, but it is certainly continually taking place in the laboratory of nature, under the directing law of the great Creator of this beautiful earth.

The speculations of men, through all ages, have leaned towards this idea, as is shown by the theory of the four elements,—Air, Fire, Earth, and Water,—of the ancients, the three,—Salt, Sulphur, and Mercury,—of the alchemists, and the refined speculations of Newton and Boscovich on the ultimate constitution of matter. All experimental inquiry points towards a similar conclusion. It is true we have no direct evidence of any elementary atom actually undergoing a change of state; but when we regard the variations produced by electrical influence, the changes of state which arise from the power of heat, and the physical alterations produced by light, it will be difficult to come to any other conclusion than that the particles of matter known to us as ultimate are capable of change, and consequently must be far removed from positively simple bodies, since the real elementary atom, possessing fixed properties, cannot be supposed capable of undergoing any transmutation. Allotropism could not occur in any absolutely simple body.

It will now be evident that in all chemical phenomena we have the combined exercise of the great physical forces, and evidences of some powers which are, as yet, shrouded in the mystery of our ignorance. The formation of minerals within the clefts of the rocks, the decomposition of metallic lodes, the germination of seeds, the growth of the plant, the development of its fruit and its ultimate decay, the secret processes of animal life, assimilation, digestion, and respiration, and all the changes of external form, which take place around us, are the result of the exercise of that principle which we call chemical.

By chemical action plants take from the atmosphere the elements of their growth; these they yield to animals, and from these they are again returned to the air. The viewless atmosphere is gradually formed into an organized being, the lordly tree upon whose branches the fowls of the air have their homes, and the human animal, exalted by being charged with a spiritual soul: yet the tree and the man alike are gradually resolved again into thin air. The changes of the mineral world are of an analogous character; but we cannot trace them so clearly in all their phenomena.

The planet on which we live began its course charged with a fixed quantity of physical force, and this has remained constant to the present moment, and will do so to the end of time. By influences external to this earth the balance of these forces is continually disturbed; and in the effort to restore the equilibrium, we have the production of all the varied forms of matter, and the manifestation of each particular physical principle or power. As motion and attraction, balanced against each other, maintain the earth in her elliptical orbit, so the opposition of forces determines the existence of the amorphous rock, the light-refracting crystal, the fixed and flowering plant, and the locomotive animal.

An eternal round of chemical action is displayed in nature. Life and death are but two phases of its influences. Growth and decay are equally the result of its power.