Gun cotton presents some peculiar phenomena which may merit brief attention. This peculiar compound is prepared by the action of nitric acid on cotton fibre. The general appearance of the cotton is not altered, but a remarkable physical change has taken place. It is now soluble in ether, and forms a gelatinous compound:—it explodes violently at a temperature which is insufficient for the combustion of gunpowder. Indeed, from, as it would appear, slight electrical disturbances taking place in the gun cotton itself, it not unfrequently explodes spontaneously. These fearful disturbances of the forces which hold bodies in combination are explained with difficulty. May it not be, that an enormous quantity of the calorific and chemical principles is held in a state of extreme tension around the particles of the compound, and that the equilibrium being destroyed, the whole is developed in destructive rapidity?
The fact of great heat being evolved during the conversion of a body from a solid to a gaseous state, as in the explosion of gunpowder or gun cotton, which is a striking exception to the law of latent heat, as it prevails in most cases, admits of no more satisfactory explanation.
As mechanical force produces calorific excitation, so we find that every movement of sap in vegetables, and of the blood and fluids in the animal economy, causes a sensible increase of heat. The chemical processes constantly going on in plants and animals are another source of heat, in addition to which nervous energy and muscular movement must be regarded as producing the caloric which is essential to the health and life of the latter. Digestion has been considered as a process of combustion; and the action between the elements of food, and the oxygen conveyed by the circulation of the blood to every part of the body, regarded as the source of animal heat; and, without doubt, it is one great source, although it can scarcely be regarded as the only one.[75]
The vis vitæ, or vital power, influences the delicate and beautiful system of nerves; and as life (an essence of the rarest and most subtile order, a diffusive influence) runs through them, from the brain to the extremities of the members of the body, it sets those tender threads in rapid vibration, and heat is developed. By this action, the circulation of the blood is effected; the muscle is maintained in an elastic condition, ready to perform the tasks of the will; and through these agencies is the warm and fluid blood fitted to receive its chemical restoratives in the lungs, and the stomach to support changes for which it is designed—chemical also—by which more heat is liberated. Was digestion—Eremacausis, as the slow combustion produced by combination with oxygen is called—the only source of animal heat, why should the injury of one filmy nerve place a member of the body for ever in the condition of stony coldness? Or why, chemical action being most actively continued after a violent death, by the action of the gastric juices upon the animal tissues, should not animal heat be maintained for a much longer period than it is found to be after respiration has ceased?[76]
In studying the influences of caloric upon the conditions of matter, we must regard the effects of extreme heat, and also of the greatest degrees of cold which have been obtained.
There are a set of experiments by the Baron Cagniard de la Tour, which appear to have a very important bearing on some conditions that may be supposed to prevail in nature, particularly if we adopt the view of a constantly increasing temperature towards the centre of our earth. If water, alcohol, or ether, is put into a strong glass tube of small bore, the ends hermetically sealed, and the whole exposed to a strong heat, the fluid disappears, being converted into a transparent gas; but, upon cooling, it is again condensed, without loss, into its original fluid state.[77] In this experiment, fluid bodies have been converted into elastic transparent gases with but small change of volume, under the pressure of their own atmospheres. We can readily conceive a similar result occurring upon a far more extensive scale. In volcanic districts, at great depths, and consequently under the pressure of the superincumbent mass, the siliceous rocks, or even metals, may, from the action of intense heat, be brought into a fluid, or even a gaseous condition, without any change of volume, since the elastic force of heat is opposed by the rigid resistance of the pressure of the surrounding rocks. Some beautiful experiments by Mr. Hopkins, of Cambridge, have proved that the temperature necessary to melt a body must be considerably elevated as the mechanical pressure to which it is subjected is increased.
Directly connected with the results of Cagniard de la Tour are a yet more remarkable set of phenomena, which have been investigated by M. Boutigny,[78] and generally known as the “spheroidal condition” of bodies. If water is projected upon very hot metal it instantly assumes a spheroidal form—an internal motion of its particles may be observed—it revolves with rapidity, and evaporates very slowly. If a silver or platinum capsule, when brought to a bright red heat, is filled with cold water, the whole mass assumes the spheroidal state, the temperature of the fluid remaining considerably below the boiling point, so long as the red heat is maintained. If we allow the vessel to cool below redness in the dark, the water then bursts into active ebullition, and is dissipated into vapour with almost explosive violence. An equal quantity of water being projected into two similar vessels, over the fire, one cold and the other red hot, it will be found that the water in the cold vessel will boil and evaporate long before that in the one which is red hot.
Another form of this experiment is exceedingly instructive. If a mass of white hot metal is suddenly plunged into a vessel of cold water, the incandescence is not quenched, the metal shines with a bright white light, and the water is seen to circulate around, but at some distance from the glowing mass, being actually repelled by calorific agency. At length, when the metal cools, the water comes in contact with it, and boils with energy.
A result similar to this was observed by Perkins, but its correctness most unjustly doubted. Having made an iron shell containing water, and carefully plugged up, white hot, it was found that the steam never exerted sufficient force to burst the vessel, as it was expected it would do. He caused a hole to be drilled into the bottom of the white-hot shell, and he was surprised to find that no water flowed through the orifice, until the iron was considerably cooled, when it issued forth with violence in the form of steam. Here we have the Cagniard de la Tour state first induced, and the calorific repulsion of the spheroidal state supervenes. If water is poured upon an iron sieve, the wires of which are made red hot, it will not percolate; but on cooling, it runs through rapidly. M. Boutigny, pursuing this curious inquiry, has recently proved that the moisture upon the skin is sufficient to protect it from disorganization, if the arm is plunged into baths of melted metal. The resistance of the surfaces is so great, that little elevation of temperature is experienced.[79] Professor Plücker, of Bonn, has stated that by washing the arm with ether previously to plunging it into melted metal, the sensation produced, while in the molten mass, is that of freezing coldness.