By the agency of the electricity of the voltaic battery, we are enabled to produce the most intense heat with which we are acquainted, and by a peculiarly ingenious arrangement Mr. Grove has succeeded in resolving water by the mere action of heat into its constituent elements—oxygen and hydrogen gases. That this decomposition is not due to the voltaic current, but to the heat produced by it, was subsequently proved by employing platina heated by the oxy-hydrogen flame.[69]
This interesting question has been examined with great care by Dr. Robinson of Armagh, who has shown that, as the temperature of water is increased, the affinity of its elements is lessened, until at a certain point it is eventually destroyed. This new and startling fact appears scarcely consistent with our knowledge that a body heated so as to be luminous has the power of causing the combination of the elements of water with explosive violence.[70] But as this acute experimental philosopher somewhat boldly but still most reasonably inquires: “Is it not probable that, if not light, some other actinic power (like that which accompanies light in the spectrum, and is revealed to us by its chemical effects in the processes of photography) is evolved by the heat, and, though invisible, determines, in conjunction with the affinity, that atomic change which transforms the three volumes of oxygen and hydrogen into two of steam?”[71]
This speculation explains, in a very satisfactory manner, some results which were obtained by Count Rumford, in 1798. In a series of experiments instituted for the purpose of examining “those chemical properties of light which have been attributed to it,” he has shown that many cases of chemical decomposition occur in perfect darkness, under the influence of heat, which are precisely similar to those produced by exposure to the sun’s rays.[72]
It must, however, be remembered, that both solar light and heat are sometimes found in direct antagonism to actinic power, and that the most decided chemical changes are produced by those rays in which neither heat nor light can be detected. The most remarkable phenomena of this class will be explained under the head of actinism.
One of the most curious relations which as yet have been discovered between light and heat is, that, the temperature at which all bodies become incandescent, excepting such as are phosphorescent, is uniform. The point on the thermometer (Fahrenheit’s scale) when the eye by perfect repose is enabled to detect the first luminous influence, may probably be regarded as, or very near, 1000°. Daniel has fixed this point at 980°, Wedgwood at 947°, and Draper at 977°.[73] Dr. Robinson and Dr. Draper, by independent observations, have both arrived at the conclusion, that the first gleam of light which appears from heated platina is not red, but of a lavender gray, the same in character of colour as that detected by Sir John Herschel among the most refrangible rays of the solar spectrum.[74]
It must be admitted, that the question of the identity, or otherwise, of light and radiant heat, is beset with difficulties. Many of their phenomena are very similar—many of their modes of action are alike: they are often found as allied agencies; but they as frequently exhibit extreme diversity of action, and they may be separated from each other.
We have now examined the physical conditions and properties of this most important element, and we must proceed to learn something of the means by which it may be developed, independently of its solar source.
This extraordinary principle exists in a latent state in all bodies, and may be pressed out of them. The blacksmith hammers a nail until it becomes red hot, and from it he lights the match with which he kindles the fire of his forge. The iron has by this process become more dense, and percussion will not again produce incandescence until the bar has been exposed in fire to a red heat. The only inference we can draw from this result is, that by hammering the particles have been driven closer together, and the heat driven out; now further hammering will not force the atoms nearer, and consequently no additional quantity of heat can be developed; the iron is made hot in a fire, it absorbs heat, the particles are restored to their former state, and we can now again by hammering develope both heat and light. The Indian produces a spark by the attrition of two pieces of wood. By friction, two pieces of ice may be made to melt each other; and could we, by mechanical pressure, force water into a solid state, an immense quantity of heat would be set free. By the condensation of hydrogen and oxygen gases, pulverulent platinum will become glowing red-hot, and, with certain precautions, even the compact metal, platinum, itself; the heat being derived from the gases, the union of which it has effected. A body passing from the solid to the fluid state absorbs heat from all surrounding substances, and hence a degree of cold is produced. The heat which is thus removed is not destroyed—it is held combined with the fluid; it exists in a latent state. Fluids, in passing into a gaseous form, also rob all surrounding bodies of an amount of heat necessary to maintain the aëriform condition. From the air or from the fluid, this heat may, as we have shown above, be again extracted. Locked in a pint measure of air, there exists sufficient heat to raise several square inches of metal to glowing redness. By the compression of atmospheric air this may be shown, and with a small condensing syringe a sufficient quantity of heat may be set free to fire the Boletus igniarius, which, impregnated with nitre, is known as amadou. We are acquainted with various sources from which heat may be developed for artificial purposes: the flint-and-steel is an example of the production of heat by mechanical force, and the modern lucifer-match, of the combined action of friction and chemical affinity. These of themselves would admit of a lengthened discourse; but it is necessary that we carefully examine some of the less familiar phenomena of heat under the influences of changes of chemical condition.
If spirits of wine and water are mixed together, a considerable degree of heat is given out, and by mixing sulphuric acid and water, an infinitely larger quantity. If sulphuric acid (oil of vitriol) and spirit of wine, or nitric acid (aquafortis) and spirits of turpentine, at common temperatures, be suddenly mixed, so much heat is set free as to ignite the spirit. In each of these instances there is a condensation of the fluid. In nearly all cases of solution, cold is produced by the absorption of the heat necessary to sustain the salt in a liquid form; but when potash dissolves in water, heat is given out, which is a fact we cannot yet explain. If potassium is placed on water, it seizes the oxygen of the water and sets fire to the hydrogen gas liberated by the heat produced in the change of form. Antimony and many other metals thrown into chlorine gas ignite and burn with brilliancy: the same phenomenon takes place in the vapours of iodine or bromine. Many chemical combinations, as the chloride of potassium and sulphur explode with a blow; whilst the slightest friction occasions the detonation of the fulminating salts of silver, mercury, and gold. Compounds of nitrogen and chlorine, or iodine, are still more delicately combined—the former exploding with fearful violence on the contact of any oleaginous body, and the latter by the smallest elevation of temperature: both of them destroying the vessels in which they may be contained.