Heat is the most universally manifested effect, no union of elements being unattended by it. Almost all the methods by which heat is artificially produced, either in the animal economy, or for the purpose of warming, depend on chemical force. Our fires owe their heating power to the chemical force which combines the carbon and hydrogen of the fuel with the oxygen of the air, and the intense heat of the electric arc is indirectly the result of the chemical action in the battery, or of the oxidation of the fuel under the boiler of the engine which drives the electro-magnetic engine. Whence is this heat derived, and how can we explain its production? Formerly, when heat was supposed to be a substance, known as caloric, the atoms were thought to hold a quantity of heat latent, and that when the atoms united, the compound, having less capacity for holding heat, the excess became sensible. We now know that this explanation is untrue; heat is only a form of vibration, and when sensible is generally the result of arrested motion. When a bullet strikes an iron target, its motion being suddenly arrested, an equivalent amount of heat is developed, which may suffice to melt the lead. Friction, or impeded motion, can develop heat, as we see in many processes of every-day life, as in the striking of a match. Electricity struggling to pass through a thin platinum wire, meets with resistance, and the wire becomes hot enough to melt even such an infusible metal. Hence, we should naturally look for a similar cause for the heat developed in chemical action, as, for example, when hydrogen, and chlorine, or hydrogen and oxygen unite. There is a common idea that the chemical force is itself transformed into heat. But the force which brings the atoms together is constantly acting with undiminished activity in keeping the atoms together, and as the transmutation of one force into another necessarily implies the cessation of the former one, this explanation must fall to B millimetre in sectional area will bear a weight of 184 lbs., or in the diamond, whose particles are almost indissolubly bound to one another, it is easy to imagine that in the closer contact of atomic cohesion a force may be exerted quite equal to explain all the stability met with in compounds. The difference in properties between a compound and those of the elements of which it is composed may seem too great to be explained by the mere contact of the atoms. But all the properties of matter appear to depend on molecular arrangement; and seeing the total alteration of the properties of phosphorus when transformed from the yellow into the red modification—a change embracing not only physical, but even chemical properties; when colour, hardness, transparency, specific gravity, and even solubility, oxidizability, and action on the animal economy, are all changed, we cease to wonder at the change in properties when a new molecule is formed with two or more differing elements with differing properties. As with molecular cohesion, so with atomic: heat is the opposing agent. The tenacity of metals diminishes as the temperature rises, until the solid is resolved into a liquid, and the liquid finally into a gas. So with chemical compounds; some can only exist at the lowest temperatures which we can produce, others require but a touch before the bonds of cohesion are destroyed, and the elements go off beyond the bounds of their mutual attraction. As we raise the temperature, more and more chemical compounds are disassociated, some splitting into simpler compounds, but at last separating into their elements, until in heat far surpassing any of man's device, in the solar furnace, where heat beyond the imagination of man struggles with atomic cohesion, compounds such as we know them seem to be impossible, and we seem to behold indications of the decomposition of what we call elements. The decomposition by heat of the molecules of a compound led chemists to foresee that the molecules of elements would fare no better; and we have seen that bromine and iodine, and perhaps chlorine, have been already almost brought into freedom from their mutual bonds. Yet heat often brings about chemical combination, of which it appears to be the antagonist. Hydrogen and chlorine remain side by side without combination, until heat or light causes the dormant force to act. This I look upon as a strong argument in favour of the molecular theory. The atoms in the molecule of hydrogen, and those of chlorine, are held together by a force which, although much weaker than those which unite the atoms of hydrogen when combined with chlorine, are still strong enough to prevent the atoms from coming within the range of their mutual attraction; but heat weakens these bonds, the atoms are removed farther apart, until they are sufficiently free from their former bonds to unite in others more stable, because exerted between diverse elements. Again, mercury and oxygen at ordinary temperatures are without action on one another. By heat the bonds which unite the molecule of oxygen are loosened, the vapourised mercury comes within the range of action, and solid oxide of mercury is formed, in in which the molecular attraction resulting in the formation of a solid body no doubt acts in increasing the stability of the compound. So far, heat acts as an aid to combination, but raise the temperature higher, and the compound ceases to be. Not heat alone, but also that related form of vibration, light, can bring about this loosening of atomic attraction. Hydrogen and chlorine, under the influence of sunlight, or other light possessing what is known as actinism, will combine as if heat had been applied, and it must act, as does heat, by diminishing the attractive force in the molecules of hydrogen and chlorine, so as to enable the atoms of the two elements to enter into the range of their mutual action. This appears to be the rational explanation, for the action of light, like that of heat, is generally antagonistic to the force which we call chemical. In photography, advantage is taken of the decomposing action of light, under which silver salts are reduced to silver, and chlorine, bromine, or iodine. The influence of light in affecting the molecular arrangement of bodies is well shown in a new pigment composed of a mixture of more or less oxidized sulphide of zinc with sulphate of barium. This pigment, when exposed to bright sunlight, becomes dark coloured, but resumes its perfect whiteness in the dark. Here the change is not apparently a chemical one, but due to an alteration in the molecule of one of the constituents. The action of light in vegetation is also a decomposing action, as under its influence carbon dioxide and water lose oxygen. Electricity must be looked upon as another antagonistic force, for under its influence nearly all soluble compounds are decomposed, whilst it scarcely ever seems to promote chemical action, except where the heat evolved by the electric spark acts like heat derived from other sources, or where some secondary action results from the chemical action of the elements liberated by the decomposing effect of the electricity. In all these cases we must remember that at the lowest temperature which we can attain, there is still heat acting on the molecule. It is probable that, even in the molecule, this prevents absolute contact, and that the atoms are vibrating under its influence within a certain range, increasing as the temperature rises, or as other vibrating actions are added to those of heat, until the atoms at last swing so far as to escape from the sphere of mutual attraction, and go off to form other molecules. The effects of chemical force are very striking, heat, light, and electricity being among the accompanying phenomena. Heat is the most universally manifested effect, no union of elements being unattended by it. Almost all the methods by which heat is artificially produced, either in the animal economy, or for the purpose of warming, depend on chemical force. Our fires owe their heating power to the chemical force which combines the carbon and hydrogen of the fuel with the oxygen of the air, and the intense heat of the electric arc is indirectly the result of the chemical action in the battery, or of the oxidation of the fuel under the boiler of the engine which drives the electro-magnetic engine. Whence is this heat derived, and how can we explain its production? Formerly, when heat was supposed to be a substance, known as caloric, the atoms were thought to hold a quantity of heat latent, and that when the atoms united, the compound, having less capacity for holding heat, the excess became sensible. We now know that this explanation is untrue; heat is only a form of vibration, and when sensible is generally the result of arrested motion. When a bullet strikes an iron target, its motion being suddenly arrested, an equivalent amount of heat is developed, which may suffice to melt the lead. Friction, or impeded motion, can develop heat, as we see in many processes of every-day life, as in the striking of a match. Electricity struggling to pass through a thin platinum wire, meets with resistance, and the wire becomes hot enough to melt even such an infusible metal. Hence, we should naturally look for a similar cause for the heat developed in chemical action, as, for example, when hydrogen, and chlorine, or hydrogen and oxygen unite. There is a common idea that the chemical force is itself transformed into heat. But the force which brings the atoms together is constantly acting with undiminished activity in keeping the atoms together, and as the transmutation of one force into another necessarily implies the cessation of the former one, this explanation must fall to B |