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

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the ground. To what source can the heat, then, be due, except to the arrested motion of the atoms? Flying together with a force varying according to the attractive power they strike, and heat is evolved, greater as the velocity is greater. We thus see in the brilliant light, consequent on the intense heat when iron burns in oxygen, the result of the collision of incalculable atoms of oxygen striking the atoms of iron, just as heat is produced when the blacksmith hammers a bar of cold iron into redness. With such a simple explanation of the heat produced in chemical action, which accords with the production of heat in cases in which we can experimentally demonstrate the cause, it seems almost unnecessary to look for more recondite causes. The amount of heat appears to depend on the force exerted and the stability of the resulting compound. To quote again from Professor Williamson's address, "It has been proved that the heat of combination affords a measure of its force; and we know that in giving off heat particles of matter undergo a diminution of motion. We see, accordingly, that substances capable of exerting great force by their combination, are those which can undergo a great diminution of the velocity of their internal motions, and reciprocally. The force of chemical combination is evidently a function of atomic motion."

As to electricity as evolved in the battery, I do not venture to suggest a cause, profoundly ignorant as we are with respect to a force which may be evolved in such different ways as by the friction of a glass rod, where no chemical action can be supposed, or from the revolution of a coil of covered wire in front of a magnet, where there is just as little. I feel myself utterly unequal to the task of explaining how it is that when zinc is dissolved in dilute sulphuric acid, no perceptible electricity is evolved, whilst if a plate of platinum is put in, electricity is manifested. When the explanation is arrived

at, I fancy that it will not be found in any transformation of the force which brings and holds atoms together, but in a result of the motion so produced, akin to that which produces heat.

Amongst the effects ascribed to chemical force of late years, is that in combination with other forces it is competent to explain the mystery of life. It seems now to be considered as the greatest factor in that concourse of powers of which the resultant is vitality. At one time electricity was the favourite agent, and in my opinion there was more to be said for it than for chemical force. Under its influence muscular movements and a faint image of vitality can be given to a recently dead body, but when life is extinct, and chemical force comes into play without check, the whole tendency is to disintegration. Then the complex compounds which exist in the living body come under the influence of external conditions, the unity of the structure is dissolved, bodies more and more simple are formed, until at last nothing remains but some gases, water, and mineral ash. Chemical actions do take place under the direction of life, but they are actions which we can seldom imitate, and in one remarkable instance, the reduction of carbonic acid with evolution of free oxygen, we are unable to repeat the result which a simple vegetable cell presents every moment. There are plenty of chemical actions in the animal and vegetable. The heat of the human body is kept up by the oxidation of carbon and hydrogen, just as the heat of a furnace is. But there is difference even here; this oxidation takes place at a temperature at which out of the body we cannot produce it so completely or in anything like the same time, if indeed, without the intervention of living organisms of low type we can perform it at all. The conversion of starch into sugar is a purely chemical action, but in the laboratory we must use heat and strong acids to