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 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

perform the operation, whilst the plant does it without the latter, and at a far lower temperature. If we borrow from the plant and use diastase from malt, or from the animal, and employ saliva, we may, indeed, produce the change under the conditions in which the plant or the animal can, but these fermentations are another mystery, and chemistry knows nothing of the manufacture of the ferment. In all these actions, which we can in some sort imitate, chemical force is exerted on formed and dead compounds. It can oxidize and destroy effete matters in the organism, but in the building up of structure, and assimilation of nutriment, we stand in the presence of some directing and controlling power which uses chemical force as a servant, but which can in no way be confounded with it. So man can utilise this force; he can accomplish with it wonders which may not unnaturally excite visionary hopes of dispensing with the aid of nature; but it is the directing mind and hand which bring about these marvels. Chemical force in itself tends to equilibrium and rest. Complex compounds are in a state of unstable equilibrium, and tend to dissolution. They are subject to resolution into more stable, because more simple, compounds, and if this were all, the final result would be death and silence. According to the teaching of modern science, there was a time when the materials of the earth were in a state of intensely heated vapour; as this cooled, simple compounds, as carbonic acid and water, silica, and the oxides of the metals, would be formed. But these are also the most stable and unchangeable compounds, and whence came the tendency to complexity and consequent instability, with life as the crowning point of the edifice. We have not the faintest knowledge of anything in chemistry that can do this. Man, with the inherited wisdom of ages, and every resource in the way of apparatus, and materials manufactured for him in the laboratory of nature, has hitherto not even approached the preparation of

what has been called the physicial basis of life. He cannot even find out its true composition, and although the simplest living matter is a complex union of albumenoids, fat, and mineral substances, as shown by analysis, we do not even know whether these may not be nothing but the products into which the living mass resolves itself when it dies. Yet this mass of organic complexity, comprising six or eight elements, or even more, if the mineral constituents are all essential, which, without life, tends to instant dissolution, we are bidden to assume at some unknown time, and under some unknown and, I suppose, unreplaceable conditions, did manage to get formed and forthwith live. It may be so, but chemical force must not be invoked as the agent, for it knows nothing of it. Under the direction of mind, chemical force can work wonders. It has already won triumphs in the manufacture of colouring materials which rival, or even excel, those of nature in brilliancy, and now it is learning to make the finished products of nature. Alizarine, the colouring matter of the madder, and indigo, the product of several different plants, have both been manufactured, and although the latter is still dearer than the natural product, it is presented in a form in which it can be profitably used. It must not be forgotten that every chemical substance of this nature derived from the animal or vegetable kingdom, which has hitherto also been produced artificially, is only an organic substance, in that it was naturally produced by an organism. It was not a living part of that organism, but a product of it, either like urea, a waste material, the final result of the wear of nitrogenous parts of the body, or like conine, an active principal medicinally, but, so far as we know, fulfilling no useful or at least necessary part in the hemlock plant. There is no reason, however, for putting any limit to the result of chemical work in this direction. Every definite chemical substance whose composition can be expressed in a chemical

formula, stands forth to be imitated, or rather reproduced, by the art of man. Nor do I think that man is wise whọ puts any limits as to what may or may not be attempted. It is as rash to say beforehand that a thing cannot be done, as to say that it can. The doing of it is the only proof that the thing is possible, and the only refutation of the charge that it is impossible. But there is little need at present to take down barriers; one has only to read the newspapers to see that those, at least, who know very little of the subject, be it chemistry, electricity, or any other force which has just received a novel application, have the most liberal expectation of the wonders which loom before them, and I fear must feel disappointed that the realisation of their dreams is not forthwith brought about.

In this review of our knowledge of chemical force, I fear that you may feel disappointed at the negative character of the result. I have no new idol to set up for worship, in lieu of the old ones; no new theory of the cause of the rush of atoms and their mutal cohesion to offer. Lucretius might be excused for endowing his atoms with mechanical means of fastening themselves together, such as hooks and spikes, and having set his atoms aimlessly falling through space, accounting for the formation of compounds by the atoms becoming entangled with one another. Those with many hooks thus gave hard and solid compounds, whilst those which were comparatively smooth cohered but slightly, so as to form liquids or gases. In those days theory was rampant; experimental philosophy was comparatively unknown; if things were in some way accounted for, not absolutely absurd, all was well. But there is something from which the human mind revolts in the formation of a universe, with its nice adaptations, and the numberless arrangements for use and beauty in the