The

vening space together occupy equal portions of space. atoms of an element, with certain exceptions, such as mercury, cadmium, and zinc, do not exist separately in the free state, but are joined together probably in pairs, and in some cases in fours. These groups of atoms, whether composed of atoms of the same element, or of different elements, are called molecules, and are the smallest portions of an element or of a compound which can exist in the free state. Thus representing the atom of the element hydrogen by the symbol H, the molecule is 2 H, or as it is generally written, H2, and if the atom of chlorine is represented by Cl, 2 Cl, or Cl2, is its molecule. If hydrogen and chlorine unite or combine together to form hydrochloric acid, the smallest particle which can exist must be represented by HCl, or one atom of hydrogen and one atom of chlorine united form a molecule of hydrochloric acid. A considerable force holds the atoms of a molecule together, although this force varies to an enormous extent, and this force, whether exerted between atoms of the same element, or between those of differing elements, is the force whose nature I propose to discuss.

It is almost universally admitted that when a mass of matter is heated, the molecules of which it is composed remove farther apart, and the mass expands; on cooling, contraction ensues by the reverse action. It has not been so generally supposed that the atoms in the molecule in like manner approximate and recede. Yet there is reason to believe that the atoms, even in the closest union, are not in absolute contact, and that the cause which prevents that contact is heat. If an absolute zero of heat could be reached, such contact might result, but at present we know of nothing approaching this. We have long known that by the action of heat molecules of compounds separate into their constituent elements; some so readily that they are only stable at exceedingly low temperatures, others can bear higher and

higher degrees of heat before yielding, whilst some compounds, and indeed many, can stand the action of the hottest fires attainable by artificial means. This must be so, for, with the exception of the electric arc, our means of producing intense heat depend on the production of compounds such as water and carbonic acid gas. But the new physical science of spectrum analysis enables us to obtain evidence of what is going on in the sun, and we find in that orb elements which, on the earth by our imperfect appliances, we cannot separate from one another by heat, coexisting without combination in that fierce heat, whose intensity is all but unimaginable.

Recent researches by V. Meyer and J. M. Crafts have shown that disassociation can take place between the atoms of a molecule, when that molecule contains atoms of only one element. The researches were made on chlorine, bromine, and iodine, at high temperatures. With regard to chlorine, there is some disagreement as to the results, but bromine and iodine, at temperatures above 800°C, begin to disassociate. The experiment with iodine gave the most accordant results. The vapour of the element, which at 450°C is 8.74 times as heavy as air at the same temperature, at 1400°C is only 5.28 times as heavy. There can be little doubt that if higher temperatures could be applied, further separation would take place, until the density was 4.37, or corresponding to I in the free state, instead of I. At the same temperatures carbon dioxide and hydrochloric acid gas showed no sign of decomposition. Spectrum analysis gives evidence which has some weight in the same direction. When the spectrum of a glowing vapour is taken at varying temperatures, it is not found to be the same at them all. This difference is most probably due to some change in the molecules, and as all the evidence tends to show greater simplicity in a molecule as the result of increased heat, the

decomposition of the molecule into its constituent atoms is the most probable explanation of the altered spectrum.

The separation by heat of the atoms of the same element in a molecule, is what might naturally have been expected, as the force which holds like atoms together is generally looked upon as comparatively slight, much slighter, indeed, than I believe it to be. The question returns, What is this A great name in chemistry, Berzelius, gave the answer, Electricity. In an able review of the electro-chemical theory, unfortunately not printed in our Transactions, which one of our vice-presidents, Dr. J. Campbell Brown, once gave, when discussing the composition of ozone, this was fully shown to be incorrect; but as his remarks may have been forgotten, or not heard by some of you, and as this theory, in a modified form, has been brought before the Chemical Society, by that distinguished physicist, Professor Helmholtz, I will endeavour to lay it fairly before you. Berzelius's idea was the simple and fascinating one that the elements might be arranged in a series, of which one extreme was positive electrically, and the other negative, whilst the intervening elements were negative to those above them in the list, and positive to those below. The force of combination was strongest between those most widely separated in the list, and weakest between those which lay near to one another. Reasoning from the experiment in electricity, that if two pith balls are electrified, one positively and the other negatively, they will, when brought near, mutually attract one another, and adhere together so long as the electricity is undischarged, he supposed that the ultimate atoms of matter were in some unexplained way charged with amounts of electricity varying in kind and quantity in different elements, and that what could be seen in the pith balls was the picture of the union of atoms. Thus was explained the unequal stability of compounds, the production of heat attendant on

chemical combination, &c. This theory at once fails to account for the formation of molecules of the same element. If two atoms of hydrogen be equally charged with the same kind of electricity, say positive, they ought to repel one another, and thus make the formation of a molecule of hydrogen impossible. Similarly, two atoms of oxygen, both charged equally with negative electricity, should repel one another. Again, one finds a difficulty in understanding how, if in a mass of oxygen each atom is strongly charged with negative electricity, the mass gives no reaction of free electricity, but acts as if it were electrically neutral. The theory is also inconsistent with what we know of substitution. A molecule of marsh gas, CH, can have its positive hydrogen replaced by negative chlorine, until tetrachloride of carbon remains, yet the electrical state of the mass remains unchanged so far as external indications go. Helmholtz suggests a view which, so far as I know, is novel, and certainly is what he calls it, "startling." He says, "If we accept the hypothesis, that the elementary substances are composed of atoms, we cannot help concluding that electricity also, positive as well as negative, is divided into definite elementary portions, which behave like atoms of electricity." In opposition to Berzelius, he holds that the atoms of any element may be charged or united with either kind of electricity, so that the electrical neutrality of free hydrogen results from one atom of hydrogen charged positively being united with one charged negatively. This gets rid of the difficulty, and accounts for the formation of molecules of the same element. It is now necessary to examine the evidence as to the union of elements being due to the union of opposite electricities.

If hydrochloric acid is decomposed by an electrical current, hydrogen is set free at one pole and chlorine at the other. We can conceive that each element thus becomes

charged with electricity. But by the assumption just made, half of the atoms of hydrogen are charged with positive and half with negative electricity. They are all evolved at the negative pole, in consequence of which they should all have the same kind of electricity. But as in a galvanic circuit there is a current of positive electricity passing one way, and an equal current of negative electricity passing in the opposite, it is conceivable that the electric charges may be so communicated to the atoms that an equal number may be charged with each kind. If, however, the union of the electric charges is the cause of the union of the atoms in hydrochloric acid, how does the charging of the atoms with electricity bring about their separation ? Also the atoms of hydrogen evolved are charged, one with positive and one with negative electricity, to form a molecule, and the same is the case with the atoms of chlorine in its molecule. Thus the atoms of hydrogen are held together by certain charges of electricity, and form a molecule, and the atoms of chlorine are held together by equal charges to form their molecule, so that the difficulty arises, how there can be any union of hydrogen with chlorine, as the molecule of hydrogen should offer just as much resistance to separation as the molecule of hydrochloric acid. Also, hydrochloric acid can be decomposed by other means than electricity, for the hydrogen can be set free by the action of a metal, and chlorine by passing the gas over heated peroxide of manganese, and the gases thus obtained will combine just as readily, and with as much force, as if they had been separated by electricity. We must therefore suppose that the charge of electricity is inseparable from the atom, and that no influence such as heat or contact with other bodies is able to remove it. The atoms of each element must thus, at their first formation, have been endowed with their electrical charges, half being positively and half negatively charged. But on this assumption we