philosophy during the last five-and-twenty years. Its bearings upon the question of the origin of the "elements" have been worked out in the Presidential Address I have already referred to. Mr. Crookes, like Mr. Lockyer before him, in seeking to apply to this question of the genesis of the "elements" the same principles of evolution which Laplace has already applied to the creation of the heavenly bodies, and which Lamarck, Darwin, and Wallace have applied to that of the organic world, is again appealing to the law of continuity. The mind which holds that nature is one harmonious whole is fain to believe that the probability that the elements have originated by chance, and are eternally selfexistent, is just as remote as that the animals and plants of to-day are primordially created things. I think, in what I am now saying I may fairly claim to be reflecting the opinion on this matter of every philosophic thinker of to-day. Nay, more: you must allow that the germ which has been kept alive for so many centuries, and which has come down to us through the brains of a succession of thinkers like Leucippus, Aristotle, Lucretius, Bacon, Newton, Dalton, and Graham, has become quickened and endowed, by the light which modern science has shed upon it from all sides, with a vitality which will persist and strengthen.

Having thus traced the development of the idea held by Graham of the essential oneness of matter, let us spend the few remaining moments in considering, in the most general way, how the science of the last twenty-five years has worked out and extended his conceptions concerning the properties of the atom and its mode of motion.

The treatment which "the few grand and simple features of the gas," to quote Graham's phrase, has received at the hands of Clausius, Clerk Maxwell, Helmholtz, Sir William Thomson, and of a score of workers in this country and on the Continent who have been actuated by their influence, has served to dispel much of the metaphysical fog which has

enshrouded the notion of the atom, and to-day we are able to reason about atoms, as physical entities, having extension and figure, and to speak of their number and dimensions and peculiarities of movement, with a confidence based on wellascertained facts. We have, of course, not yet attained to a complete molecular theory of gases. But we know the relative masses of the molecules of various gases, and we have calculated in miles per second their average velocity. The phenomena of diffusion indicate that the molecules of one and the same gas are all equal in mass. For, as was pointed out by Clerk Maxwell, if they were not, Graham's method of using a porous septum would enable us to separate the molecules of smaller mass from those of greater, as they would stream through porous substances with greater velocity. We should thus be able to separate a gas, say hydrogen, into two portions, having different densities and other physical properties, different combining weights, and probably different chemical properties of other kinds. As no chemist has yet obtained specimens of hydrogen differing in this way from other specimens, we conclude that all the molecules of hydrogen are of sensibly the same mass, and not merely that their mean mass is 66 a statistical constant of great stability." (See Art. "Atom," Encyclopædia Britannica, 9th ed.) This line of argument has, it seems to me, an important bearing upon a question which has been raised by Marignac, Schützenberger, and others, and which has again been raised by Mr. Crookes in the address I have already referred to. Mr. Crookes thinks that it may well be questioned whether there is an absolute uniformity in the mass of every ultimate atom of the same chemical element, and that it is probable that our atomic weights merely represent a mean value, around which the actual atomic weights of the atoms vary within certain narrow limits, or, in other words, that the mean mass is "a statistical constant of great stability." The facts of diffusion would seem to lend no support to such a supposition,

REESE LIBRARY

OF THE

Graham was still living when Loschmidt published what Exner calls his epoch-making paper "On the Size of the Air Molecule." Although the numerical estimate which Loschmidt deduced from the mean free path of the molecules and their volume has now only an historical interest, it has exercised a profound influence on the development of molecular physics in demonstrating that in dealing with molecules we are dealing with masses of finite dimensions, and further, that these dimensions are by no means immeasurably small. The very manner in which Loschmidt stated his conclusions was well calculated to rivet attention. He showed that these magnitudes, small as they are, are yet comparable with those which can be reached by mechanical skill. The German optician Nobert has ruled lines on a glass plate so close together that it requires the most powerful microscope to observe the intervals between them; he has drawn, for example, as many as 4000 lines in the breadth of a millimeter —that is, about 112,000 lines to the inch. Now, if we assume with Maxwell that a cube whose side is the 4000th of a millimeter is the smallest volume observable at present, it would follow from Loschmidt's calculations that such a cube would contain from 60 to 100 millions of molecules of oxygen or nitrogen; and if we further assume that the molecules of organised bodies contain on an average 50 "elementary" atoms, it further follows that the smallest organised particle visible under the microscope contains about 2 million molecules of organic matter And as at least half of every living organism is made up of water, we arrive at the conclusion that the smallest living being visible under the microscope does not contain more than about a million organic molecules. I could have wished, had time permitted, to dwell a little upon the intensely interesting questions which such a conclusion at once raises. In the article "Atom" in the Encyclopædia Britannica, from which I have quoted, you will find Clerk Maxwell points out its relation to physiological theories, and especially to the

doctrine of Pangenesis. "Molecular science," says Maxwell, "forbids the physiologist from imagining that structural details of infinitely small dimensions can furnish an explanation of the infinite variety which exists in the properties and functions of the most minute organisms."

In the year following Graham's death Sir William Thomson still further developed the modes of molecular measurement, and from a variety of considerations, based upon the kinetic theory of gases, upon the thickness of the films of soap bubbles, and from the electrical contact between copper and zinc, he arrived at estimates which, although sensibly different from that of Loschmidt, are still commensurable with it. In a lecture at the Royal Institution, given about four years ago, he extends the lines of his argument and arrives at the conclusion that in any ordinary liquid, transparent solid, or seemingly opaque solid, the mean distance between the centres of contiguous molecules is less than the one five-millionth and greater than the one thousandmillionth of a centimeter; and in order to give us some conception of the degree of coarse-grainedness implied by this conclusion, he asks us to imagine a globe of water or glass, as large as a football, to be magnified up to the size of the earth, each constituent molecule being magnified in the same proportion. The magnified structure would be more coarse-grained than a heap of small shot, but probably less coarse-grained than a heap of footballs (Nature, 19th July 1883).

Here, I think, we may leave the subject, at all events for to-night. I am painfully conscious that I have left unsaid much that ought to have been said, and possibly said some things that might well have been left unsaid. But my main purpose will have been served if I have succeeded in indicating to you Graham's position as an atomist, and in showing you how his ideas respecting the constitution of matter have germinated, and, like the seed which fell upon good ground, have borne fruit an hundredfold.

IX

FRIEDRICH WÖHLER

A LECTURE DELIVERED AT THE ROYAL INSTITUTION, ALBEMARLE STREET, ON FRIDAY EVENING, 15TH FEBRUARY 1884.

IT seems fitting that these walls, which have vibrated in sympathy with that brilliant eulogy of Liebig, which Professor Hofmann pronounced some nine years ago, should hear something of him whose life-long association with Liebig has exercised an undying influence on the development of scientific thought. The names of Friedrich Wöhler and Justus Liebig will be linked together throughout all time. The work which they did in common marks an epoch in the history of chemistry. No truer indication of the singular strength and beauty of their relations could be given than is contained in a letter from Liebig to Wöhler, written on the last day of the year 1871. "I cannot let the year pass away," writes Liebig to Wöhler, "without giving thee one more sign of my existence, and again expressing my heartfelt wishes for thy welfare and the welfare of those that are dear to thee. We shall not for long be able to send each other New-Years' greetings, yet, when we are dead and mouldering, the ties which have united us in life will still hold us together in the memory of men as a not too frequent example of faithful workers who, without envy or jealousy, have zealously laboured in the same field, linked together in the closest friendship."