of those who held a realist creed and believed in the existence of something—unknown in form perhaps, but still something-corresponding to our mental pictures in general, had doubts about the 'reality' of the individual atom. While they accepted the atoms as convenient conceptual entities which led to results in accordance with experience when from hypothetical atomic properties those of matter in bulk were deduced, they felt no confidence that atoms stood for more than a system of symbols which had hitherto led to consistent results on integration.

But now, by the genius of Prof. Rutherford on the one hand and of Mr C. T. R. Wilson on the other, we have two distinct methods of making visible the effect of one single atom of matter shot out of that amazing form of automatic gun, a speck of a salt of radium. Moreover, it has been shown that a third older method, due to Sir William Crookes, gives yet another means of detecting an atomic effect. In a very real sense these results form a consummation of the atomic theory. Rising from a correct, though vague and unverified guess of a Greek philosopher, whose chief merit lay in the perception that matter must be either continuous and homogeneous or discontinuous and atomic, through the useful and necessary working hypothesis of Dalton's chemistry based on definite experimental evidence, the atomic theory has stood the final test; and the atom has become as real as any other conception which corresponds to something that can affect our senses and be made appreciable to them.

At this stage, then, it seems appropriate to give a short summary of the history of the theory of atoms, and to trace the varying importance that has been allotted to it throughout the ages. For the atomic theory has had many uses. It has been the subject matter of philosophical dispute; it has served as a weapon in religious controversy; it has become an essential part in a theory of chemical combination applicable to industriál processes; and it is often put forward as a final statement of the essence of the underlying phenomena on which men have built up an explanation of the whole world of material manifestation. No one of these aspects is more fundamental than the other; and the exponent of any parti

cular phase has much to learn by keeping in mind the many-sided nature of the hypothesis.

We appear to owe the conception of an atom as well as the word by which it is expressed to the philosophers of Greece-the first men of science and conscious students of Nature known to history. The earliest Greek atomist seems to have been Leucippus, of whom little trace save tradition survives. His disciple Democritus is a less shadowy figure. Democritus was born at Abdera about 460 B.C., and there, after many wanderings, he returned to live and die. Our knowledge of his philosophy is derived chiefly from Aristotle's account of it-an unfavourable one-and from the poem of the Roman Lucretius, where, interwoven with the additions and modifications due to Epicurus, it is described in detail and given its philosophic import.

It is unfair and misleading to compare the atomic philosophy of the Greeks with the definite chemical theory developed by Dalton and Avogadro in the early years of the 19th century out of the physical speculations of Boyle and Newton. The modern theory arose naturally as an exact and limited working hypothesis, framed to explain or co-ordinate a number of facts obtained by the experimental method. The ground had been prepared for it by countless patient investigators; and at once it was seen to bring order and intelligibility into a host of disconnected phenomena. It was a scientific hypothesis, and had all the sharply-cut though narrowly circumscribed field of usefulness characteristic of its nature. It had little immediate philosophic bearing even of an indirect kind, and we are not aware that the sleepless vigilance of theologians detected in it any attack on religion. But the whole object and meaning of the theory of Leucippus and Democritus and Epicurus at the time when it was put forth was primarily and ultimately philosophic and religious. It was the first great attempt at a rationalist explanation of the Universe, the first recorded effort to trace mechanism instead of caprice in the workings of Nature, both in the physical and in the biological realm. It was an attack on the received dogmas of religion; an onslaught at once on the Homeric gods of Olympus, and on those formless, distressful

deities in the background, which had been developed by the southern aboriginal race out of the more primitive cults of magic and tabu before the invasion of the conquering northern Achaeans and Dorians had introduced their anthropomorphic and friendly gods born and bred in regions beyond the mountains.

The attempt to explain Nature in terms of known mechanical principles, with the object of doing away with the need of direct divine interposition, is, then, the task of the Greek atomists. Of course, from the philosophic point of view, ancient or modern, there remains the ultimate insufficiency of all physical explanation. Such explanation is but the expression of one unknown in terms of another, which seems understood merely because it is so familiar that the mystery is no longer consciously apprehended. But then, as always, the less clear-sighted champions of orthodoxy took up positions which were unnecessarily exposed to attack, and denounced Democritus and Epicurus, in a way that must have assured those philosophers that the success of their endeavours to undermine the traditional beliefs was only a matter of time and opportunity. But here and now we are concerned less with the ultimate religious and philosophic import of the atomic theory than with the details of the earliest speculation about the structure of matter. If matter be infinitely subdivisible, argued the atomists, subdivisible into an unending series of smaller and yet smaller particles, all of the same nature, and with the same properties as matter in bulk, no explanation of the immediate properties of the various kinds of matter as revealed by our senses is possible; we have to accept these properties as fundamental and ultimate incomprehensibles. But if, in a process of subdivision, we come at one point to structures which can be subdivided no further-individual structures the properties of which are not those of matter in bulk-we drive back the incomprehensible at least one step. We may hope to explain the immediately perceptible properties of matter in terms of the interactions of those ultimate atoms.

Moreover, since movement is possible, since division. can take place, there must exist empty space or void. If all nature were full, nothing could move or separate, because nothing could give way to make room for any

thing else. Thus between the ultimate particles there is void; and atoms and void are the essence of things. 'According to convention,' says Democritus, 'there are a sweet and a bitter, and according to convention there is colour; in truth, there are atoms and a void.' The atoms are imagined as eternal, unchangeable, 'strong in solid singleness,' far smaller than the senses can perceive. They are in perpetual motion through the void, and link together to produce more complex structures, forming the different elements of which bodies are composed. 'Bodies are partly first-beginnings of things, partly those which are formed of a union of first-beginnings.' Thus, by varieties of form, arrangement and position of the atoms, the different properties of different kinds of matter may be explained.

This scheme, it will be seen, is a true scientific hypothesis; but experimental and observational knowledge had hardly begun, and it was impossible to test the hypothesis by deducing the properties of different bodies and comparing the deduction with observation. Nor was there any means of verifying the existence of that void in which the atomic properties were to manifest themselves. Even the gaseous condition, wherein the disengaged molecules begin to show some of their typical effects, remained without physical meaning for another two thousand years. A few vague guesses, it is true, were made, such as that things which look hard and dense must consist of particles more hooked together and be held in union because compacted throughout with branchlike elements... [while] those things which are liquid and of fluid body ought to consist more of smooth and round elements.' But beyond this nothing could be done; and the promising hypothesis, with no ground of experience in which to take root, and no need of immediate verification to direct the path of progress, disappeared when Greek philosophy was still in its prime, and became a mere lifeless speculation, useful only as a specimen in a museum of conceptual curiosities, where it stood ready on the shelves to suggest one of the more fruitful ideas of modern experimental science.

Aristotle denied the existence of void on the grounds that it was inconceivable to the human mind, and with it rejected the remainder of the tenets of the Greek atomists.

He held matter to be infinitely divisible into parts similar to itself, and-probably with a reminiscence of the eastern doctrines of dualism-sought to explain its perceptible differences by the combinations of pairs of the qualities, such as hot and cold, wet and dry, which, when added to matter, gave rise to the four elements, air, earth, fire and water. Aristotle's authority was enough to overthrow the atomic theory, and, save for its glorification in the poem of Lucretius, it almost vanished for two thousand years.

After the loss of ancient learning during the Dark Ages, the first task of later medievalism and of the Renaissance was the recovery of Greek literature and philosophy. It is part of the irony of history that the dawn of modern science saw in Aristotle the supreme achievement of ancient knowledge; men went to him instead of to Nature for their facts, and accepted his views instead of making experiments and of framing hypotheses of their own. Hence we may readily forgive the blind fury felt for Aristotle and all his works by such men as Bruno and Paracelsus, as well as the more measured denunciation of Lord Chancellor Bacon, in all of whom the new spirit of critical experiment was fermenting.

References to the atomic philosophy of Democritus and Epicurus are frequent in the writings of Bacon, who saw in it a promising attempt to explain the ultimate structure of matter; and it was accepted by Pierre Gassendi (1592–1655), the eminent French philosopher, who did much to undermine the authority of Aristotle and to make acceptable the astronomical teaching of Copernicus and Kepler. But it is in the writings of Robert Boyle (1627–1691) that we first meet an example of the modern method. Boyle defines elements in true empirical fashion as those which are found to be 'primitive and simple or perfectly unmingled bodies,' and regards 'perfectly mixed' bodies as being compounded of elements. The number of elements and of compounds is simply a matter of experiment. The deeper problem of the constitutions of the elements, of the ultimate structure of matter itself, Boyle recognises to be a question of speculation only in the then state of knowledge. He is willing to