258 



NA TURE 



[August 26, 1909 



may be called my scientific life has been spent in Canada, 

 and "I owe much to this country for the unusual facilities 

 and opportunity for research so liberally provided by one 

 of her great Universities. Canada may well regard with 

 pride her Universities, which have made such liberal 

 provision for teaching and research in pure and applied 

 science. .'\s a physicist, I may be allowed to refer in 

 particular to the subject with which I am most intimately 

 connected. After seeing the splendid home for physical 

 science recently erected by the University of Toronto, and 

 the older but no less serviceable and admirably equipped 

 laboratories of McGill University, one cannot but feel that 

 Canada has recognised in a striking manner the great 

 value attaching to teaching and research in physical 

 science. In this, as in other branches of knowledge, 

 Canada has made notable contributions in the past, and 

 we may confidently anticipate that this is but an earnest 

 of what will be accomplished in the future. 



It is my intention to-day to say a few words upon the 

 present position of the atomic theory in physical science, 

 and to discuss briefly the various methods that have been 

 devised to determine the values of certain fundamental 

 atomic magnitudes. The present time seems very oppor- 

 tune for this purpose, for the rapid advance of physics 

 during the last decade has not only given us a much 

 clearer conception of the relation between electricity and 

 matter and of the constitution of the atom, but has pro- 

 vided us with experimental methods of attack undreamt 

 of a few years ago. hx. a time when, in the vision of the 

 physicist, the atmosphere is dim with flying fragments of 

 atoms, it may not be out of place to see how it has fared 

 with the atoms themselves, and to look carefully at the 

 atomic foundations on which the great superstructure of 

 modern science has been raised. Every physicist and 

 chemist cannot but be aware of the great part the atomic 

 hypothesis plays in science to-day. The idea that matter 

 consists of a great number of small discrete particles 

 forms practically the basis of the explanation of all proper- 

 ties of matter. As an indication of the importance of this 

 theory in the advance of science it is of interest to read 

 over the Reports of this .Association and to note how many 

 addresses, either wholly or in part, have been devoted to 

 a consideration of this subject. .\mongst numerous 

 examples I may instance the famous and oft-quoted lecture 

 of Maxwell on Molecules, at Bradford in 1873 ; the dis- 

 cussion of the Kinetic Theory of Gases by Lord Kelvin, 

 then Sir William Thomson, in Montreal in 1884; and the 

 Presidential .Address of .Sir .Arthur Riicker in 1901, which 

 will be recalled by many here to-day. 



It is. far from my intention to discuss, except with 

 extreme brevity, the gradual rise and development of the 

 atomic theory. From the point of view of modern science, 

 the atomic theory dates from the work of Dalton about 

 1805, who put it forward as an explanation of the com- 

 bination of elements in definite proportions. The simplicity 

 of this explanation of the facts of chemistry led to the 

 rapid adoption of the atomic theory as a very convenient 

 and valuable working hypothesis. By the labour of the 

 chemists matter was shown to be composed of a number 

 of elementary substances which could not be further de- 

 composed by laboratory agencies, and the relative weights 

 of the atoms of the elements were determined. On the 

 physical side, the mathematical development of the kinetic 

 or dynamical theory of gases by the labours of Clausius 

 and Clerk Maxwell enormously extended the utility of this 

 conception. It was shown that the properties of gases 

 could be satisfactorily explained on the assumption that 

 a gas consisted of a great assemblage of minute particles 

 or molecules in continuous agitation, colliding with each 

 other and with the walls of the containing vessel. 

 Between encounters the molecules travelled in straight 

 lines, and the free path of the molecules between collisions 

 was supposed to be large compared with the linear 

 dimensions of the molecules themselves. One cannot but 

 regard with admiration the remarkable success of this 

 statistical theory in explaining the general properties of 

 gases arid even predicting unexpected relations. The 

 strength and at the same time the limitations of the 

 theory lie in the fact that it does not involve anv definite 

 conception of the nature of the molecules themselves or 

 of the forces acting between them. The molecule, for 



NO. 20;8, VOL. 81] 



example, may be considered as a perfectly elastic sphere 

 or a Boscovitch centre of force, as Lord Kelvin preferred 

 to regard it, and yet on suitable assumptions the gas 

 would show the same general statistical properties. We 

 are consequently unable, without the aid of special sub- 

 sidiary hypotheses, to draw conclusions of value in regard 

 to the nature of the molecules themselves. 



Towards the close of the last century the ideas of the 

 atomic theory had impregnated a very large part of the 

 domain of physics and chemistry. The conception of atoms 

 became more and niore concrete. The atom in imagination 

 was endowed with size and shape, and unconsciously in 

 many cases with colour. The simplicity and utility of 

 atomic conceptions in explaining the most diverse pheno- 

 mena of physics and chemistry naturally tended to enhance 

 the importance of the theory in the eyes of the scientific 

 worker. There was a tendency to regard the atomic 

 theory as one of the established facts of nature, and not 

 as a useful working hypothesis for which it was e.xceed- 

 ingly difficult to obtain direct and convincing evidence. 

 There were not wanting scientific men and philosophers • 

 to point out the uncertain foundations of the theory on ' 

 which so much depended. Granting how useful molecular 

 ideas were for the explanation of experimental facts, what 

 evidence was there that the atoms were realities and not 

 the figments of the imagination? It must be confessed 

 that this lack of direct evidence did not in any way detract 

 from the strength of the belief of the great majority of 

 scientific men in the discreteness of matter. It was not 

 unnatural, however, that there should be a reaction in 

 some quarters against the domination of the atomic theory 

 in physics and in chemistry. A school of thought arose 

 that wished to do away with the atomic theory as the 

 basis of explanation of chemistry, and substitute as its 

 equivalent the law of combination in definite proportions. 

 This movement was assisted by the possibility of explain- 

 ing many chemical facts on the basis of thermodynamics 

 without the aid of any hypothesis as to the particular 

 structure of matter. Everyone recognises the great 

 importance of such general methods of explanation, but 

 the trouble is that few can think, or at any rate think 

 correctly, in terms of thermodynamics. The negation of 

 the atomic theory has not, and does not, help us to make 

 new discoveries. The great advantage of the atomic theory 

 is that it provides, so to speak, a tangible and concrete 

 idea of matter which serves at once for the explanation 

 of a multitude of facts and is of enormous aid as a work- 

 ing hypothesis. For the great majority of men of science 

 it is not sufficient to group together a number of facts on 

 general abstract principles. What is wanted is a concrete 

 idea, however crude it may be, of the mechanism of the 

 phenomena. This may be a weakness of the scientific 

 mind, but it is one that deserves our sympathetic con- 

 sideration. It represents an attitude of mind that appeals, 

 I think, very strongly to the .Anglo-Saxon temperament. 

 It has no doubt as its basis the underlying idea that the 

 facts of nature are ultimately explicable on genera! 

 dynamical principles, and that there must consequently 

 be some type of ni<'chanism capable of accounting for the 

 observed facts. 



It has been generally considered that a decisive proof 

 of the atomic structure of matter was in the nature of j 

 things impossible, and that the atomic theory must of 

 necessity remain an hypothesis unverifiable by direct 

 methods. Recent investigations have, however, disclosed 

 such new and powerful methods of attack that we may 

 well ask the question whether we do not now possess 

 more decisive evidence of its truth. 



Since molecules are invisible, it might appear, for ex- 

 ample, an impossible hope that an experiment could be 

 devised to show that the molecules of a fluid are in that 

 state of continuous agilatiori which the kinetic theory leads 

 us to suppose. In this connection I should like to direct 

 your attention for a short time to a most striking pheno- 

 menon known as the " Brownian movement," which has 

 been closely studied in recent years. Quite apart from its 

 probable explanation the phenomenon is of unusual 

 interest. In 1S27 the English botanist Brown observed 

 by means of a microscope that minute particles like spores 

 of plants introduced into a fluid were always in a state 

 of continuous irregular agitation, dancing to and fro in 



