Seftembeb 3, 1909] 



SCIENCE 



291 



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 more concrete. The atom 

 in imagination was endowed with size and 

 shape, and unconsciously in many cases 

 with color. The simplicity and utility of 

 atomic conceptions in explaining the most 

 diverse phenomena of physics and chem- 

 istry naturally tended to enhance the im- 

 portance 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 exceedingly 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 un- 

 natural, however, that there should be a 

 reaction in some quarters against the dom- 

 ination 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 chem- 

 istry, and substitute as its equivalent the 

 law of combination in definite proportions. 

 This movement was assisted by the possi- 

 bility of explaining many chemical facts on 

 the basis of thermodynamics without the 

 aid of any hypothesis as to the particular 

 structure of matter. Every one recognizes 

 the great importance of such general meth- 

 ods of explanation, but the trouble is that 

 few can think, or at any rate think cor- 

 rectly, 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 

 scientists it is not sufficient to group to- 

 gether a number of facts on general ab- 

 stract 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 

 consideration. 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 imderlying idea 

 that the facts of nature are ultimately ex- 

 plicable on general dynamical principles, 

 and that there must consequently be some 

 type of mechanism 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 things impos- 

 sible, and that the atomic theory must of 

 necessity remain a hj^jothesis 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 example, an impossible hope 

 that an experiment could be devised to 

 show that the molecules of a fluid are in 

 that state of continuous agitation which the 

 kinetic theory leads us to suppose. In this 

 connection I should like to draw your at- 

 tention for a short time to a most striking 

 phenomenon known as the "Brownian 

 movement," which has been closely studied 

 in recent years. Quite apart from its 



