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SCIENCE 



[N. S. Vol. XXX. No. 766 



discuss briefly the various methods that 

 have been devised to determine the values 

 of certain fundamental atomic magnitudes. 

 The present time seems very opportune 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 

 provided us with experimental methods of 

 attack undreamt of a few years ago. At a 

 time when, in the vision of the physicist, 

 the atmosphere is dim with flying frag- 

 ments 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 can 

 not 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 properties of matter. As an indication 

 of the importance of this theory in the ad- 

 vance 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 considera- 

 tion of this subject. Amongst numerous 

 examples I may instance the famous and 

 oft-quoted lecture of Maxwell on "Mole- 

 cules, ' ' at Bradford in 1873 ; the discussion 

 of the "Kinetic Theory of Gases" by Lord 

 Kelvin, then Sir "William Thomson, in 

 Montreal in 1884; and the presidential 

 address of Sir Arthur Eucker 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 combination of ele- 



ments in definite proportions. The sim- 

 plicity 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 labor 

 of the chemists matter was shown to be 

 composed of a number of elementary sub- 

 stances which could not be further decom- 

 posed by laboratory agencies, and the rela- 

 tive weights of the atoms of the elements 

 were determined. On the physical side, 

 the mathematical development of the kin- 

 etic or dynamical theory of gases by the 

 labors of Clausius and Clerk Maxwell enor- 

 mously extended the utility of this concep- 

 tion. 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 traveled in straight lines, and the 

 free path of the molecules between col- 

 lisions was supposed to be large compared 

 with the linear dimensions of the molecules 

 themselves. One can not but regard with 

 admiration the remarkable success of this 

 statistical theory in explaining the general 

 properties of gases and 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 any 

 definite conception of the nature of the 

 molecules themselves or of the forces acting 

 between them. The molecule, for example, 

 may be considered as a perfectly elastic 

 sphere or a Boscovitch center 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 subsidiary hypotheses, to draw con- 

 clusions of value in regard to the nature of 

 the molecules themselves. 



Towards the close of the last century the 



