8 LarmOR, Physical Aspect of the Atomic Theory. 



that the apparently continuous substances around us are 

 divisible without limit, and afterwards to wonder what 

 sort of evidence it is that has suggested the contrary 

 conclusion. Such evidence must have an essentially 

 chemical flavour : its gist has been strikingly expressed 

 by Newton in the argument of which the conclusion has 

 been quoted. It is true that the mathematical physicists 

 of the beginning of the nineteenth century were 

 accustomed to conduct their investigations in terms of 

 atoms or particles of bodies : but for their purposes, these 

 terms were hardly much more than the embodiment of 

 the fact that exact reasoning requires numerical expression, 

 and therefore, the mathematical resolution of the media 

 with which it is concerned into infinitesimal geometrical 

 parts.* On a rather higher plane must however be placed 

 the (subsequent) electric atoms of W. Weber, which may be 

 held to have been somewhat unduly discredited by the 

 destructive criticism of Helmholtz. 



The Daltonian Atoms. 



Early in the nineteenth century the time had come 

 for the translation of these dim physical perceptions into 

 secure experimental knowledge. Perhaps the new feature 



*An exception must be made at any rate in the case of Young, as will 

 appear in connexion with optical dispersion, and, as Lord Rayleigh has 

 remarked, in connexion with capillarity. In a letter to Arago (Jan. 12, i8i7)> 

 he reports : — " I have been reconsidering the theory of capillary attraction 

 and have at last fully satisfied myself with respect to the fundamental 

 demonstration of the general law of superficial contraction, which I have 

 deduced in a manner at once simple and conclusive from the action of a 

 cohesive force extending to a considerable number of particles within -a given 

 invisible distance. This solution has very unexpectedly led me to form an 

 estimate, something more than merely conjectural, though not fully 

 demonstrative, of the magnitude of the ultimate atoms of bodies ; in water 

 for instance, about " io~*cm. in diameter [modern estimates being nearer 

 lO-^cm.]. Young's "Works," ed. Peacock, vol. i., p. 382 ; also article 

 *' Cohesion," loc. cit. p. 462. 



