SOME ASPECTS OF THE ATOMIC THEORY 



By Prof. FREDERICK SODDY, M.A., F.R.S., University, Aberdeen. 



Either matter must occupy space continuously or it must exist 

 in the form of discrete particles. The historical origin of the 

 Atomic Theory of matter is to be found in the choice between 

 the two possible answers to these mutually exclusive alternatives. 

 But this is little more its real origin than is Prout's celebrated 

 guess that all elements are built up of hydrogen the real origin 

 of our present far-reaching conclusions as to the structure of 

 atoms. The true origin of the Atomic Theory is recognised 

 universally to have been during the first decade of last century 

 in Dalton's discovery of the simple laws of chemical combination, 

 though, even to the discoverer himself, the laws of gaseous 

 behaviour, upon which later the totally distinct but inextricably 

 interwoven Molecular Theory was to be based, undoubtedly 

 played a part in directing the interpretation he put upon 

 these laws. 



Henceforth science was to deal no longer with atoms as the 

 end results of a purely mental process of the subdivision of 

 matter, a process which must of necessity have an end if matter 

 does not occupy space continuously, but with atoms of definite 

 mass, determinable simply and exactly relatively, that is, the 

 mass of any one kind of atom in terms of that of any other. 

 Absolutely, of course, these masses have only been precisely 

 determined in the present century. In the words of Sir J. J. 

 Thomson in his recent Romanes Lecture, " Dalton traced the 

 atoms of the different elements in all their migrations from one 

 compound to another by means of their weight ; this was a 

 quality they could neither change nor disguise ; until quite 

 recently, however, this was about the only quality of the atom 

 of which this could be said." And, it may be added, it was the 

 difference of the weights of atoms of different elements which 

 made the test of value. The long and now admittedly mistaken 

 efforts on the part of what may be termed the thermodynamical 

 school of physical chemists, especially in Germany, to dispense 

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