2 Meldrum, Development of the Atomic Theory. 



the essential homogeneity of each pure substance, that it 

 is composed of molecules of only one type, absolutely 

 alike. Once it is postulated that only one kind of aggre- 

 gation into molecules occurs, e.g., that in water there is 

 only one way in which the hydrogen attaches itself to the 

 oxygen, the laws of definite and multiple proportions are 

 self-evident." ^ 



Undoubtedly this principle, " a definite molecule for 

 each substance," is common to the various systems of 

 chemistry of the nineteenth century. Yet the principle 

 was not necessarily advanced first by Dalton. I have 

 already shown (in the third paper of this series) that 

 William Higgins expounded a definite chemical atomic 

 theory in a book which he published in the year 1789. 

 Further, the words, a "definite molecule for each substance," 

 give, as will presently appear, an unexceptionable state- 

 ment of the theory contained in Higgins' book. 



The two theories, Higgins' and Dalton's, led their 

 authors, in a remarkable degree, to the same results. 

 This is proved by the following table, the formulae in 

 which reveal at once the ideas which Higgins and Dalton 

 had regarding the molecules of the substances in question : 



The great similarity between these results is to be 

 explained in only one way. The two theories have in 

 common, as a guiding principle, the rule that atoms of 



' Manchestei' Mefiioirs, 1908, $2, No. 10, p. 9. 



