12 MeldrUM, DevelopDient of the Atomic Theory. 



particulars and instances, Dalton does not explain, nor 

 is it obvious that anyone can explain, how he was to 

 test the sizes of atoms without some kind of chemical 

 theory. One may either assume that different atoms 

 have the same size, and act accordingly, or one can 

 endeavour to test the position, by obtaining data regard- 

 ing atoms, on the basis of some hypothesis as to the way 

 in which they combine chemically. 



It has been shown in this paper that Dalton, so far as 

 the formation of the chemical theory is concerned, did 

 not act on the belief that atoms of different kinds have 

 the same size. Again, the author has already shown, in 

 the paper on Dalton's physical atomic theory, that the 

 chemical theory was formed first and the conclusion that 

 "atoms" of different gases were different in size was 

 come to afterwards. 



This is the order that was to be expected in the 

 nature of the case. Moreover, there is nothing in the 

 note-books to show that the chemical theory was devised 

 except for its own sake. The testing of the sizes of 

 atoms was an afterthought. The connection between the 

 sizes of atoms and the diffusion of gases was not con- 

 sidered till a year after the chemical theory had been 

 formed. 



The cxperiuieiits of August ^th, i8oj. 



The chief matter that continues to be doubtful is the 

 exact way in which Dalton arrived at the law of multiple 

 proportion. The author, after a careful consideration of 

 the evidence, can come to no other conclusion than that 

 it was Dalton's experiments on the combination of nitric 

 oxide and oxygen that aroused his attention, and made 

 him apply his physical theory to the purposes of chemistry. 



