Manchester Mcuioirs, Vol. h. (191 1), No. 0. 5 



I became convinced that different gases have not their 

 particles of the same size." ' 



(2) Debus argues, from a phrase in Thomas Thomson's 

 first sketch of the atomic theory, that Dalton was still in 

 1804 a believer in the hypothesis. This is the phrase, 

 ^' the density of the atoms." 



The ''density of the atoms'^ 



The interpretation of this phrase is open to questionj 

 and Roscoe and Harden do not agree with Debus on the 

 matter. But neither they nor any of the parties to the 

 controversy seem to be aware that Dalton put exactly 

 the same construction on the phrase as Debus, and at the 

 same time repudiated the opinion which it attributed to 

 him. " It is rather amusing to me to observe the different 

 manners in which a cursory view of the atomic system 

 strikes different observers. Dr. Thomson . . , used the 

 phrase density of the atoms indifferently for iveight of the 

 nionis, thereby implying that all atoms are of the same 

 size, and differ only in density ; but he has since very 

 properly discontinued the use of the phrase."^ 



It is, of course, impossible that a statement, made 

 by Dalton in 18 14, can be taken to prove that he did not 

 use a misleading expression in a conversation held ten 

 years earlier. He may have used the phrase in question 

 in his interview with Thomson, or Thomson may have 

 originated the phrase. These are the two possibilities. 

 But the matter is not one of high importance. There are 

 far stronger arguments than this statement of the year 

 1 8 14 can be, against the opinion held by Debus. 



" "New System of Chemical I'hilo.soijhy," iSoS, pp. 187-1S8. 

 * Ann. of Phil., vol. 3, p. 175, 1814. 



