Manchester Memoirs, Vol. Iv. (1910), No. 3. 9 



notes for it still exist in his own handwriting, and were 

 found, along with his notebooks, in the rooms of the Man- 

 chester Literary and Philosophical Society. He begins 

 by discussing his physical atomic theory, which aimed at 

 explaining the diffusion of gases. He entertained two 

 diffusion hypotheses, the first of which originated in 1801, 

 while an amended hypothesis, he says, was formed in the 

 year 1805. He had not at first "contemplated the effect 

 oi difference of size in the particles of elastic fluids." On 

 consideration, he " found that the sizes must be different," 

 and subsequently arrived at a different explanation of the 

 mechanism of diffusion from the one he at first suggested. 



He then introduces the subject of the chemical atomic 

 theory : — " The different sizes of the particles of elastic 

 fluids under like circumstances of temperature and pres- 

 sure being once established, it became an object to deter- 

 mine the relative sizes and zveigkts, together with the 

 relative ?z«w<^^r of atoms in a given volume. This led the 

 way to the combination of gases . . . other bodies 

 besides elastic fluids, namely, liquids and solids, were 

 subject to investigation, in consequence of their combining 

 with elastic fluids. Thus a train of investigation was laid 

 for determining the number and weight of all chemical 

 elementary principles which enter into any sort of com- 

 bination one with another." ■" 



This narrative is certamly right on a vital matter. It 

 recognises that Dalton had been using a physical atomic 

 theory, from which he passed to a chemical one. Here 

 there is a common ground of objection to the com- 

 munications made by Dalton to Thomson and Henry 

 respectively. They both ignore the connection, which 

 certainly existed, between the physical and chemical 

 theories. Thomson did not feel this defect, but Henry 



■-" Roscoe and Harden, Op. cit., pp. i6 — 17, 



