Manchester Mevioirs, Vol. h. {\gi i), No. 5. 17 



September 14th, 1804, and this is the approximate date of 

 the amended diffusion theory. 



Dalton probably influenced by Thomson and Gotigh. 



Up to this time Dalton had given no sign of anything 

 but the fullest confidence in his original theory. He had 

 defended it eagerly, and, as late as June, had been 

 encouraged in his belief by the accession of William 

 Henry to his side. What then could have induced 

 Dalton, not a very impressionable man, to reconsider the 

 matter ? 



It may be assumed, in the absence of any positive 

 information on the subject, that the change was due 

 partly to Thomas Thomson and partly to John Gough. 

 As has already been mentioned more than once in these 

 papers, Thomson visited Manchester with the express 

 object of discussing the mixed gases theory with Dalton. 

 Now everything goes to show that Thomson made a 

 considerable impression on him and won his confidence. 

 He explained the chemical theory to Thomson in 

 detail, and afterwards mentioned Thomson's opinions 

 regarding mixed gases, although adverse to his own, with 

 the utmost respect.-^ Consequently one can well believe 

 that Thomson's scepticism regarding the original mixed 

 gases theory began to shake his confidence in it. Again, 

 John Gough had written two letters, which appeared in 

 Nicholson's Journal^ criticising the theory. The criticism 

 was effective, for Dalton, although he continued to main- 

 tain his theory, made no answer at the time to Gough's 

 argument regarding the velocity of sound. Gough's letters 

 are dated July i6th and August 23rd, 1804, respectively. 

 The interview between Dalton and Thomson occurred on 

 ■-" "New System of Chemical Philosophy," 1808, p. 72. 



