Manchester Memoirs, Vol. iv. ( 1 9 1 1 ), No. 0. 1 7 



they agree with the laws of definite and multiple pro- 

 portions. 



In the second place Dalton was at an advantage over 

 other workers, in having a theory to which he could refer 

 facts. Something more is needed than important facts, 

 one must have the eye to perceive their importance. 

 Charles Darwin gives an illustration of this when he 

 admits he once walked along a valley, full of the plainest 

 indications of glacial action which he absolutely failed to 

 notice. " On this tour I had a striking instance how easy 

 it is to overlook phenomena, however conspicuous, before 

 they have been observed by anyone. We spent many 

 hours in Cwm Idwal, examining all the rocks with extreme 

 care, as Sidgwick was anxious to find fossils in them ; but 

 neither of us saw a trace of the wonderful glacial pheno- 

 mena all around us ; we did not notice the plainly scored 

 rocks, the perched boulders, the lateral and terminal 

 moraines. Yet these phenomena are so conspicuous that, 

 as I declared many years afterwards — a house burnt 

 down by fire did not tell its story more plainly than did 

 this valley." "' 



This is not a fanciful argument, but one that can be 

 amply justified by facts. Chemists did not go on making 

 analyses conscientiously without sometimes obtaining 

 data in good agreement with the law of multiple propor- 

 tion. But they quite failed to perceive the significance of 

 the data. Dalton himself was able afterwards triumph- 

 antly to point out more than one such case, which had 

 escaped the notice of the chemist concerned. He quotes 

 Bostock's analyses of the acetate and superacetate of 

 lead :—"< 



^" "Life and Letters of Charles Darwin," 3 vols., 1887, vol. i, p. 57. 

 '■'' Nicholson'' s /our., vol. 11, p. 75, 1805 ; vol. 29, p. 150, iSii. 



