Manchester Memoirs, Vol. xlvii. (1903). ^o.W. 7 



In order that a new thought shall be acceptable, 

 certain prerequisite conditions must be fulfilled. If the 

 ground is not prepared, the seed cannot be fruitful ; if 

 men are not ready, no harvest will be reaped. Only 

 when the time is ripe, only when long lines of evidence 

 have begun to converge, can a new theory command 

 attention. Dalton's opportunity came at the right 

 moment, and he knew how to use it well. Elements 

 had been defined ; the constancy of matter was estab- 

 lished ; pneumatic chemistry was well developed, and 

 great numbers of quantitative analyses awaited inter- 

 pretation. The foundations were ready for the master 

 builder, and Dalton was the man. His theory could at 

 once be tested by the accumulated data, and when that 

 had been done it was found to be worthy of acceptance. 



It is not my purpose to discuss in detail the processes 

 of Dalton's mind. The story is told in his own notebooks, 

 which have been given to the public by Roscoe and Harden,* 

 and it has been sufficiently discussed by others. We now 

 know that Dalton was thoroughly imbued with the 

 corpuscular ideas of Newton, and that, when studying the 

 diffusion of gases, he was led to the belief that the atoms 

 of different substances must be different in size. Upon 

 applying this hypothesis to chemical problems, he dis- 

 covered that these differences were in one sense measure- 

 able, and that to every element a single, definite, 

 combining number, the relative weight of its atom, could 

 be assigned. From this, the law of definite proportions 

 logically followed, for fractions of atoms were inadmissible ; 

 and the law of multiple proportions, which Dalton worked 

 out experimentally, completed the generalisation. The 



* " A new view of the origin of Dalton's Atomic Theory," etc. By Sir 

 Henry E. Roscoe and Arthur Harden. London, 1896. 



See also Debus, in Zeils. Pliysikal. Chem., Bd. 20, p. 359, and a 

 rejoinder by Roscoe and Harden in Bd. 22, p. 241. 



