232 MEMOIR OF DE. DALTON, AND 



we find no difficulty, either physical or metaphysical. He 

 persuades us to go, leads us and describes all to us in a few 

 sentences, when volumes of persuasion written before had not 

 been sufficient to induce men to turn their eyes. His descrip- 

 tions are rigid as well as picturesque. Some persons would 

 apply to them the word material, and still more the word 

 mechanical. It was by following rigidly the mechanical pro- 

 perties of his atoms that he arrived at his results. To those 

 who read his works, it will be clear that his mind became 

 gradually more confirmed in this course. 



In reading what he says, at p. 49, we see him plainly verging 

 towards his theory, and also the nature of his struggle, which 

 is in no respect similar to that of any other inquirer. He there 

 first says, that he is inquiring into the relative weights of the 

 ultimate particles of bodies. This idea had never suggested 

 itself as practical to any one before Dalton, nor am I aware 

 that it has ever been claimed. Having made some advance 

 in this inquiry, he made it the starting point of all that he 

 advanced on atomic chemistry and the theory of proportion. 



He was not in haste to publish his theory, but told it openly 

 to Dr. Thomson, in 1804; this, then, is the date of the com- 

 plete discovery, as Dr. Thomson published an abstract of 

 it at once. Some persons unacquainted with this have advanced 

 an argument against him, his own book containing the sub- 

 ject not having been published till 1808. A well known 

 quotation from Dr. Thomson's history says, " Mr. Dalton 

 informed me that the atomic theory first occurred to him 

 during his investigations of olefiant gas and carburetted 

 hydrogen gas, at that time imperfectly understood, and the 

 constitution of which was first fully developed by Mr. Dalton 

 himself. It was obvious from the experiments which he made 

 upon them, that the constituents of both were carbon and 

 hydrogen, and nothing else. He found, further, that if we 

 reckon the carbon in each the same, then carburetted hydrogen 

 contains exactly twice as much hydrogen as olefiant gas does. 



