PHILOSOPHICAL WRITINGS OF THE LATE DB. DALTON. 29 



Stances, however, which put an end to his mathematical 

 correspondence impelled him to make greater exertions in the 

 more interesting branches of experimental physics. He now 

 possessed greater facilities for conducting his researches ; and 

 by bringing his well-trained mathematical genius to bear upon 

 the results of his manipulations, he succeeded in penetrating 

 the secrets of nature, and in a few years changed the whole 

 aspect of chemical philosophy. Mr. Dalton was elected a mem- 

 ber of this society in 1794, and read his first essay on a pecu- 

 liarity of his own ** Vision of Colours," in October of the same 

 year. This paper was printed in the fifth volume of the first 

 series of the Memoirs, and constitutes the earliest record of what 

 is now technically termed " Daltonism," or "colour blindness," 

 by the medical profession. In a subsequent volume h6 discusses 

 with much acuteness the difficult problem of the diffusion of 

 gases ; and, by means of well-conducted experiments, succeeds 

 in developing a series of fundamental principles respecting 

 their specific gravities, which have not only been found of the 

 highest importance to practical chemists, but have formed the 

 basis of all subsequent investigations. His contributions to 

 the various scientific journals are, on the whole, tolerably 

 extensive, and many of them are of a very valuable character ; 

 but all of these must yield in importance to the ^* New 

 System of Chemical Philosophy" which contains his grand 

 discovery of the atomic theory. He had turned his attention 

 to this theory as early as 1803, and was enabled to convince 

 Dr. Thomson of its truth in 1804 :— by 1807 he had satisfied 

 both Wollaston and Davy that definite proportions constitute 

 of necessity the true composition of matter, and this opinion 

 had already become the prevailing doctrine of the continental 

 philosophers. " It was Newton," says a recent writer, " who 

 first put the conception of atoms into clear hypothetical con- 

 nexion with the phenomena of chemistry ; but it was Dalton 

 that imparted enlargement, vitality, and fertility to the perti- 

 nent and memorable thought of the astronomer-royal of the 



