146 MEMOIR OF mi. DALTON, AND. 



Stance that induced me to broach Dr. Cullen's name along 

 with Black and Bergman. 



n* * ♦ As to Dr. Black, I consider myself as acquainted 

 with his opinions, because I attended his lectures ; and there 

 are thousands in Great Britain who did the same, and who 

 cannot but recollect the facts that I shall state. Dr. Black 

 taught that bodies combine in definite proportions, and he 

 explained double decomposition by means of diagrams, not, 

 indeed, the same as those of Mr. Higgins, but much simpler 

 and more elegant. I have been informed by Prof. Robison, 

 that he employed these diagrams from the very beginning of 

 his career, as a professor. One of them is given in page 554, 

 Vol. I., of his printed lectures. I have no doubt that all 

 similar diagrams, published in London, by Fordyce, &c., 

 were derived from the same source. Now, could the doc- 

 trine of definite proportions be taught, and could double de- 

 composition be explained in this way (I quote Dr. a b 

 Black's explanation), let the bodies A and B be O 10 

 united with a force, 10 ; and the bodies C and D 

 with a force, 6. Suppose the attraction of A for 

 C to be 8, and that of B for D to be 9, if we mix # 6 O 

 these bodies, A will unite with C, and B with D. c d 

 To me they conveyed just as much of the atomic theory as 

 the perusal of Mr. Higgin's book did." 



Dr. Robison edited the lectures of Black in 1803, and in a 

 note gives the above diagram and some judicious remarks, 

 shewing, at the same time, that although definite proportion 

 "was taken for granted, no general law to account for it had 

 been given. 



But the question cannot be as to whether Dr. Cullen 

 discovered the atomic theory, (indeed, this extract might 

 have been brought on somewhat later), but whether Dr. 

 Cullen had so far advanced our knowledge of matter as to be 

 the first who gave out the ideas of single and double elective 

 attractions, such as have been attributed to Bergman. 



