334 On the Discovery of the Atomic Theory. [May, 



a little too far. The science of chemistry Lad not advanced far 

 enough, and too itv/ accurate analyses were known, to admit any 

 attempt to determine the weight of atoms. Ihis was the case also 

 at the lime that Mr. Higgins wrote ; and I am not aware of any 

 attempt in his book to determine the weiglit of a single atom : but 

 Cullen, Black, and Bergman, h id made some steps towards the 

 atomic theory, and these by no means unimportant ones. 



The chemical works of Dr. Cullen niiglit soon be looked over j 

 for I am not aware of any thing vvKicii he wrote on the subject 

 except a few pages on the cold produced by the evaporation of 

 ether. My kliowledge of his opinions was derived from the late 

 Professor Robison_. of Edinburgh, who had the means of informa- 

 tion, and, as he was a particular friend and great admirer of Dr. 

 Black, was entitled to credit. Now he informed me that Dr. 

 Black's explanation of double decompositions, which he annually 

 gave in his Class, had been originally broached by Dr. Cullen. 

 This was the circumstance that induced me to introduce Dr. 

 Cullen's name along with Black and Bergman. There is one man 

 still alive who is probably acquainted with Dr. Cullen's chemical 

 lectures, and with tlie materials which they furnished to Dr. Black j 

 I mean Mr. Watt, of Birmingham. 



As to Dr. Black, I consider myself as acquainted with his opi- 

 nions, because 1 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 

 Professor Robison that he employed these diagrams from the very 

 beginning of his career as a Professor. One of them is given in 

 page 541, vol. i. of the printed edition of Dr. Black's lectures. 1 

 have no doubt that all similar diagrams published in London by 

 Fordyce, &c. were derived fioni tins original source. Nay, I even 

 su-pect that the diagrams of Mr. Higgins himself might be traced 

 to tiie same origin. Now could the doctrine of definite proportions 

 be taught, and could double decomposition be 

 explained, in this way (for 1 quote Dr. Black's 

 explanation) ? Let the bodies A and B be 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 

 these bodies A will unite with C, and B with D — 

 I say could these opinions and explanations be 

 given by any one who did not believe the atomic c D 



theory, at least to a certain extent? To me 

 they conveyed just as much of the atomic theory as the perusal of 

 Mr. Higgins'sbook did. 



As to Bergman, the perusal of his Treatise on Electric Attrac- 

 tions, particularly his explanation of the apparent anomalies, his 



