HISTORY OP THE ATOMIC THEORY. 145 



These lectures shew him to have been an exceedingly clear 

 and rational expounder of science. With good common sense 

 he waits for more knowledge when science fails, fully shewing 

 why he became famous, although he published very little. 

 Reasoning on the state of things at the time, he sayg, " It 

 appears, then, that we know of no physical element, nor any 

 chemical principle, nor are we acquainted with any body which 

 has fixed and permanent qualities." 



He afterwards adds, " Having laid down and demonstrated 

 this fundamental proposition, viz., that the changes of the 

 qualities of bodies are all of them produced by combination 

 or separation, I now proceed to inform you that combination 

 depends upon attraction^ that is, the attraction of cohesion, 

 whereby the small particles of bodies very near each other are 

 disposed to approach, and in a certain contiguity to remain 

 coherent together." 



He then goes on to explain simple elective attraction and 

 double elective attraction by diagrams, like those below, 

 where the lines ought to be drawn straight from C to B, and 

 from A to D. This appears to be earlier than Bergman, who 

 at that time had published nothing on chemistry. I can find 

 no internal evidence of their being written later than they 

 profess to be, the binding itself being old. 



Dr. Cullen was professor of chemistry at Glasgow, and 

 Dr. Black attended his lectures, before being appointed his 

 successor, on the removal of Dr. Cullen to Edinburgh, in 

 1756. In the Annals of Philosophy, Vol. III., p. 554, Dr. 

 Thomson says : — " My knowledge of Dr. Cullen's opinions 

 was derived from the late Professor Robison, of Edinburgh, 

 who had the means of information, and, as he was a particular 

 friend and great admirer of Black, is entitled to credit. Now, 

 he informed me that Dr. Black's explanation of double de- 

 compositions, which he annually gave in his class, had been 

 originally broached by Dr. Cullen. This was the circum- 

 U 



