LAVOISIER. 249 



these great men, then, Priestley made the discovery in 



1774, Scheele in 1775, Lavoisier neither in 1774 nor in 



1775, nor ever except by receiving the information from 

 " the true and first discoverer thereof, which, at the time, 

 others did not use."""" 



There can be no doubt whatever that it was the dis- 

 covery of oxygen gas which suggested to M. Lavoisier 

 his theory of combustion. He had previously made the 

 important step of explaining the calcination of metals, 

 so far, at least, as shewing that it was the union of the 

 metals with air absorbed, though he was wholly mis- 

 taken as to the air which they gave out on reduction, 

 and had a most imperfect notion of the change which 

 their calcination produced on the air in which the process 

 took place ; but now he was enabled, by Dr. Priestley's 

 discovery, to shew that the air absorbed is oxygen gas; 

 while Dr. Black's great doctrine of heat, which he also 

 called to his assistance, enabled him to perceive that the 

 gas, on becoming fixed, parted with its latent heat, and 

 assumed a solid form. A felicitous idea of Macquer's, 

 which M. Lavoisier cites, ('Mem./ 1777, p. 572,) that 

 calcination is only a slow combustion, may have given 

 rise to his theory of this operation ; but he had also, in 

 his experiments on phosphorus and sulphur, shewn the 

 absorption of oxygen by those bodies in burning ; and as 

 the doctrine of Dr. Black shewed how much heat was 

 evolved on a gaseous body becoming fixed and solid, we 

 may suppose that these experiments, which he laid before 

 the Academy in the spring of I777f, led him to his 



* Words of our Patent Act, 21 James I. 



f In his Memoir on Phlogiston in the volume for 1783, he speaks 

 of his theory of combustion as having been "published in 1777." If 



