14 BLACK. 



because the capacity of water for heat is nineteen times 

 that of gold. So twice as much heat is required to 

 raise water to any given point of sensible heat as to 

 raise mercury, the volumes of the two fluids compared 

 being equal. 



The true doctrine of combustion, calcination of 

 metals, and respiration of animals, which Lavoisier 

 deduced from the experiments of Priestley and Scheele 

 upon oxygen gas, and of Cavendish on hydrogen gas, 

 and which has changed the whole aspect of chemical 

 science, was founded mainly upon the doctrines of 

 latent and specific heat. It was thus the singular fe- 

 licity of Black to have furnished both the pillars 

 upon which modern chemistry reposes, and to have 

 furnished them so long before any one attempted to 

 erect the superstructure, that no doubt could by any 

 possibility arise respecting the source of our increased 

 knowledge, the quarter to which our gratitude should 

 be directed. Fixed air was discovered in 1752, and 

 fully explained to the world in 1754 and 1755. 

 Latent heat was yearly, from 1763, explained to nu- 

 merous classes of students, before whom the experi- 

 ments that prove it were performed by the author's 

 own hands. Cavendish made his experiments on in- 

 flammable air in 1766; Priestley began his in 1768, 

 first publishing in 1772 ; and he discovered oxygen in 

 1774, in which year the nature of combustion was first 

 explained by Lavoisier, a boy at school when fixed air 

 was discovered, and having made no experiments nor 

 written any one line upon chemical subjects for seven 

 years after latent heat was discovered. 



But we shall form a more striking idea to ourselves 

 of the revolution which Black thus effected in che- 

 mistry, if we attend a little to the state of that science 

 in general before he began his labours. We have 

 already seen the low condition of the knowledge then 

 possessed respecting aeriform fluids ; the general condi- 

 tion of the science was in the same proportion humble. 



