340 BLACK. 



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 177'2 ; 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 con- 

 dition of the science was in the same proportion 

 humble. 



The celebrated ' Preliminary Discourse' to the ' En- 

 cyclopedic' makes hardly any mention of chemistry 

 among the sciences; and in the ' Arbre Encyclopedique,' 

 on which the authors (D'Alembert and Diderot) plume 

 themselves much, we find it not very distinctly repre- 

 sented, or in very good company. It is termed the 

 science of interior and occult qualities of bodies, its 



