BLACK. 335 



that the article on 'Magnesia,' published in 1765, dog- 

 matically asserts Black to be in error when he de- 

 scribes Epsom salts as yielding that earth, " because," 

 says the author, " those salts are purely Seidlitian," 

 "entierement Seidlitiens" (vol. x. p. 858). In fact, Ep- 

 som salts, magnesia, limestone, and sea-water are the 

 great sources from which all magnesia is obtained. 

 The first of these substances is in truth only a com- 

 bination of magnesia with sulphuric acid. 



The other discoveries to which Black's led were as 

 slowly disseminated as his own. Oxygen gas had been 

 discovered, in August, 1774, by Priestley, and soon 

 after by Scheele without any knowledge of Priestley's 

 previous discovery ; yet in 1777 Morveau, who wrote the 

 chemical articles in the ' Supplement,' never mentions 

 that discovery, nor the almost equally important dis- 

 covery of Scheele, chlorine, made in 1774, nor that of 

 azote, discovered .by Rutherford in 1772, nor hydro- 

 gen gas, the properties of which had been fully inves- 

 tigated by Cavendish as early as 1766. Lavoisier's 

 important doctrine, well entitled to be called a dis- 

 covery, of the true nature of combustion, had likewise 

 been published in 1774 in his ' Opuscules,' yet Mor- 

 veau doggedly adheres to his own absurd theory of 

 the air only being necessary to maintain those oscilla- 

 tions in which he holds combustion to consist ; and 

 finding that the increase of weight is always the result 

 of calcination as well as combustion, he satisfies him- 

 self with making a gratuitous addition to the hypo- 

 thesis of phlogiston, and supposes that this imaginary 

 substance is endowed with positive levity ; nor does he 

 allude to the experiments of Lavoisier on gases, on 



