X04 HISTORT of the SOCIETK 



Before Joseph Black offered himfelf a candidate for a de- 

 gree in medicine, he had aheady made his difcovery of Fixed 

 Air ; that is to fay, of an elaftic fluid, which being fixed in cal- 

 eareous and alkahne fubftances, is difpelled from them in their 

 calcination by fire, or effervefcence with acids, leaving a refi- 

 duum, which, in the abfence of this air with which it had been 

 combined, becomes cauftic ; a quality which it retains until the 

 air of which it was deprived is again reftored. Pure chalk, or 

 pure calcareous matter of any form, in this experiment, lofes 

 about two- fifths of its weight, or, mofe accurately, forty-one 

 parts in the hundred. Here, accordingly, is a ready way of af- 

 faying limeftone or marl, to afcertain its purity or its value, to 

 the hufbandman in particular, who would employ it to improve 

 his foil. 



The air obtained in this experiment has its peculiar qualities. 

 Befides that of being fixable in flone, it is heavier than common 

 air, its fpecific gravity being nearly double. Flame is extinguifh- 

 ed, and animals are fufFocated in it equally as in water. By this 

 difcovery, notwithftanding the ingenious and copious obferva- 

 tions of Dr Hales and others, on the fubjedl of what were call- 

 ed Factitious Airs, it was referved for Black to give a fpur 

 and a new diredion to the refearches of fcience. 



From this beginning, great progrefs has been made in diftin- 

 gui£hing varieties of elaftic fluid, and in afcertaining the compo- 

 fition of the atmofphere itfelf. Chemiftry was. become a favou- 

 rite ftudy in France, as it bad long been in Germany, and the 

 report of a new. fpecies of air was every where received with avi- 

 dity. The experiments, of Black were repeated, and the infe- 

 rences confirmed by ingenious, men both at home and abroad ; 

 though by fome in Germany the refult was at firft contelled, 

 and a controverfy arofe, in which it does not appear that Black 

 took any part, nor does any doubt now remain of the fail, as fta- 

 ted by him, or of the inferences he drew from it *. 



An 

 * Vid. Opufcles PJiyfiques et Chimiques par M. Lavoisier, 



