44 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



close of these experiments was slightly less dense than 

 ordinary air, and did not allow a candle to burn in it, or 

 even the smallest spark to continue glowing. This inactive 

 portion of the air was separated by no fewer than sixteen 

 different methods, but the recovery of the "lost air," as 

 Scheele called it, was a problem of much greater difficulty. 

 Scheele actually obtained it from a variety of substances, 

 amongst which was nitric acid, thus confirming the view of 

 Hooke and Mayow as to the close relationship existing 

 between nitre and air. He found that it could be prepared 

 most readily either from nitre (saltpetre) or from common 

 RED PRECIPITATE (obtained by dissolving mercury in nitric 

 acid and heating the residue until the product became red) 

 which he proved to be identical with calcined mercury (red 

 oxide of mercury). 



Having separated this PURE FIRE-AIR Scheele repeated 

 the experiments which he had carried out with ordinary air 

 and proved that the substances which diminished the 

 volume of ordinary air when left or burnt in it, were 

 capable of absorbing " fire-air " entirely. For example, 

 phosphorus, heated in a closed flask filled with " fire-air," 

 burnt with remarkable brilliancy ; on allowing the flask 

 to cool and opening it under water, it was found that 

 water entered the bottle and filled it almost completely. 

 Scheele showed that his " fire-air" (Lavoisier's "oxygen ") 

 was slightly denser than air, and soluble in water. He also 

 proved it to be essential to the breathing of animals, and 

 the growth of plants. By mixing it with 3 J times its volume 

 of " foul air," left after burning phosphorus in a closed 

 vessel, there was obtained a gas which in every respect 

 resembled common air. Scheele rightly concluded that 

 common air is a mixture of " fire-air " with about four times 

 its volume of " foul air." 



Azote. The residue left after removing the oxygen from 

 air was described by Priestley as " phlogisticated air," by 



