120 ESSAYS BIOGRAPHICAL AND CHEMICAL 



of glass : in brass, for if anything of the nature of machinery, 

 such as pumps, stirrers, etc., is required, brass is perhaps 

 the most convenient material; in clay, for vessels are 

 wanted which will withstand a high temperature ; and of 

 recent years silica glass, made from fused rock-crystal, has 

 proved of great use, for it can be worked before a blow-pipe 

 fed with coal-gas and oxygen. 



But to return to the discovery of oxygen. Priestley 

 heated oxide of mercury, or, as he called it, ' red precipi- 

 tate,' in a retort, and collected the escaping gas; and he 

 found that a candle burned in it much more brightly than 

 in air; and, moreover, after having found that a mouse 

 could live in it longer than in the same volume of air, 

 confined in a bottle, he breathed it himself and found 

 that its effect was pleasant and exhilarating. 



Similar experiments were made by Scheele with the 

 same result; but Scheele went much further. Having 

 noticed that a number of substances had the property of 

 making combustible bodies, such as wood, flour, and 

 charcoal, deflagrate, or burn more brilliantly when mixed 

 with them, he heated these substances, and found that 

 they too evolved oxygen gas. Among the substances 

 were red-lead, black oxide of manganese, nitre, and many 

 others ; so he established a general rule that those sub- 

 stances which can be mixed with charcoal to make a kind 

 of gunpowder will evolve oxygen when heated. 



It thus became known that air contained a gas, amount- 

 ing to about a fifth Scheele says a sixth of its bulk, 

 possessing the property of making combustible objects 

 burn with greater vigour. Flame, therefore, was caused 

 by the action of oxygen, as the new gas was called later, 

 with combustible bodies. 



It would take too long to consider the curious doctrine 

 of 'phlogiston,' an immaterial effluvium which was sup- 

 posed to escape when bodies burn ; I can merely mention 



