AIR AND OXYGEN 



29 



(1777) heated mercury in a retort (Fig. 16), the neck of 

 which projected into a jar standing in a larger dish of mercury. 

 The air, thus enclosed within the jar and the retort, during 

 twelve days lost one-fifth of its volume. Simultaneously, red 

 particles of mercuric oxide accumulated on the surface of the 

 mercury in the retort. The residual gas, nitrogen, no longer 

 supported life or combustion. The oxide, on being heated more 

 strongly, by itself, gave off a gas whose volume exactly corre- 

 sponded with the shrinkage undergone by the enclosed air, and 

 this gas possessed in an exaggerated degree the properties which 

 the air had lost. The proof that oxygen was a component of 

 the atmosphere was therefore complete. Later, Lavoisier, in 

 the mistaken belief that the new element was an essential con- 

 stituent of all sour substances, named it oxygen (Greek, acid- 

 producer) . 



Occurrence. As we have seen, nearly 50 per cent of terres- 

 trial matter is oxygen. Water contains about 89 per cent, the 

 human body over 60 per cent, and common materials 

 like sandstone, limestone, brick, and mortar more than 

 50 per cent of this element. One-fifth by volume 

 (nearly one-fourth by weight) of the air is free oxy- 

 gen. 



Preparation of Oxygen. 1. The oxygen of com- 

 merce is now made chiefly from liquefied air. The 

 liquid oxygen boils at 182.5, but the nitrogen boils 

 at an even lower temperature ( 194). Since the 

 liquid air has a temperature of about 190, some- 

 what above that of boiling nitrogen, the latter 

 evaporates much more freely than does the oxygen. FlG 17 

 After a time, when the remaining liquid is almost pure 

 oxygen (96 per cent), the gas coming off is compressed by pumps 

 into the steel cylinders (Fig. 17) in which it is sold. In medicine, 



