42 SMITH'S INTERMEDIATE CHEMISTRY 



Uses of Oxygen. In the foregoing sections we have referred 

 to its use in breathing, its role in decay, which is a beneficient 

 process because it removes much useless matter which might 

 otherwise cause disease, and its value in the disposal of sewage. 

 Power and heat for commercial purposes are almost all obtained 

 by the burning of coal, in which oxygen from the air plays a large 

 part. If we had to purchase the oxygen as well as the coal, we 

 should require at least three tons of oxygen for every ton of coal. 



Oxygen in cylinders and oxygen generators are used to restore 

 the supply in the atmosphere of submarine boats, and to assist 

 the respiration of aviators at very great altitudes. 



Substances Indifferent to Oxygen. Finally, since the 

 atmosphere contains so large a proportion of oxygen, substances 

 which do not oxidize and, when heated, do not burn, have many 

 uses. Gold, silver, and platinum are of this kind (p. 36), and are 

 used for ornaments. The last is used for crucibles in which bodies 

 are heated in the laboratory. Although iron burns in pure oxygen, 

 it does not oxidize rapidly in the air even when heated, and so is 

 used for making vessels for cooking and in constructing fireproof 

 buildings. 



Substances, already fully oxidized, are naturally not com- 

 bustible. Of this nature are sandstone, granite, brick, porcelain, 

 glass, and water. All these are, therefore, fireproof. More- 

 over these substances do not give off oxygen when heated. It is 

 this inactivity which renders glass and porcelain suitable materials 

 for laboratory apparatus, since they experience no change in 

 weight when heated. 



Exercises. 1. What percentage by weight of free oxygen is 

 obtained by heating: (a) mercuric oxide, (6) potassium nitrate, 

 (c) potassium chlorate? At $1.50, $0.15, and $0.15 per kilogram, 

 respectively, which is the cheapest source of oxygen? 



2. Using the data on p. 34, calculate the weight of oxygen dis- 

 solved by 100 c.c. of water at under one atm. pressure. 



