294 SMITH'S INTERMEDIATE CHEMISTRY 



Thus, the observed density of the air is precisely that which we 

 find by calculation from the known proportions and several densi- 

 ties of the components. The solubility of each gas is observed to 

 be the same as if it were alone present. 



Again, when liquefied air is allowed to evaporate in a suitable 

 apparatus, the nitrogen, being more volatile, can be separated 

 almost completely from the oxygen. When the oxygen is, in turn, 

 allowed to evaporate, the carbon dioxide and water remain as 

 solids, frozen by the low temperature. 



Finally, the exact proportions can not be represented by a chemical 

 formula. This shows that the law that, in chemical compounds, 

 the proportions can be represented by multiples of the atomic 

 weights by whole numbers (p. 77), does not apply to air. 



In spite of the fact that air is a mixture, the composition of the 

 air is remarkably uniform and constant. The uniformity is due 

 to constant mixing by the winds. The steadiness of the com- 

 position from year to year is due to the fact that, although decay 

 and combustion continually remove oxygen and add carbon diox- 

 ide, vegetation as continually consumes the latter and restores 

 the former (p. 396). The mass of carbon dioxide in the whole 

 atmosphere of the planet, about 2450 thousand million tons, is so 

 great that the amounts added and removed by the agencies just 

 mentioned are small by comparison. 



( Liquefaction of Gases^- The principle now used in liquefy- 

 ing gases depends on the fact that, although a perfect gas, when 

 expanding into a vacuum, should suffer no fall in temperature, 

 since it does no work, ordinary gases do become cooled very 

 slightly. The work which they do in expanding in such circum- 

 stances is done in overcoming the cohesion between their mole- 

 cules (p. 91), so that a tearing apart of the substance, which 

 consumes heat, has to take place. Since this cohesion becomes 

 more conspicuous the lower the temperature, the cooling effect of 

 expansion becomes greater and greater as the temperature falls. 



