THE ATMOSPHERE 27 



gas would occupy under new conditions of temperature and 



pressure becomes simple. To take an example : Suppose a 



gas measures 1000 c.c. at a temperature of 15° C. and under 



a pressure of 740 mm. of mercury, and it is desired to 



calculate the volume it will occupy at 20° C. and 7G0 mm. 



Consider the effect of change of temperature. Tiie absolute 



temperature of 15° is 273 + 15 = 288. The absolute 



temperature of 20° is 273 + 20 = 293. The volume is 



directly proportional to the absolute temperature; .*. the 



203 

 volume will be 1000 x -^^ if the pressure were constant. Next 



ronsider the influence of pressure. The pressure changes from 

 740 to 760 mm. ; therefore the volume will be diminished ; it 



9QQ 740 

 will thus be 1000 x ^r^^ x 7^77, = 1004-3 c.c. 



Chemical Composition of the Air.— Air is a mechanical 



mixture of several gases, some of which are invariably present, 

 though in variable proportion, while others are sometimes 

 practically absent. 



The chief gases are : — 



Nitrogen. Ammonia. 



Oxygen. Nitric acid or oxides 



Argon. of nitrogen. 



Carbon dioxide. Ozone. 



Water vapour. 



• 



Nitrogen is the largest and least variable constituent, its 

 amount in dry air being about 78 per cent, by volume, or 

 75-5 per cent, by weight. Although so abundant, it plays very 

 little part in the processes going on in the air ; indeed, its 

 chief function may almost be said to be that of a diluent. 

 Certain crops have been proved to assimilate the free 

 nitrogen by the aid of tubercles on their roots, and some very 

 low forms of plant-life are believed to be able directly to 

 utilise free nitrogen, but the great majority of plants have 



