Minor Constituents of Air 241 



shows that the air passage has been too rapid, and that 

 all the moisture in the air has not been absorbed in C. 

 T' should also show no appreciable gain in weight. Its 

 function is to prevent backward diffusion of the moisture- 

 laden air from the aspirator into either T or C. 



It will be found that, even when 10 litres of air have 

 been passed through the tubes, the gain in weight is 

 small, and hence every care must be taken in order to 

 obtain a good result. 



Further, it must be clearly understood that, by 

 this method, only the average amount of moisture present 

 in the air around the apparatus during the whole time 

 of the experiment can be ascertained, and not the actual 

 amount present at any given instant. 



163. Besides the substances already mentioned 

 there are numerous other bodies found in very small 

 quantities in the air, especially in the neighbourhood 

 of towns and cities. The table below gives the 

 percentage composition by volume of what may be 

 considered an average sample of ordinary air. 



Per cent. Per cent. 



Nitrogen 76*95 Hydrogen .. .. 0'019 ? 



Oxygen .. ........ .. 20'66 Ammonia ^ 



Aqueous vapour .. T40 Nitric acid 



A rvrv^TTi t f 1 races. 



Argon 0'94 Hydrogen peroxide | 



Carbon dioxide . . . . 0'034 Ozone ? 



In addition to the above, traces of sulphur com- 

 pounds, such as sulphur dioxide and sulphuretted 

 hydrogen, are found in the air of towns where coal and 

 coal-gas, both of which contain a little sulphur, are 

 used. Further, ordinary air is never entirely free from 

 small solid impurities, such as dust and soot particles, 

 and small living organisms known as bacteria. 



1 64. Since carbon dioxide is being continually formed 

 by the respiration of animals and by the burning of fuel, 



H. D. s. 16 



