THE STORY OF THE LUNGS, SKIN AND KIDNEYS 81 



is greater than that represented in the chimney. The 

 open fireplace, although an extremely wasteful appli- 

 ance in the matter of coal consumption seeing that 

 a very large proportion of the heat passes up the 

 chimney, nevertheless possesses the advantage of 

 assisting materially the ventilation of a room. A 

 current of air passing up the chimney displaces so 

 much of the air in the room which is renewed from 

 the outside, and thus a certain circulation of air is 

 maintained. 



AIR QUANTITIES. Allusion has been made to the 

 difference between the composition of the air 

 breathed in and that expired. We may assume that 

 the air inspired contains in one hundred parts about 

 twenty-one parts of oxygen, seventy-nine parts of 

 nitrogen, and '04 parts of carbonic acid. On the other 

 hand air which is breathed out has naturally its oxygen 

 diminished owing to so much of the gas having been 

 absorbed by the tissues, the amount being about 

 fifteen parts. The quantity of nitrogen does not alter, 

 this gas being inert, and serving apparently for the 

 purpose of diluting the oxygen; whilst the carbonic 

 acid given forth is naturally increased and is repre- 

 sented by other four parts in the hundred parts of air. 

 Summing up the differences between air breathed in 

 and air breathed out, we may say that the air breathed 

 in contains much oxygen, little carbonic acid, a certain 

 amount of watery vapour, and is of the temperature 

 of the surroundings. That which is exhaled on the 

 other hand contains less oxygen, more carbonic acid, 

 is of the temperature of the blood from which it has 

 passed, this temperature being 98*4 degrees, and in 

 addition contains a certain amount of the organic 

 matter already noted. In a rough way it may be 

 said that the air we expire contains five per cent less 



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