CHANGES OF COMPOSITION IN RESPIRED AIR. 823 



of carbonic acid, ammonia, chlorides, urates, and even some albuminoid 

 substance, and it readily undergoes decomposition. 



The changes in the composition of the expired air are regulated by 

 that of the inspired air. The composition of the atmosphere in free 

 space is singularly uniform in different localities, and at different alti- 

 tudes. By weight, supposing it to be dry, it contains nearly 77 parts 

 of nitrogen, and 23 of oxygen. Besides these, its essential constitu- 

 ents, it contains a small percentage of carbonic acid, disengaged into 

 it by terrestrial agencies, partly physical, such as volcanoes and 

 springs, partly chemical, as the decomposition of carbonaceous matter, 

 but chiefly organic, as from the respiration of plants ; the atmosphere 

 also presents minute traces of nitric acid, ammonia, and carburetted 

 hydrogen, from the decomposition of animal and vegetable substances. 

 In towns it often contains sulphuretted hydrogen and sulphurous acid, 

 from the combustion of coal ; in the neighborhood of chemical works 

 it may also be charged with chlorine, mineral acids, and metallic sub- 

 stances. Under certain circumstances, a very pure air contains the 

 important substance known as ozone, which is now usually regarded as 

 a modification of oxygen. It is most abundant in sea air, in early 

 morning, and, in England, with southwest or west winds ; it is almost 

 absent with east winds, and is quite so in the centre of large towns, 

 and in the atmosphere of dwellings. 



By volume, dry air consists, in round numbers, of 80 volumes of 

 nitrogen, and 20 of oxygen, or of 4 volumes of the former to 1 of the 

 latter. A closer analysis gives 79 volumes of nitrogen to 21 of oxy- 

 gen. The quantity of carbonic acid gas averages .04 volumes per 

 cent., or, as it is commonly expressed, 4 parts in 10,000. 



The effect of a single respiration on the composition of the air 

 breathed, is first, to remove from 100 volumes of air about 5 volumes 

 of oxygen, i. e., about Jth its normal quantity of that gas ; and, sec- 

 ondly, to add to it about 4 volumes per cent, of carbonic acid gas. 

 Besides this, however, the quantity of nitrogen is slightly increased ; 

 and ammonia, carburetted hydrogen, certain salts, organic matter, and 

 various undetermined volatile substances, are added to the air in the 

 respiratory process. 



The annexed Table from Vierordt shows the percentage composi- 

 tion in volumes of air before, and after, it has been once breathed ; 

 the air being, in both cases, supposed to be perfectly dry. The minute 

 trace of carbonic acid gas in unbreathed air, only .04 per cent., is here 

 neglected. 



Atmospheric Air. Air once breathed. 



Nitrogen, ; 79.2 . . . 79.3 



Oxygen, 20.8 . . . 15.4 



Carbonic Acid, . . . . . . . .4.3 



Loss, . . . . . . . . . .1 



100 100 



During a single respiration, therefore, 5.4 parts of oxygen disap- 

 pear, being absorbed by the lungs; whilst only 4.3 parts of carbonic 

 acid are exhaled from those organs. Moreover, a minute quantity of 



