COMPOSITION OF THE AIR. 413 



other. Though, as should naturally be expected, the doc- 

 trines of this great observer have been modified with the 

 advances in science, he developed facts which will stand for- 

 ever, and which have served as the starting point of all 

 our knowledge on this subject. From that time physiol- 

 ogists began to look on respiration as consisting in the appro- 

 priation of oxygen and the exhalation of carbonic acid; 

 and now the seat of this process is only changed from the 

 lungs to the tissues. From the limited knowledge of the 

 intimate phenomena of nutrition which obtained in his day, 

 Lavoisier could not be expected to entertain any other view 

 than that the carbonic acid produced was the result of the 

 direct union of oxygen with carbon in the blood. It is only 

 since investigations have made manifest the great complexity 

 of the processes of nutrition, that some are unwilling to be- 

 lieve that carbonic acid is produced in as simple a way as 

 it appeared to Lavoisier. 1 



Composition of the A.ir. Pure atmospheric air is a 

 mechanical mixture of T9*19 parts of nitrogen with 20*81 

 parts of oxygen (Dumas and Boussingault). 2 It contains in 

 addition a very small quantity of carbonic acid, about one 

 part in 2,000 by volume, and traces of ammonia. The air 

 is never free from moisture, which is very variable in quan- 

 tity, being generally more abundant at a high than at a low 

 temperature. In 1840, Schonbein discovered in the air a pecu- 

 liar odorous principle called ozone, which he conceived to be a 

 compound of oxygen and hydrogen (HO 3 ), but which is now 

 pretty well shown to be an allotropic form of oxygen. The 



1 The applications of the discoveries of Lavoisier to the production of animal 

 heat will be taken up in connection with that phenomenon. 



2 Some chemists suppose that the oxygen and nitrogen in the air are in a con- 

 dition of feeble chemical combination. However that may be, it is certain that 

 in respiration it is the oxygen which is absorbed by the blood, and which carries 

 on the function. The nitrogen seems to act simply as a diluent, thus providing 

 that the blood in the lungs shall be exposed to but a certain quantity of the re- 

 spiratory principle. 



