RESPIRATION. 191 



inspired air is allowed to remain in the lungs; for although, during 

 ordinary respiration, the expired air is always saturated with watery 

 vapor, yet when respiration is performed very rapidly the air has scarcely 

 time to be raised to the highest temperature, or be fully charged with 

 moisture ere it is expelled. 



The quantity of water exhaled from the lungs in twenty-four hours 

 ranges (according to the various modifying circumstances already men- 

 tioned) from about 6 to 27 ounces, the ordinary quantity being about 9 

 or 10 ounces. Some of this is probably formed by the chemical combi- 

 nation of oxygen with hydrogen in the system ; but the far larger pro- 

 portion of it is water which has been absorbed, as such, into the blood 

 from the alimentary canal, and which is exhaled from the surface of the 

 air-passages and cells, as it is from the free surfaces of all moist animal 

 membranes, particularly at the high temperature of warm-blooded ani- 

 mals. 



6. A small quantity of ammonia is added to the ordinary constitu- 

 ents of expired air. It seems probable, however, both from the fact 

 that this substance cannot be always detected, and from its minute 

 amount when present, that the whole of it may be derived from decom- 

 posing particles of food left in the mouth, or from carious teeth or the 

 like ; and that it is, therefore, only an accidental constituent of expired 

 air. 



7. The quantity of organic matter in the breath is increased and is 

 about 3 grains in about twenty-four hours. (Ransome.) 



Method of Experiment. The following represents the kind of experi- 

 ment by which the foregoing facts regarding the excretion of carbonic 

 acid, water, and organic matter, have been established. 



A bird or mouse is placed in a large bottle, through the stopper of 

 which two tubes pass, one to supply fresh air, and the other to carry off 

 that which has been expired. Before entering the bottle, the air is made 

 to bubble through a strong solution of caustic potash, which absorbs the 

 carbonic acid, and then through lime-water, which, by remaining limpid, 

 proves the absence of carbonic acid. The air which has been breathed 

 by the animal is made to bubble through lime-water, which at once be- 

 comes turbid and soon quite milky from the precipitation of calcium 

 carbonate ; and it finally passes through strong sulphuric acid, which, 

 by turning brown, indicates the presence of organic matter. The 

 watery vapor in the expired air will condense inside the bottle if the 

 surface be kept cool. 



By means of an apparatus sufficiently large and well-constructed, 

 experiments of the kind have been made extensively on man. 



Methods by which the Respiratory Changes in the Air are 



effected. 



The method by which fresh air is inhaled and expelled from the 

 lungs has been explained. It remains to consider how it is that the 



