RESPIRATORY GAS-INTERCHANGE. 



349 



to be the same, for the volume of CO 2 is equal to that of the O it 

 contains, if set free. The volume CO 2 given off is, however, only 

 about 4.38, to 4.5 volumes of O taken in, so that part of the O 

 must be used in some other way than in the manufacture of CO 2 . 

 7. The expired air is also said to contain traces of the following 

 impurities : (1) ammonia, (2) hydrogen, (3) carburetted hydrogen 

 (CH 4 ), (4) organic matter. These, and probably other impurities, 

 give the breath its peculiar odor and noxious properties, for an 

 atmosphere rendered "stuffy" by expired air is much more inju- 

 rious to health than an atmosphere in which a similar deficiency 

 of O or excess of CO 2 has been artificially produced by chemical 

 means ; this fact ought to be remembered when calculating the 

 ventilation required for hygienic purposes. The following Table 

 may assist in comparing the atmosphere with the expired arr: 



About |th of the O which is used does not take part in the pro- 

 duction of the CO 2 , but this proportion may vary greatly. Thus 

 the estimation of the CO 2 can give no sure guide to the amount of 

 O taken up ; and each gas has to be estimated separately if an 

 accurate measurement be required. 



The average amount per diem may be said to be : 



Carbon dioxide 

 Oxygen . 

 Water 



given off about 800 grammes, 

 consumed about 700 " 

 given off about 500 " 



The amounts of O taken up and of CO 2 given off differ in dif- 

 ferent individuals and in the same individuals under varying 

 circumstances, among which the following may be enumerated : 



1. Increase in the rapidity or the depth of respiratory move- 



