CHAPTER XIX. 



THE CHEMISTKY OF EESPIKATION. 



THE simplest way to investigate the study of the gas-interchange 

 that takes place in the lungs between the air and the blood is to 

 compare the composition of the expired air with that of the atmo- 

 sphere, and from the alteration found to have taken place in the 

 tidal stream we can arrive at the changes which the air under- 

 goes during its journey in and out of the air- passages, and we can 

 then examine the venous and arterial blood in order to ascertain 

 the change the blood undergoes in becoming arterial. 



The atmosphere is made up of a mixture of nitrogen and oxy- 

 gen, with a variable amount of moisture and a minute proportion 

 of carbonic acid. 



The following table gives the volume* of the gases in dried air: 



Oxygen, .... 20.96 per cent., or about 21 per cent. 

 Nitrogen, . . . 79.02 " " 79 " 



Carbonic dioxide, . 0.02-0.06 " " 4 parts in 10,000. 



The amount of moisture contained in the air is very variable, 

 and depends in a great measure upon the temperature and the 

 direction of the wind. The dampness of the air depends upon 

 the temperature, so that air containing the same absolute amount 

 of moisture may be relatively dry or damp, according as the tem- 

 perature rises or falls. As a general rule the air is relatively dry, 

 that is to say, it does not contain so much moisture as it is capa- 

 ble of taking up in the form of aqueous vapor at its ordinary tem- 

 perature. At certain times of the day the air may be saturated 

 owing to a sudden fall of temperature. 



The temperature of the air which we breathe of course varies 



* On account of the difference in the atomic weights, the atmosphere being 

 only a mechanical mixture of the gases, the proportion by weight is slightly 

 different, being about Oxygen 23 per cent., Nitrogen 77 per cent. 



