THE LUNGS AND RESPIRATION. 217 



The composition of expired air may then be tabulated 

 as follows: 



Nitrogen 79 per cent. 



Oxygen 15 " 



Carbon Dioxide 4 "' 



Water in increased amounts. 

 Organic substances. 



2. The Amount of Oxygen Consumed in One Day and 

 the Amount of Carbon Dioxide Eliminated in the same 

 Period. As the amount of oxygen which is taken up by 

 the blood while in the lung is five per cent, of the entire 

 air, we are easily able to calculate, knowing the amount we 

 breathe in at each breath and the number of breaths in a 

 given time, just how much oxygen under ordinary pressure 

 we can consume in, say a day. Such calculations give as 

 the daily oxygen consumption twenty to twenty-five cubic 

 feet. The carbon dioxide is a little over four per cent, and 

 in a day amounts to from sixteen to eighteen cubic feet. 



To all students who have taken even elementary courses 

 in chemistry, oxygen and carbon dioxide are things of 

 knowledge. For the sake of persons who have had no 

 training in chemistry it may be in place here to refer briefly 

 to the nature of these two substances. 



Oxygen is a gas which forms about a fifth of the ordi- 

 nary atmosphere, and is that element of the atmosphere 

 which makes burning possible. It is ordorless, tasteless, 

 colorless, and so not easily perceived by the senses, yet 

 forming an integral part of the very atmosphere in which we 

 move, and chemically familiar to everybody as the element 

 in the atmosphere which as the draft we lead into stoves and 

 lamps to make combustion possible, it needs no further ex- 

 planation. Pure oxygen very materially increases the energy 

 of combustion, and in an atmosphere of pure oxygen even 

 such an apparently incombustible thing as a steel wire may 

 be made to burn easily. In the air, however, the oxygen is 

 very much diluted with the inert gas called nitrogen which 

 serves, therefore, in this process of respiration as a mere 

 diluting agent and plays no active role itself. 



