328 CHANGES IN THE AIR. [BOOK n. 



body; but the amount of carbonic acid given out by the skin is, as 

 we shall see, very slight (10 grms. or even less), so that 800 grms. 

 may be taken as the average production of carbonic acid by an 

 average man. The quantity however, both of oxygen consumed 

 and of carbonic acid given out, is subject to very wide variations ; 

 thus in Pettenkofer and Voit's observations, the daily quantity of 

 carbonic acid varied from 686 to 1285 grms., and that of the 

 oxygen from 594 to 1072 grms. These variations and their causes 

 will be discussed when we come to deal with the problems of 

 nutrition. 



5. Besides carbonic acid, expired air contains various im- 

 purities, many of an unknown nature, and all in small amounts. 

 Traces of ammonia have been detected in expired air, even in that 

 taken directly from the trachea, in which case its presence could 

 not be due to decomposing food lingering in the mouth. When 

 the expired air is condensed by being conveyed into a cooled 

 receiver, the aqueous product is found to contain organic matter, 

 and rapidly to putrefy. The organic substances thus shewn to be 

 present in the expired air are the cause in part of the odour 

 of breath. It is probable that many of them are of a poisonous 

 nature; for an atmosphere containing simply 1 p.c. of carbonic 

 acid (with a corresponding diminution of oxygen) has very little 

 effect on the animal economy, whereas an atmosphere in which the 

 carbonic acid has been raised to 1 p.c. by breathing, is highly 

 injurious. In fact, air rendered so far impure by breathing that 

 the carbonic acid amounts to '08 p.c. is distinctly unwholesome, 

 not so much on account of the carbonic acid, as of the accom- 

 panying impurities. Since these impurities are of unknown 

 nature and cannot be estimated, the easily determined carbonic 

 acid is usually taken as the measure of their presence. We have 

 seen that the average man loads, at each breath, 500 c.c. of air 

 with carbonic acid to the extent of 4 p.c. He will accordingly at 

 each breath load 2 litres to the extent of 1 p.c.; and in one hour, 

 if he breathe 17 times a minute, will load rather more than 2000 

 litres to the same extent. At the very least then a man ought to 

 be supplied with this quantity of air hourly; and if the air is to be 

 kept fairly wholesome, that is with the carbonic acid reduced 

 below *1 p.c., he should have more than ten times as much. 



