552 NATURE OF EXPIRED AIR. [BOOK 11. 



and expired air arises from the fact that the volume of any given 

 quantity of carbonic acid is equal to the volume of the oxygen 

 consumed to produce it ; the slight falling short of the expired air 

 <is due to the circumstance that all the oxygen inspired does not 

 reappear in the carbonic acid expired, some having formed within 

 the body other combinations. 



341. Besides carbonic acid, expired air contains various 

 substances which may be spoken of as impurities, 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, which, from 

 the presence of micro-organisms, introduced in the inspired air, 

 is very apt 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 some of them are of a 

 poisonous nature, either poisonous in themselves as coming direct 

 from and produced in some way or other in the pulmonary 

 apparatus, or poisonous as being the products of putrefactive 

 decomposition ; for various animal substances and fluids give rise 

 by decomposition to distinct poisonous products, known as 

 ptomaines, and it is possible that some of the constituents of 

 expired air are of an allied nature. In any case the substances 

 present have a deleterious action, 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 dis- 

 tinctly unwholesome, not so much on account of the carbonic acid, 

 as of the accompanying impurities. Since these impurities are of 

 unknown nature and cannot be estimated, the easily determined 

 carbonic acid is usually taken as an indirect 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 considerably below *1 p.c., he should have even more 

 than ten times as much. 



