442 NATURE OF EXPIRED AIR. [Book ii. 



Hence, when an animal is made to breathe in a confined space, 

 the air is absolutely diminished in volume. The approximate 

 equivalence in volume between inspired 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 cir- 

 cumstance that all the oxygen inspired does not reappear in 

 the carbonic acid expired, some having formed within the body 

 other combinations. 



§ 272. 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 constitu- 

 ents 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 distinctly unwholesome, not so much 

 on account of the carbonic acid, as of the accompanying impuri- 

 ties. 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 breathes 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. 



