302 RESPIRATION 



substitution of well-purified gas for coal in fires, or by smokeless 

 combustion of coal, the trouble might be avoided, and indeed has 

 been much diminished within recent years. 



Lower organisms, and particularly plants, are on the whole 

 far more sensitive to impurities in air and other changes in en- 

 vironment than higher animals, and particularly man. The real 

 reason for this is that between the living tissue elements and the 

 outside environment higher organisms possess an internal en- 

 vironment which is not only highly developed, but is maintained 

 with an efficiency which increases with the scale in development. 

 Plants are extremely sensitive to the particulate and other impuri- 

 ties in air and the obstruction of light by smoke and opaque fogs. 

 But few trees and plants can flourish in the air of a town or in- 

 dustrial area. The traces of acid and other impurities present in 

 the air can act more or less directly on their tissue elements, which 

 have very little between them and the external environment. 



Air of Occupied Rooms. In rooms of all kinds where men are 

 present the composition of the air becomes altered, owing to res- 

 piration and evaporation and to any gas or oil lamps which may 

 be burning. Both respiration and lamps consume oxygen and pro- 

 duce CO 2 and moisture. The combustion in the lamps is perfect, 

 so that no CO passes into the air; and unless the gas is badly 

 purified from sulphur the products of combustion have very little 

 unpleasant effect apart from what may be due to heat. It was 

 formerly supposed that some volatile toxic substance is given off 

 in the breath; but the experimental evidence in support of this 

 belief was found to be fallacious, and all attempts to demonstrate 

 the existence of such a substance have failed. Some of the most 

 striking evidence on the subject is afforded by experience in sub- 

 marines, in which a limited volume of air is quite commonly re- 

 breathed until after a few hours a light will not burn and 3 per cent 

 or more of CO 2 may be present. Provided the air remains cool, as 

 it does in a temperate climate owing to the cooling influence of the 

 water, the only effects observed are those due to CO 2 . 



Even in the most crowded and ill-ventilated rooms the pro- 

 portion of CO 2 seldom rises above 0.5 per cent, with, of course, a 

 corresponding drop in the oxygen percentage. From the account 

 already given of the physiology of breathing it is evident that a 

 difference of this order in the composition of the air is in itself of 

 no appreciable importance. The breathing simply becomes very 

 slightly deeper and the composition of the alveolar air and 



