VENTILATION 155 



necessary in order to keep up the proportion of oxygen and 

 to keep down the proportion of carbon dioxid. 



186. Ventilation problems. More recent experiments show 

 that under ordinary conditions the air contains neither a breath 

 poison nor a dangerous proportion of carbon dioxid, even when 

 the ventilation is decidedly bad. Nor is there danger that the 

 percentage of oxygen will fall below a safe limit. It seems that 

 the chief problems in ventilation are (i) to keep the air at a 

 suitable temperature, (2) to regulate the moisture and dust in 

 the air, and (3) to prevent the air from stagnating. 



187. Skin temperature. The body radiates heat and trans- 

 pires moisture all the time. Up to a certain temperature we 

 may remain comfortable by increasing perspiration and trans- 

 piration ; that is, evaporation of water from the surface of the 

 body keeps the temperature of the skin down. An excess of 

 moisture in the air stops evaporation, and we become uncom- 

 fortably hot. Or if the temperature becomes too high, evapora- 

 tion cannot go on fast enough to leave us comfortable. But this 

 discomfort that the skin feels when it is hot and damp is of the 

 same kind, only greater in degree, in the case of the lungs. 

 A hot, stuffy room interferes with the breathing because it inter- 

 feres with the radiation of heat and the transpiration of water 

 inside the lungs. The oppression felt in a poorly ventilated 

 room has apparently nothing to do with the amount of carbon 

 dioxid or with the lack of oxygen. Bad odors are offensive 

 and may interfere with one's breathing on that account. 1 



The drowsy effects of a badly ventilated room are due to the con- 

 gestion of the skin capillaries and the corresponding depletion of the 

 blood vessels that supply the brain and the muscles, quite as much as 

 to the influence upon the breathing. 



1 In the famous Black Hole of Calcutta, in which so many people lost their 

 lives, the victims were supposed to have died of lack of oxygen. It seems 

 probable that in such cases death really results from " heat-stroke," due to the 

 excessive humidity (from the perspiration and the lung transpiration of the 

 people) and the high temperature (from the heat radiated by the bodies and 

 not carried off). 



