RESPIRATION : THE GASEOUS INTERCHANGES 373 



starvation or be suffocated, as such a mode of death is called, as 

 surely, though not quite so fast, as if he were put under the re- 

 ceiver of an air-pump and all the air around him removed. Hence 

 the necessity of ventilation to supply fresh air in place of that 

 breathed, and clearly the amount of fresh air requisite must be 

 determined by the number of persons collected in a room; the 

 supply which would be ample for one person would be insufficient 

 for two. Moreover, fires, gas, and oil lamps, all use up the oxygen 

 of the air and give carbon dioxid to it, and hence calculation 

 must be made for them in arranging for the ventilation of a 

 building in which they are to be employed. 



In order that air be unwholesome to breathe, it is by no means 

 necessary that it have lost so much of its oxygen as to make it 

 difficult for the Body to get what it wants of that gas. The evil 

 results. of insufficient air-supply are rarely, if ever, due to that 

 cause even in the worst-ventilated room for, as we shall see here- 

 after, the blood is able to take what oxygen it wants from air 

 containing comparatively little of that gas. The headache and 

 drowsiness which come on from sitting in a badly ventilated room 

 appear to be due chiefly to the high percentage of water-vapor 

 present under such circumstances, and the want of energy and 

 general ill-health which result from permanently living in such 

 surroundings are probably the result of a slow poisoning of the 

 body by absorption of gaseous substances given off to the air, not 

 from the lungs, but from the skin in evaporating sweat and from 

 the alimentary tract. The idea, formerly held very generally, 

 that volatile poisons are given off by the lungs in quantities too 

 small for chemical detection, has been largely abandoned partly 

 because of the failure of the most careful experiments to demon- 

 strate any such substances, but more because there are enough 

 injurious materials given off from other channels of the body to 

 explain all the ill effects of insufficient ventilation. 



That the air of rooms occupied by persons becomes injurious 

 long before the amount of carbon dioxid in it is sufficient to do 

 any harm has been abundantly demonstrated. Breathing air 

 containing one or two per cent of that gas produced by ordinary 

 chemical methods does no particular injury, but air containing 

 one per cent of it produced by respiration is decidedly injurious, 

 because of the other things present in it at the same time. Carbon 



