376 THE HUMAN BODY. 



tively little of that gas. The headache and drowsiness 

 which come on from sitting in a badly ventilated room, and 

 the want of energy and general ill-health which result from 

 permanently living in such, are dependent on a slow poison- 

 ing of the Body by the reabsorption of the things elimi- 

 nated from the lungs in previous respirations. What these 

 are is not accurately known; they doubtless belong to those 

 volatile bodies mentioned above, as carried off in minute 

 quantities in each breath; since observation shows that the 

 air becomes injurious long before the amount of carbon 

 dioxide in it is sufficient to do any harm. Breathing air 

 containing one or two per cent of that gas produced 

 by ordinary chemical methods does no particular injury, 

 but breathing air containing one per cent of it produced 

 by respiration is decidedly injurious, because of the other 

 things sent out of the lungs at the same time. Carbon 

 dioxide itself, at least in any such percentage as is com.- 

 monly found in a room, is not poisonous, as used to be 

 believed, but, since it is tolerably easily estimated in air, 

 while the actually injurious substances evolved in breath- 

 ing are not, the purity or foulness of the air in a room is 

 usually determined by finding the percentage of carbon, 

 dioxide in it: it must be borne in mind that to mean muck 

 this carbon dioxide must have been produced by breathing; 

 the amount of it found is in itself no guide to the quantity 

 of really important injurious substances present. Of 

 course when a great deal of carbon dioxide is present the 

 air is irrespirable: as for example sometimes at the bottom 

 of wells or brewings ats. 



In one minute (p. 366) .5 X 15 = 7.5 liters (0.254 cubic 

 feet) of air are breathed and this is (p. 374) vitiated with 

 carbon dioxide to the extent of rather more than four per 

 cent; mixed with three times its volume of external air, 

 it would give thirty liters (a little over one cubic foot) 

 vitiated to the extent of one per cent, and such air is not 

 respirable for any length of time with safety. The result 

 of breathing it for an evening is headache and general 

 malaise; of breathing it for weeks or months a lowered 

 tone of the whole Body less power of work, physical or 



