402 THE HUMAN BODY. 



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 commonly 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 breathing 

 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 much 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 brewing-vats. 



In one minute .5 X 15 = 7.5 liters (0.254 cubic feet) of 

 air are breathed and this is 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 breath- 

 ing it for weeks or months a lowered tone of the whole Body 

 less power of work, physical or mental, and less power of 

 resisting disease; the ill effects may not show themselves at 

 once, and may accordingly be overlooked, or considered scien- 

 tific fancies, by the careless; but they are nevertheless there 

 ready to manifest themselves. In order to have air to breathe 

 in a fairly pure state every man should get for his own 

 allowance at least 23,000 liters of space to begin with 

 (about 800 cubic feet) and the arrangements for ventilation 

 should, at the very least, renew this at the rate of 30 litres 

 (one cubic foot) per minute. The nose is, however, the best 

 guide, and it is found that at least five times this supply of 

 fresh air is necessary to keep free from odor a small room 

 inhabited by one adult. In the more recently constructed 

 hospitals, as a result of experience, twice the above minimum 

 cubic space is allowed for each bed in a ward, and the re- 

 placement of the old air at a far more rapid rate is also 

 provided for. 



Ventilation does not necessarily imply draughts of cold 

 air, as is too often supposed. In warming by indirect radia- 



