374 THE HUMAN BODY 



dioxid 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 in- 

 jurious substances also present are not, the purity or foulness of 

 the air in a room is usually determined by finding the percentage 

 of carbon dioxid in it: it must be borne in mind that to mean 

 much this carbon dioxid 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 dioxid is present the air is irrespirable : as 

 for example sometimes at the bottom of we^ls or brewing-vats. 



In one minute .5X15 = 7.5 liters (0.254 cubic feet) of air are 

 breathed and this is vitiated with carbon dioxid 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 breath- 

 ing it for an evening is headache and general malaise; of breath- 

 ing it 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 scientific fancies, by 

 the careless; but they are nevertheless there ready to manifest 

 themselves. In order to have air to breathe in an even moder- 

 ately 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 liters (one cubic foot) per minute. 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 replacement of the old air at a far more rapid rate, 

 100,000 liters per hour per person, is also provided for. 



Ventilation does not necessarily imply draughts of cold air, as 

 is often supposed. In warming by indirect radiation (the ordi- 

 nary hot-air furnace) it may readily be secured by arranging, in 

 addition to the registers from which the warmed air reaches the 

 room, proper openings at the opposite side, by which the old air 

 may pass off to make room for the fresh. An open fire in a room 

 will always keep up a current of air through it, and is the healthiest, 



