\i RESPIRATION 127 



Ventilation. Air which has been breathed till it contains 

 more than .2 |>er cent of CO., is injurious, not so much on 

 account of the CO., present, but on account of the poisonous 

 nature of the organic matter which is given off by the lungs. In 

 one hour a man, by quiet respiration, will vitiate 3000 cubic feet 

 of air to this extent. Thus the air in a good-sized room, 1 8 feet 

 by 1 8 feet and I o feet high, containing 3240 cubic feet, should be 

 totally renewed every hour. Air is also -vitiated by lights in 

 the room ; a cubic foot of coal-gas produces when burnt 2 cubic 

 feet of CO.,, besides some sulphur dioxide and other substances. 

 One gas-burner consuming 3 cubic feet per hour of gas 

 would vitiate 3000 cubic feet of air to the extent of .2 per cent 

 of CO., in an hour. It is estimated that in the crowded rooms 

 in many small houses in England there is often less than 500 

 cubic feet of air space for each person. The air in such a 

 room should be completely changed six times in an hour to 

 keep the air pure enough to be breathed without injurious 

 effect. 



The lungs, then, are a source of loss, the body con- 

 stantly losing weight by the carbon lost as carbonic acid, 

 and by the hydrogen lost as water in the breath. Water 

 is also lost, as we shall see, in two other ways : by the 

 skin as perspiration, and by the kidneys as urine, in which, 

 compounds containing another important element, nitrogen, 

 are lost to the body. The lungs supply oxygen only. The 

 carbon, nitrogen, and hydrogen, and the other elements 

 the body requires, are united with one another and with 

 oxygrn to form the various substances of which the fowl we eat 

 consists, and this is the only means by which these elements 

 arc supplied. Let us consider now how the body yains these 

 elements by means of the food. 



