CHANGES IN THE AIR BY RESPIRATION. 281 



If collected and examined, after passing through the lungs, the air is 

 found to have become altered in the following particulars: first, it has 

 lost oxygen; secondly, it has gained carbonic acid; and thirdly, it has 

 absorbed the vapor of water. The most important of these changes are 

 its diminution in oxygen and its increase in carbonic acid. 



Diminution of Oxygen. According to the researches of Valentin, 

 Yierordt, Regnault, and Reiset, the air loses during respiration, on an 

 average, five per cent, of its volume of oxygen. At each inspiration, 

 therefore, about 16 cubic centimetres of oxygen are removed from the 

 air and absorbed by the blood ; and, as we have seen that the daily 

 quantity of air used in respiration is about 10,000 litres, the entire quan- 

 tity of oxygen thus consumed in twenty-four hours is not less than 500 

 litres. This is, by weight, 715 grammes, or rather more than one pound 

 and a half avoirdupois. 



In consequence of this diminution in oxygen, air which has once been 

 breathed is less capable, both of supporting combustion and of serving 

 for respiration, than before. If an animal be confined in a limited space, 

 the air becomes poorer in oxygen as respiration goes on ; and when its 

 proportion has been reduced to a certain point, the animal dies by suf- 

 focation, because the substance which is essential to life is no longer 

 present in sufficient quantity. Different kinds of animals are affected in 

 different degrees of intensity by a given diminution in the proportion of 

 atmospheric oxygen. Cold-blooded animals, in which respiration is 

 naturally a comparatively slow process, may continue to breathe when 

 only a very small quantity of oxygen is present ; and it has been found 

 that electrical fishes, as well as slugs and snails, may continue respira- 

 tion until they have completely exhausted the oxygen in the water or 

 the air in which they are confined. But in species where the respira- 

 tion and circulation are carried on with activity, as in birds, in quad- 

 rupeds, and in man, a partial reduction of the oxygen is sufficient to 

 cause death. If the carbonic acid exhaled be absorbed by an alkaline 

 solution, so that the purity of the air be maintained, it is found that 

 a sparrow dies in an hour when its proportion of oxygen has been 

 gradually reduced to 15 per cent. ; and a mouse dies in five minutes 

 when the oxygen is reduced to 10 per cent.; 1 the remainder of the air 

 in both cases consisting of nitrogen. In man, also, asphyxia is almost 

 immediately produced when the proportion of oxygen has fallen to 10 

 per cent. 



As a candle flame is also extinguished in an atmosphere deprived of 

 oxygen, this is sometimes employed as a test to determine whether it 

 be safe to enter an atmosphere the composition of which is doubtful. 

 In bread-rooms and beer-vats, where the process of fermentation has 

 been going on, in old wells which have been for a long time closed, or 

 in any newly opened underground cavity or passage, the atmosphere is 

 frequently so poor in oxygen that suffocation would at once follow if 



1 Milne-Edwards, Lec.ons sur la Physiologie. Paris, 1857, tome ii. p, 638. 

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