242 FUNCTIONS OF NUTRITION. 



dies in five minutes when it is reduced to 10 per cent. ;* the remainder 

 of the air in both cases consisting of nitrogen. In man 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 test is sometimes employed to determine whether it be 

 safe to enter an atmosphere of doubtful composition. In bread-rooms 

 and beer-vats, where fermentation has been going on, in wells which 

 have been for a long time closed, or in old underground cavities or 

 passages, the atmosphere is frequently so poor in oxygen that it would 

 be unsafe to enter them without precaution. A lighted candle is, 

 accordingly, let down into the suspected cavity, and if sufficient oxygen 

 be present, it continues to burn ; if not, it is extinguished. 



This test is the more valuable, because the proportion of oxygen 

 necessary for the combustion of a candle is greater than that required 

 for the immediate support of respiration. A candle is extinguished 

 when the air contains only IT per cent, of its volume of oxygen, while 

 less than this may still serve a short time for respiration. According 

 to Milne-Edwards, a man may respire in an atmosphere which is in- 

 sufficient to support combustion ; and we have repeatedly seen pigeons 

 continue to breathe in air in which a candle flame was immediately 

 extinguished. 



But although an atmosphere containing from 10 to 17 per cent, of 

 oxygen is not at once fatal to man, it is still unfit for continued breath- 

 ing, and after a time its deleterious effects would become manifest. A 

 complete renewal of the deteriorated air, in such cases, is essential to 

 the perfect performance of respiration. 



Increase of Carbonic Acid. The expired air usually contains, in 

 man, about 4 per cent, of its volume of carbonic acid, which it has ab- 

 sorbed in the lungs. Rather less than 13 cubic centimetres of this gas 

 are, therefore, given off with each ordinary expiration ; and as 10,000 

 litres of air are inhaled and discharged during twenty-four hours, this 

 will give 400 litres of carbonic acid as the amount expired per day. 

 This quantity is, by weight, 786 grammes, or rather less than one 

 pound and three-quarters avoirdupois. 



The exhalation of carbonic acid by respiration varies, for the most 

 part, in a similar way, with the absorption of oxygen. In general, it 

 may be said, as the result of many trustworthy observations, both in 

 animals and man, that the carbonic acid exhaled during a given time, 

 is increased by muscular exertion, or any other physiological activity 

 of the system, and is diminished by quietude, during sleep, and in a 

 state of inanition. 



These facts were first established, particularly for the human subject, 

 by Scharling,f who found that the quantity of carbonic acid exhaled 



* Milne-Edwards, Leons sur la Physiologic. Paris, 1857, tome ii., p. 638. 

 f Annales de Chimie et de Physique. Paris, 1843, tome viii., p. 490. 



