310 RESPIRATION OF ANIMALS. 



or later,, are applied to uses by which some branch of 

 human labour is assisted, the necessities of man's con- 

 dition relieved, and the amenities of life advanced. 



The respiration of animals is an instance of the same 

 kind of chemical phenomena as we discover in ordinary 

 combustion. In the lungs the blood becomes charged 

 with oxygen, derived from the atmospheric air, with which 

 it passes through the system, performing its important 

 offices, and the blood is returned to the lungs with the 

 carbonic acid formed by the separation of carbon from 

 the body which is thrown off at every expiration. It 

 will be quite evident that this process is similar to that 

 of ordinary combustion. In man or animals, as in the 

 burning taper, which is aptly enough employed by 

 poets as the symbol of life, we have hydrogen and 

 carbon, with some nitrogen superadded ; the hydrogen 

 and oxygen form water under the action of the vital 

 forces ; the carbon with oxygen produces carbonic acid, 

 and, by a curious process, the nitrogen and hydrogen 

 also combine to form ammonia.* 



All the carbon which is taken into the animal economy 

 passes, in the process of time, again into the atmosphere, 

 in combination with oxygen, this being effected in the 

 body, under the catalytic power of tissues, immediately 

 influenced by the excitation of nervous forces, which are 

 the direct manifestations of vital energy. The quantity 

 of carbonic acid thus given out to the air is capable of 

 calculation, with only a small amount of error. It 

 appears that upwards of fifty ounces of carbonic acid 

 must be given off from the body of a healthy man in 

 twenty-four hours. On the lowest calculation, the popu- 

 lation of London must add to the atmosphere daily 

 4,500,000 pounds of carbonic acid. It must 'also be re- 

 membered that in every process for artificial illumination, 

 and in all the operations of the manufactures in which 



* See note, ante, On the Chemical Theory of Respiration. 



