8(38 INFLUENCE OF THE MEPHITTC GASES 



But he adds, " those who wish to refrain from 

 breathing should avoid action, which exhausts the 

 air in the lungs of its vital principle more quickly, 

 and charges it with lad matter." (Phil. Mag. 3rd 

 series, vol. iii. p. 243.) It is very true that the 

 vital principle is exhausted by exertion, not, 

 however, in the lungs, for there it is obtained, and 

 imparted to the blood, but in the muscles, where 

 it is expended in causing their action. 



Since the time of Cullen, it has been generally 

 supposed that the mephitic gases produce dis- 

 ease and death, by operating as positive poisons 

 on the nervous system, and not primarily on the 

 blood. But carbonic acid, nitrogen, and hydro- 

 gen cannot be regarded as positive poisons, for 

 the obvious reason, that they are at all times 

 present in the lungs. And I shall proceed to 

 show, that they produce their deleterious effects 

 by diminishing respiration, and preventing the 

 due arterialization of the blood ; that as blood 

 is the fountain of life, and the substratum of the 

 solids, it must be primarily affected by what- 

 ever impairs the chemical function of the lungs, 

 in which it is generated. When animals are im- 

 mersed in carbonic acid, or any of the mephitic 

 gases, they die in about the same time as if con- 

 fined in a vacuum, or as when respiration is sus- 

 pended by strangulation or immersion under 

 water. Under such circumstances, birds die 

 sooner than mammalia, and the latter sooner 

 than reptiles or fishes, because respiration is more 



