EFFECTS OF RESPIRATION ON THE AIR. 531 



strate that an exhalation of carbonic acid may continue for a considerable period 

 (in cold-blooded animals especially), during which the animal is breathing an 

 atmosphere in which no oxygen exists. Thus it was shown by Spallanzani, 1 

 that Snails might be kept for a longtime in Hydrogen, without apparent injury 

 to them } and that during this period they disengaged a considerable amount 

 of Carbonic acid. Dr. Edwards 3 subsequently ascertained that, when Frogs 

 were kept in hydrogen for several hours, the quantity of carbonic acid exhaled 

 was fully as great as it would have been in atmospheric air, or even greater ; 

 this latter fact, if correct, may be accounted for by the superior displacing 

 power which (on the laws of the diffusion of gases) hydrogen possesses for car- 

 bonic acid. Collard de Martigny 3 repeated this experiment in Nitrogen, with 

 the same results. In both sets of experiments, the precaution was used of com- 

 pressing the flanks of the animal, previously to immersing it in the gas, so as to 

 expel from the lungs whatever mixture of oxygen they might contain. These 

 experiments have been since repeated by Miiller and Bergemann, who took the 

 additional precaution of removing, by means of the air-pump, all the atmospheric 

 air that the lungs of the frog might previously contain, together with the car- 

 bonic acid that might exist in the alimentary canal. They found, in one of 

 their experiments, that the quantity of carbonic acid exhaled in hydrogen was 

 nearly a cubic inch in 6i hours; and, in another, that nearly the same amount 

 was given off in nitrogen, though this required rather a longer period. It ap- 

 pears from the table of their results, 4 that the amount was not ordinarily greater 

 in the experiments which were prolonged for twelve or fourteen hours, than in 

 those which were terminated in half the time ; hence it may be inferred that 

 the quantity which the blood is itself capable of disengaging is limited, and that 

 the absorption of oxygen is necessary to enable carbonic acid to be set free from 

 the body. It is impossible, however, for an adult Bird or Mammal to sustain 

 life for any considerable time in an atmosphere deprived of oxygen ; since the 

 greatly-increased rapidity and energy of all their vital operations, necessitate a 

 much more constant supply of this vivifying agent than is needed by the inferior 

 tribes ; and, as we shall presently see, the capillary action requisite for the pas- 

 sage of the blood through the lungs will not take place without it. But Dr. 

 Edwards has shown, that young Mammalia can sustain life in an atmosphere of 

 hydrogen or nitrogen, for a sufficient length of time to exhale a sensible amount 

 of carbonic acid ; so that the character of the process is clearly proved to be the 

 same in them as in Reptiles and Invertebrata. 



568. Much discussion has taken place with regard to the degree in which the 

 proportion of Nitrogen in the air is affected by Respiration. It seems probable 

 that the absorption and exhalation of this gas are continually taking place ; but 

 that the two amounts usually nearly balance each other. 5 On the whole, how- 

 ever, there is adequate reason to believe that Nitrogen is usually given off; this 

 being the joint result of the analysis of the expired air, and of the comparison 

 of the Nitrogen given off in the other excretions with that ingested as a con- 

 stituent of the food. Of the experiments made in the former of these methods, 

 the most accurate. are those of MM. Regnault and Reiset, whose general conclu- 

 sions are as follows : (1.) That warm-blooded animals subjected to their ordi- 

 nary regimen exhale nitrogen, but never in larger proportion than l-50th, and 

 sometimes in less than l-100th, of the oxygen consumed : (2.) That in a state 

 of inanition, animals usually absorb nitrogen : (3.) That animals whose usual 



1 "Memoires sur la Respiration," traduits par Senebier, Geneve, 1804. 



2 De 1' Influence des Agens Physiques sur la Vie ;" Paris, 1824. 



3 "Recherches Experinientales," &c. in Magendie's "Journal de Physiologie," torn. x. 



4 Mailer's "Elements of Physiology," translated by Baly, p. 338. 



5 For the considerations which render this probable, see especially Dr. W. F. Edwards, 

 " On the Influence of Physical Agents on Life," Part iv. chap. xvi. sect. 2 r 3. 



