Mitchell on the Penetrat-iveness of Fluids. 311 



of his experiments, with others, fully as conclusive, leave no doubt 

 of the entire absence of carbonic acid in the blood.* 



It must, therefore, be produced in one of two modes, either by 

 the penetration of oxygen into the blood, and its union there with 

 carbon, or the exit of carbon from the blood, to unite with oxygen 

 in the air-cells. Now, as carbon is one of the most fixed substances 

 in nature, and has not been proved capable of such transmission, we 

 are, if facts be our guide, compelled to adopt the other theory, 

 which is perfectly in accordance with the laws of gaseous infiltra- 

 tion. If it be asked hcuv the carbonic acid is formed in the blood, 

 at so low a temperature, we reply, that carbonic acid is actually cre- 

 ated at a lower temperature, by the agency of infiltration, when 

 oxygen gas is imbibed by a piece of fresh cold charcoal. The dif- 

 ference in the rate of permeation, is quited sufficient to account for 

 the escape of all the carbonic acid formed by the infiltrated oxygen. 



Our theory does not account for the production of animal heat, but 

 it is presumed that no well-informed physiologist now seeks for it in 

 the action of the lungs, or the process of decarbonization. The simple 

 fact, that cold-blooded animals breathe without any increase of tem- 

 perature, proves that mere breathing to any amount will not produce 

 heat. Like all the other animal functions, that productive of heat is 

 dependent on a normal condition of blood, and is thus indirectly go- 

 verned by the act of respiration. As in cold-blooded animals, there 

 is no apparatus for producing heat, respiration does not in any way 

 influence their temperature. So in some of the cases quoted by John 

 Hunter, where blue-boys maintained a temperature preternaturally 

 great, the blood was very imperfectly decarbonized. In such cases 

 the calorific function found some novel stimulant. 



Our experiments afford ready explanations of the effect of the 

 various gases when respired. Carbonic acid not only cuts off the 

 necessary supply of oxygen, but also penetrates into the blood, and 

 passes through the route of the circulation. 



We perceive why nitrous oxide, so identical with oxygen in all 

 its chemical habitudes, should act so differently on the human sys- 

 tem. It penetrates at least sixteen times as rapidly, and probably 

 acts then solely as oxygen would do. Hence we see why it does not 



* Having filled i phial with hydrogen gas, hlood was received into it from a 

 vein, so as to exclude the agency of oxygen. When completely full of blood, the 

 phial was closed hy sheet gum elastic, and immediately subjected to the action of 

 the air-pump. Under such circumstances, no gas of any kind could be immedi- 

 ately separated from the blood ; but after coagulation was completed, a bubble 

 of air, about the size of a pin's head, was perceived beneath the membrane, and 

 that that was atmospheric air, or nitrogen, was proved by its long continuance 

 there, without apparent diminution or escape. 



VOL. II. Nov. 1831. Y 



