348 DR. STEVENS ON THE THEORY OF RESPIRATION. 



used had been carefully excluded from atmospheric air, and the hydrogen was pure ; 

 consequently the carbonic acid could have been derived from no other source than 

 the venous blood itself. 



7. A few ounces of venous blood were drawn into a double-necked bottle pre- 

 viously filled with hydrogen. After having been gently heated, the hydrogen was 

 found to contain carbonic acid. This experiment was made at Copenhagen in the 

 beginning of 1833^ by Professor Forchammer and myself; but the conclusion which 

 we drew from it respecting the existence of carbonic acid in the blood was by some 

 objected to, in consequence of the interference of heat : the air-pump experiments, 

 however, remove all such objections. 



Dr, Edwards confined some animals in an atmosphere of hydrogen, and they con- 

 tinued to live for a considerable period, during which it was found that the hydrogen 

 had acquired a portion of carbonic acid, which in some cases was equal in bulk to 

 the size of the animals. By some these experiments were considered as conclusive of 

 the evolution of carbonic acid from venous blood ; but others maintained that there 

 might have been a sufficient quantity of oxygen in the pulmonary cells to account 

 for the formation of the carbonic acid. This objection is also removed by the above 

 experiment. 



Dr. Mitchell of Philadelphia made an experiment in 1830 with hydrogen and 

 venous blood, but without obtaining any carbonic acid. Mr. G. H. Hoffman of Mar- 

 gate made a similar experiment in 1832, and obtained a sufficient quantity of car- 

 nic acid, not merely to render lime water turbid, but even to render the hydrogen 

 uninflammable. These contrary results seem to me to have arisen from Dr. Mitchell 

 having used the air-pump immediately, and before the hydrogen had time to act on the 

 blood so as to displace its carbonip acid ; whereas Mr. Hoffman agitated the hydrogen 

 with the blood, and probably allowed a sufficient time for their mutual action. 



8. A small quantity of venous blood was drawn into the double-necked bottle, 

 containing atmospheric air, the valve at the orifice of the ascending tube was closed, 

 and the orifice of the descending tube was immersed in barytic water. The bottle 

 was put under the receiver and the pump immediately used. But in this experiment 

 the barytic water was not more affected than it would have been by a similar quan- 

 tity of common air. This proves that when the blood is exposed even to common 

 air, carbonic acid cannot be obtained, when the pump is used immediately, and before 

 any change of colour in the blood has taken place ; that is, before the air has had 

 time to act upon it. 



9. A small quantity of blood was drawn into the double-necked bottle containing 

 atmospheric air, as in the last experiment. Both of the valves were closed, and after 

 agitation, the bottle was allowed to stand about an hour, during which the colour of 

 the blood changed from venous to arterial. The lower orifice was then immersed in 

 barytic water, the apparatus was put under the receiver of the air-pump, and when 

 the pump was used, the gas which escaped gave a milky appearance to the barytic 



