DR. STEVENS ON THE THEORY OF RESPIRATION. 347 



From this experiment it is obvious that serum (consequently blood or other albu- 

 minous fluid,) may absorb carbonic acid, and so retain it as not to be separable by 

 the mere removal of the pressure of the air. 



The air-pump has hitherto been used almost exclusively by the experimenters on 

 venous blood ; and those who deny the existence of carbonic acid in it, do so almost 

 entirely on such evidence. The fact of their not having thus obtained this gas is cor- 

 rect ; but there is an error in the conclusion drawn from it, which is the chief cause of 

 the difference of opinion on this subject ; for such experiments only afford a positive 

 proof that carbonic acid cannot always be obtained from blood placed under the ex- 

 hausted receiver of the air-pump : but, with respect to the existence or non-existence 

 of such acid in the blood, the proof is merely negative ; for in experiment 4. the pump 

 did not separate carbonic acid from serum previously impregnated with it : conse- 

 quently such experiments are inconclusive. 



5. Carbonic acid was introduced into an empty bladder that had been previously 

 well moistened with warm water. When the bladder was distended about one third, 

 its neck was firmly tied with a waxed thread, by which it was suspended in the centre 

 of a receiver of an air-pump. When the pump was worked, the bladder increased in 

 volume, and in a few seconds was much distended. Nearly the whole of the atmo- 

 spheric air was exhausted from the receiver, but the bladder, though apparently very 

 tense, did not burst, neither did it decrease in size. A shallow glass vessel contain- 

 ing barytic water had been placed under the same receiver, but the transparency of 

 this was not affected. Hydrogen was then transmitted into the receiver, and the 

 bladder was reduced to the same size as when first suspended under it ; but, after an 

 interval of four hours, it had become perfectly flaccid. In fact, there was scarcely a 

 particle of carbonic acid left in it, and the barytic water within the receiver contained 

 a quantity of carbonate of baryta. 



6. The double-necked bottle was carefully filled with pure hydrogen, and about 

 five ounces of blood were drawn into it from a vein in the arm, in the same manner 

 as in experiment 3. Both the orifices of the bottle were then closed, and the blood 

 and the hydrogen well agitated together. After this the lower orifice was immersed 

 in distilled water, and the bottle left undisturbed for nearly an hour, to allow the 

 hydrogen to act on the blood. The orifice of the descending tube was then immersed 

 in barytic water, the lower valve was opened, and the whole apparatus put under the 

 receiver of the air-pump. When the pump was used, the gas which was over the 

 blood passed through the barytic water, and immediately rendered it turbid. This 

 experiment seems to prove that venous blood does contain carbonic acid ; and as the 

 only difference between experiments 3 and 6 was, that in the former the pump was 

 used immediately, and before the hydrogen had time to act on the blood, whilst in 

 the latter the hydrogen was allowed to act nearly an hour, it would appear that the 

 hydrogen has some power of removing the carbonic acid, and that this removal may 

 even take place through a membrane. In the last experiment^ the blood which was 



