THEORY. 391 



surface of the lungs in a manner corresponding entirely to known 

 physical laws as well as to positive facts ; there are few theories 

 in physiology which have resulted from such numerous and care- 

 fully conducted experiments as those which Vierordt established, 

 and which he based upon the laws of absorption discovered by 

 Henry and Dalton. Henry has shown that the quantity or the 

 volume of an absorbed gas depends entirely upon the pressure 

 under which the gas above the fluid remains after the absorption 

 has been completed ; while Dalton has proved that in the mixed 

 gases the pressure of each individual gas, which, as is well known, 

 is entirely independent of that of the intermixed gases, alone 

 determines the proportion in which this gas is absorbed by a fluid. 

 If, therefore, there be more carbonic acid contained in the blood 

 than the pressure of the carbonic acid in the pulmonary vesicles 

 is able to maintain in a state of condensation, a corresponding 

 quantity will escape from the blood, until the amount of carbonic 

 acid in the blood is reduced to the number corresponding to the 

 amount which would be absorbed by blood containing no carbonic 

 acid, and exposed to a tension equal to the carbonic-acid pressure 

 on the pulmonary vesicles. The quantity of carbonic acid thus 

 passing into the pulmonary vesicles would therefore depend, in 

 part, upon the quantity of this gas condensed in the blood, and 

 in part upon the tension of the carbonic acid gas already contained 

 in the air of the pulmonary vesicles. Under the relations 

 occurring in the animal body a motion in a directly opposite 

 direction would be imparted to the oxygen. The blood, when it 

 enters the lungs, is not sufficiently saturated with oxygen, and is 

 able, under the pressure which it then experiences, to absorb a 

 larger quantity of this gas ; the tension of the oxygen contained 

 in the pulmonary vesicles is so considerable, that a portion of it is 

 transferred into the blood, and there condensed. Both gases are 

 therefore quite independent of each other, as the more correct 

 physical explanation would lead us to infer ; their interchange is 

 not effected by mutual displacement, but is determined for each gas 

 by the quantity of condensed gas in the blood, and by the tension 

 of the corresponding elastic fluid gas contained in the air of the 

 pulmonary vesicles. 



There can be no doubt whatever that this law of Dalton 

 applies perfectly and completely to the free gas contained in the 

 blood (whether mechanically combined or absorbed), and hence it 

 must constitute one of the most important factors in the inter- 

 change of gases in the lungs ; but we have already seen that a very 



