128 RESPIRATION. LECT. VI. 



is from to almost \ in arterial blood, which it is only 

 from J to even | in venous blood. 



Lastly, when we consider the means by which we ex- 

 tract the gases from the blood, such as the presence of hy- 

 drogen, or a vacuum, it becomes evident that these gases 

 are dissolved there. Hence we must admit, that the gases 

 thus disengaged from the blood, are set free by the presence 

 of other gases, obeying, in this respect, the physical laws 

 relating to the interchange which takes place between gases 

 dissolved in liquids, and those which are free. 



We have seen that the change of colour, which venous 

 blood undergoes in becoming arterial, a change effected 

 by oxygen, also takes place when oxygen is separated 

 from the blood by a membrane. It is essential, therefore, 

 to prove that these phenomena, that is, the reciprocal ac- 

 tion of the gases, and the modification which the colour of 

 the blood undergoes, are effected out of the living body, 

 through layers of membranes, and in virtue of laws which 

 are altogether physical. 



Any gas placed in a well-closed bladder, soon traverses 

 it, and filters rapidly through its pores ; while, at the same 

 time, atmospheric air is introduced in its place. If the 

 volume of the exterior gas be not infinite, compared to that 

 of the gas contained within the bladder, the interchange 

 soon ceases, and both outside and inside we shall find an 

 uniform mixture of the two gases. Place a bladder, filled 

 with water slightly acidulated with carbonic acid, under a 

 bell-glass receiver, filled with hydrogen, oxygen, or azote, 

 and a portion of carbonic acid will leave the water, and be 

 found free in the receiver ; while, at the same time, a por- 

 tion of the exterior gas will supply its place, by becoming 

 dissolved in the water. In general, two gases, one of which 

 is free, or dissolved in a liquid, and the other separated from 

 it by a membrane, mix in definite proportions. 



