XI 



KESPIEATORY EXCHANGES 



379 



the different gases are absorbed by the same liquid, varies considerably. One 

 volume of distilled water is capable of absorbing : 



Indifferent salts which are incapable of combining chemically with gases, 

 lower the absorption coefficients in watery solutions in proportion to their 

 concentrations. 



In the body it is always with gaseous mixtures in the fluids of the tissues, 

 and never with isolated gases, that we have to deal. We must, therefore, 

 investigate the absorption of gas mixtures by liquids. Since gases exert no 

 reciprocal pressure, one volume of liquid may absorb several gases successively 

 or simultaneously, in different volumes, according to the respective co- 

 efficients of absorption, and each gas absorbed is at a tension proportional 

 with the volume that it occupies in the mixture of absorbed gases. Bunsen 

 gave to this the name of partial pressure, because it represents the pressure 

 (in mm. Hg) which each gas would exert in the surrounding atmosphere, if 

 there were neither absorption nor emission of gas on the part of the liquid. 

 Since, e.g., the average pressure of atmospheric air is 760 mm. Hg, and it 

 consists in round numbers of 21 vols. per cent O 2 and 79 vols. per cent 

 N, the partial pressure of the oxygen absorption will be equal to 

 0-21 x 760 = 160 mm. Hg, and the partial pressure at which the absorption of 

 N occurs is equal to 0'79 x 760 = 600 mm. Hg. 



During absorption each gas of the mixture diffuses in the liquid in an 

 amount proportional to the difference in concentration of the adjacent layers, 

 as Graham shows for other substances in solution. Diffusion ceases and 

 absorption is complete when in all layers of the liquid, and in the atmosphere 

 with which it is in relation, complete equilibrium of tension for each of the 

 gases contained in the atmosphere, or dissolved in the liquid, is established. 



The rate of diffusion of a gas through a layer of liquid stands in direct 

 ratio with the solubility coefficient of the gas, and in inverse ratio with the 

 square root of its density. So that, <?.</., although the diffusion rate of the 

 molecules of H 2 is greater than that of the molecules of C0 2 , the latter being 

 more soluble in water than the former, more C0 2 than H 2 passes through a 

 layer of liquid in a given time. 



Generally speaking, the velocity of diffusion of a gas in a liquid is very 

 low (Duncan and Hoppe-Seyler, 1894). They found that at ordinary 

 barometric pressure and mean temperature, atmospheric air penetrates 

 extremely slowly into a given quantity of water closed on all sides, 

 save at the top where the air enters. After fourteen days of contact, 

 absorption in the lower layers of the column of water was still incomplete. 



V. The most important conclusions from the work carried out 

 under the directions of Ludwig and Pfliiger confirm the fact 

 already determined by Magnus, to the effect that the amount of 

 gas that can be extracted from arterial blood differs considerably 

 from that of venous blood. 



From the average of twelve analyses performed by Pfliiger with 

 the rapid method, it appears that the arterial blood of the dog 



