RESPIRATION 131 



greater than in the atmosphere. In the swim-bladder of fishes 

 (which is analogous to the lung of mammals) the oxygen is certainly 

 far in excess of anything that can be explained by mere diffusion. 

 The storage of oxygen, moreover, ceases when the vagus nerves 

 which supply the swim-bladder are divided. 



Some Continental observers have stated that certain noxious 

 substances are ordinarily contained in expired air, which are much 

 more poisonous than carbonic acid, but researches in this country 

 have entirely failed to substantiate this. If precautions be taken by 

 absolute cleanliness to prevent admixture of the air with exhalations 

 from skin, teeth, and clothes, the expired air only contains one noxious 

 substance, and that is carbonic acid. 



Tension of Gases in Fluids. It is necessary to understand 

 thoroughly the expression ' tension of gases in fluids ' ; we will 

 therefore go into the matter a little more fully. 



The first question which arises is, In what circumstances will a 

 gas, dissolved in a fluid, diffuse out of the fluid into the air in contact 

 with it ? or vice versa, in what circumstances will a gas diffuse out of 

 the air into the fluid, and at what rate will it do so ? 



The answer depends upon the physical constants of the fluid and 

 of the atmosphere ; and these must be determined experimentally. 



As an example the following instance may be taken : 100 c.c. 

 of water charged with 80 c.c. of carbonic acid are shaken with pure 

 air in a closed bottle of 500 c.c. capacity. The carbonic acid will 

 come out of the water at first, but as the shaking continues the 

 carbonic acid will come out more and more slowly until it entirely 

 ceases to do so. Analysis of the air and of the water in the bottle 

 would show that the water had not parted with all its carbonic acid. 

 It would be found that the water contained 16 c.c. of carbonic acid 

 dissolved in it, while 64 c.c. have diffused out into the air. 



At the end of this experiment, then, there will be in the atmosphere 

 64 c.c. of carbonic acid in a space of 400 c.c. If the whole space 

 (400 c.c.) were filled with carbonic acid its pressure would be 760 mm. 

 The partial pressure of carbonic acid in the atmosphere of the bottle 

 is therefore 760 x ^Vo mm. of mercury=122 mm. Water, therefore, 

 containing 16 volumes per cent, of carbonic acid is in equilibrium 

 with an atmosphere in which the carbonic acid exerts a pressure of 

 122 mm. This fact is stated shortly by the phrase ' when water has 

 a carbonic acid tension of 122 mm. it contains 16 per cent, of 

 carbonic acid.' 



The amount of carbonic acid which would be contained in any 

 other fluid when it was in equilibrium with an atmosphere having a 



K 2 



