THE CONTROL OF THE RESPIRATION 337 



of the partial pressure which it produces in air. The same will be 

 the case with the nitrogen of the air. The actual amount of gas which 

 becomes dissolved in the fluid, pressure and temperature being constant, 

 depends partly on the nature of the gas and partly on the nature of the 

 fluid. For example, the solubility of oxygen in water is considerably 

 different from that in a neutral oil; or, taking the same solvent, nitro- 

 gen and C0 2 do not dissolve to the same extent in water. It becomes 

 necessary, therefore, in calculating what amount of a particular gas 

 will dissolve in a particular fluid to use a figure known as the coefficient 

 of solubility of the gas that is, the amount of gas taken up by a unit 

 volume of fluid at standard temperature and pressure ; for example, to 

 say that the coefficient of absorption of nitrogen in water at C. is 

 0.0239 means that, at this temperature and at normal barometric pres- 

 sure, 1 c.c. of water will dissolve 0.0239 c.c. of nitrogen when exposed to 

 a pure atmosphere of this gas. Obviously, then, if water were exposed 

 to 79 per cent of an atmosphere of nitrogen (as in air) the amount which 



79 

 would become dissolved in each c.c. would be -r/wrx 0.0239 = 0.0189 c.c. 



In solutions containing no chemical substances with which the gas can 

 enter into combination, it is evident that the tension of the gas will be 

 proportional to the amount of gas that can be displaced or pumped out 

 from the fluid. On the other hand, when a chemical compound is formed, 

 the combined gas will exercise no direct influence on the tension, so that 

 this will be independent of the amount; in suck cases separate methods 

 will have to be used for the determination of amount and tension. Let 

 us take the case of pure water exposed to an atmosphere of C0 2 : the 

 amount of C0 2 which goes into solution will . depend entirely on the 

 pressure. If a trace of alkali is dissolved in the water, however, some 

 of the C0 2 will become combined to form carbonate, so that a much 

 larger quantity of C0 2 will be displaceable from the solution (as by 

 adding a mineral acid to it) than corresponds to the tension of C0 2 in 

 the atmosphere surrounding it. Since blood contains alkali the condi- 

 tions are analogous with those of a weak alkaline solution. 



The Tension of C0 2 and 2 in the Arterial Blood. If we were to 

 pass blood at body temperature in a very thin film over the walls of a 

 confined space containing a mixture of gases one of which was C0 2 , it 

 is evident that the percentage of C0 2 in the atmosphere contained in 

 this space would remain unchanged only when the tension of this gas in 

 the blood was the same as that in the confined atmosphere. If, on the 

 other hand, the tension of CO, in the blood should correspond to a per- 

 centage that is higher than that in the atmosphere, then C0 2 would dif- 

 fuse from the blood, and at the end of the experiment an analysis of the 



