CH. XXVI.] 



TENSION OF GASES 



363 



D -fe 



the pressure which the gas exerts on the liquid, P'. If we call the 

 tension of the gas in the liquid, T, we find that when equilibrium 

 exists, P' = T. In the case of all true solutions, therefore, we may 



T 



replace P' in our previous equation by T; therefore, Q = Kx^p. 



We thus arrive at a relation between two separate things, which 

 must be most carefully distinguished from 

 one another the quantity of the gas dis- 

 solved in the liquid and its tension. 



Measurement of Tension. Numerous 

 instruments, called tonometers, exist for 

 measuring the tension of gases in fluids. 

 Of these, the instrument which has given 

 the most trustworthy measurements of 

 the oxygen and carbonic acid tensions in 

 circulating blood is that invented by Krogh 

 (fig- 292). 



A T-shaped cannula (A) is introduced 

 into the blood-vessel, say the carotid artery ; 



the blood fills the cavity B and leaves it at Flo . 292. Krogh 's Tonometer. 

 C, so that a constant stream of blood is kept 



flowing. Into it a small bubble of air (D) is introduced. Exchange 

 of gases takes place between the bubble and the blood, and the 

 former very soon gets into equilibrium with the latter. When it 

 has done so, the bubble is withdrawn up the capillary tube E, 

 taken away, and analysed. 



As an example, suppose the bubble on analysis proved to consist 

 of 4 per cent, carbonic acid and 12 per cent, oxygen, together with 

 nitrogen and aqueous vapour. The gas in the instrument was 

 compressed by the pressure of the arterial blood (say 120 mm. of 

 mercury) in addition to the atmospheric pressure of 760 mm. of 

 mercury, and therefore the total pressure was 120 + 760 = 880 mm. of 

 mercury. Four per cent, of this would have been due to the carbonic 

 acid ; 4 per cent, of 880 is 35-2. Twelve per cent, would have been 

 due to the oxygen; 12 per cent, of 880 is 105-6. That is, the 

 carbonic acid and oxygen tensions would have been in round figures 

 35 and 106 mm. of mercury respectively. 



Measurement of the Quantity of the Gases of the Blood. 



This may be done by means of an air-pump, or by a chemical 

 method. (1) The extraction of the gases from the blood by means 

 of the mercurial air-pump depends upon the fact that blood gives off 

 all its gases when it is boiled in a vacuum. The total gas obtained 

 is first measured; then the carbon dioxide is removed by caustic 



