RESPIRATION 233 



of the piston be balanced by a counterpoise. The pressure at the 

 surface of the liquid is evidently that of the atmosphere. Now, let 

 the whole be put into the receiver of an air-pump, and the air gradu- 

 ally exhausted. Let exhaustion proceed until gas begins to escape 

 from the liquid and lies in a thin layer between its surface and the 

 piston, the quantity of gas which has become free being very small 

 in proportion to that still in solution. At this point the piston is 



A, the blood bulb ; B, the froth 

 chamber ; C, the drying tube ; D, fixed 

 mercury tube ; E, movable mercury bulb 

 connected by a flexible tube with D ; F, 

 eudiometer ; G, a narrow delivery tube ; 

 i. 2, 3, 4, taps, 4 being a three-way tap. 

 A is filled with blood by connecting the 

 tap i by means of a tube with a blood- 

 vessel. Taps i and 2 are then closed. 

 The rest of the apparatus from B to D 

 is now exhausted by raising E, with tap 

 4 turned so as to place E only in com- 

 munication with G, till the mercury fills 

 D. Tap 4 is now turned so as to con- 

 nect C with D, and cut off G from D, 

 and E is lowered. The mercury passes 

 out of D, and air passes into it from B 

 and C. Tap 4 is again turned so as to 

 cut off C from D and connect G and D. 

 E is raised, and the mercury passes into 

 D and forces the air out through G, the 

 end of which has not hitherto been 

 placed under F. This alternate raising 

 and lowering of E is continued till a 

 manometer connected between C and 4 

 indicates that the pressure has been 

 sufficiently reduced. The tap 2 is now 

 opened ; the gases of the blood bubble 

 up into the froth chamber, pass through 

 the drying-tube C, which is filled with 

 pumice-stone and sulphuric acid, and 

 enter D. The end of G is placed under 

 the eudiometer F, and by raising E, 

 with tap 4 turned so as to cut off C, 

 the gases are forced out through G and 

 collected in F. The movements re- 

 quired for exhaustion can be repeated 

 several times till no more gas comes off. 

 The escape of gas from the blood is 

 facilitated by immersing the bulb A in 

 __ water at 4o-5o C. 

 FIG. 83. SCHEME OF GAS-PUMP. 



acted upon by two forces which balance each other, the pressure of the 

 air in the receiver acting downwards, and the pressure of the gas escap- 

 ing from the liquid acting upwards. If the pressure in the receiver 

 is now slightly increased, the gas is again absorbed. The pressure at 

 which this just happens, and against which the piston is still sup- 

 ported by the impacts of gaseous molecules flying out of the liquid 

 while no pressure is as yet exerted directly between the liquid and 

 the piston, is obviously equal to the pressure or tension of the gas in 

 the liquid. 



From the above principles it follows that a gas held in solution 



