BLOOD GASES. 125 



syringe. This connection will vary with the exact shape of the head of the piston, but 

 in any case, free play must be possible at the joint. In the syringe used in this 

 laboratory the head of the piston is ring shaped and is satisfactorily connected 

 by fitting a wooden bobbin in the ring and attaching the brass strips to the end 

 of the bobbin. The syringe is surrounded by a glass cylinder containing a fairly 

 heavy mineral oil (Mobile oil) and this cylinder is attached to the iron lever so 

 that it moves up and down with the piston. 



The first step in preparing the blood pump is to get rid of all 

 the dead space in the tubing and connections. This is readily 

 accomplished, for E, by turning stopcock C so that E communicates 

 with B, raising the levelling burette (G), and simultaneously with- 

 drawing the piston of the syringe by depressing the lever until 

 about 20 mm. of mercury has collected on the top of the piston. 

 The stopcock is then turned so that B and D are connected and 

 the piston raised until all the air is expelled and mercury completely 

 fills tube D. Any drops of mercury falling from the open end of D 

 must be caught in a small beaker. The mercury left on the top of 

 the piston seals this completely during the subsequent manipula- 

 tions. 



After squeezing all air out of the tubing on the blood receiver, 

 this is connected with D, and immersed in a jug containing water 

 at 45 C. Having turned C so that B communicates with D, the 

 piston is then depressed to about the 20 c.c. mark, and while still 

 depressed the screw clip (1) is opened. About this time the blood 

 will begin to "boil" and the gases given off from it will pass into 

 the vacuum above the mercury in the syringe. C is turned so that 

 B is closed off and the piston allowed slowly to ascend. (It must 

 not be allowed to ascend too rapidly, since this might break the 

 syringe). The gas which has collected in the syringe is then ex- 

 pelled into the burette (F) by turning C so that B and E communi- 

 cate and pressing up the piston. After all the gas is out of the 

 syringe, the mercury is allowed to run into E a short distance, 

 being careful not to allow any to get into F. This first process 

 obviously removes only a small fraction of the total gas in the blood, 

 and it must be repeated several times exactly as described above, 

 until no more gas can be secured. The dislodgement of the gas 

 from the blood is greatly accelerated by warmth and by occasionally 

 removing the bulb from the water-bath and shaking briskly. 



It is now necessary to measure and analyse the evolved gas. 



