504 PRACTICAL ORGANIC AND BIO-CHEMlSTRY 



(4) Barcroffs Differential Method. 



The oxygen content and the oxygen capacity of two samples of blood, 

 arterial and venous, of a volume of i c.c. or -i c.c. can be determined by 

 the differential method which has been elaborated by Barcroft. In this 

 method either the form of apparatus used in the previous method or the later 

 pattern (Figs. 69 and 70) can be used. 



The later pattern of apparatus consists of two egg-shaped bottles fitted 

 with hollow stoppers. In that for i c.c. of blood the stoppers are prolonged 

 into the interior of the flask and terminate in small cups to hold the ferri- 

 cyanide solution. In that for T c.c. of blood there is a small receptacle in the 

 bottle to hold the ferricyanide The stoppers outside the flasks are continued 

 into capillary tubes to each of which there is fused a 3-way tap. One end of 

 each tap is fused to a small pressure gauge. The interior of each vessel can 

 be thus connected to the air or with one limb of the manometer (or with both). 



Fio. 69. (From Barcroft's " Respiratory Function of the FIG. 70. 



Blood," Cambridge University Press.) 



The apparatus is mounted on a stand upon the front of which there is a scale 

 for reading the manometer and upon the back a hook so that the two flasks, 

 which are at the same level, can be immersed in a large bath at constant 

 temperature, whilst the manometer hangs vertically outside the bath and the 

 taps are easily accessible. 



The manometer is filled to about half-way up each limb with clove oil; 

 in order that it registers properly it must be absolutely clean and dry. The 

 filling of the manometer with oil is done by removing one of the stoppers and 

 turning the other stopper so that the manometer is shut off from the other bottle 

 and the air. The oil is sucked up into a pipette, the end of the pipette is 

 put through the gap where the stopper has been taken out and pushed as far 

 as the top of the manometer. Sufficient oil is introduced to fill up the one 

 limb of the manometer and as far as the zero point of the other limb, the tap 

 on the other side being carefully opened to the air so that the oil can come 

 up to the zero point on this side of the manometer. The tap is closed and if 

 excess of oil has been put in, it is removed with a sponge on a piece of wire 

 introduced in the same way as the pipette. On opening the tap the oil will 

 occupy the proper position. The other tap is replaced. 



When the taps on both sides are closed to the air and open to the bottles 

 any difference in pressure in the two bottles is recorded by the manometer, 

 the clove oil moving down on the side of greater pressure and up on the 

 other side. The pressure is given by the difference between the two sides. 



