24 



MODE OF COLLECTING BLOOD. 



[BOOK i. 



so that its lower part is above the level of the stop-cocks of A and B. 

 These are now opened, mercury rises into the tubes, driving the air 

 which they previously contained before it; when the tubes are filled and 

 a stream of mercury is issuing from them, the stop-cocks are closed. In 

 order to determine whether the stop-cocks do not leak, the reservoir R 

 may now be held in the hands of the experimenter at thirty-five or forty 

 inches below the stop-cocks of A and B. The mercury in these tubes will 

 naturally fall at first and then remain steady: on raising the reservoir 

 cautiously the metal should however rise and fill the tubes completely. 



Fm. 8. APPARATUS FOR COLLECTING BLOOD OVER MERCURY WITHOUT ALLOWING IT 



TO COME IN CONTACT WITH AIR. 



In order to collect apart arterial and venous blood, glass cannulae, 

 to which are attached narrow elastic tubes of considerable length, are in- 



