6 Mr Bar croft, Apparatus for the 



according as the stopper is turned. Besides the passage F, the 

 stopper is bored from end to end (E, G). The orifice E is drawn 

 off into a tube which is closed by a tap H. Perhaps the 

 mechanism will be made most clear by a description of the actual 

 manipulation in the measuring of a sample of blood. The whole 

 burette, and its connections, are filled with mercury by raising the 

 reservoir J, the tap H is opened so that the mercury may rise and 

 fill the passages, G and E, in the stopper A. (The bottom of the 

 stopper has been carefully ground with sharp angles at K to 

 prevent bubbles of air getting caught at the sides.) H is then 

 closed and the stopper turned so that F is continuous with G. 

 Thus the passage F and the tube G are filled with mercury. 

 By means of a small piece of india-rubber tube G is connected 

 with the cannula which is already full of blood. F is now again 

 made continuous with C, the reservoir J is lowered a little, and 

 thus some blood, and any air that is in the cannula finds its way 

 into B. Perhaps 1 c.c. of blood may be taken at this juncture. 

 The stopper is now turned through 90° so that F becomes blind. 

 J is raised, H is opened, and when the blood and air have been 

 expelled U is closed again. After this preliminary the experiment 

 may begin. Blood is drawn into B by the process just described, 

 the surface of the mercury in J being kept at the same level as 

 that of the mercury in B, except when some special manipulation 

 is required. When about 8 c.c. has been collected, and the reading 

 on the graduated scale of B taken, that stopper is turned through 

 180°, so that F becomes continuous with D. As soon as this takes 

 place all the blood is sucked into the vacuous receiver and some 

 mercury is allowed to follow to wash the blood over. 



The reading of the blood-mercury surface on the graduated 

 scale has been taken. The question arises, "Supposing this 

 reading to be, say, 7 c.c, how much blood is actually delivered 

 into the receiver ? What constant must be added to this reading, 

 which starts from an arbitrary zero, to get the exact amount of 

 blood which is subjected to the vacuum ?" The answer to this 

 question is : The burette must be calibrated by taking out the 

 stoppers from beneath each of the froth chambers successively, 

 and allowing the blood measured in the burette to flow out of 

 the apparatus through the stopperless tap into a graduated tube. 

 The reading in the tube may then be compared with that of the 

 burette. In such a calibration the first reading in the graduated 

 tube will be fractionally lower than the successive ones. To 

 correct for this, a. small bulb is placed at the end of what will 

 hereafter be described as the " blood-main," so that a film of blood 

 may be made along the glass before the first sample is taken into 

 one of the vacuous chambers. 



Passing from the measuring apparatus to that which repre- 



