PHYSICAL METHODS IX BLOOD EXAMINATION. 



257 



the two liquids may be stirred together. The plates are then 

 put on the ends of the pipette, where they are held by springs. 

 The pipette may be rotated as before in the centrifugal ma- 

 chine, the rotation being continued until the volume occupied 

 by the corpuscles becomes constant. By using a number of 

 pipette tubes it is possible to employ different mixtures and 



d 



FIG. 25. The essential part of the Koeppe hematocrit. The measuring tube 

 a is closed by two plates, b and c, which are held fast by the springs d. The 

 tube is filled by means of a peculiar syringe shown at the right. 



soon find one in which the corpuscle volume remains normal. 

 If a series of sugar or salt solutions of known osmotic pres- 

 sure are employed, that of the blood must be taken as equiva- 

 lent to the pressure in the solution for which no change in the 

 volume of the corpuscles occurs. 



Conversely the apparatus may be, and is, frequently employed to find 

 osmotic pressures of solutions. The volume of the corpuscles is found 

 in some solution, of cane sugar for example, which has about the same 

 osmotic pressure as the blood, but which must be accurately known. 

 Then other solutions of a new substance are tested until two are found 

 which give volumes, one greater and the other less than that with the 

 sugar. A simple calculation will then give the concentration of the solu- 

 tion of the substance under comparison which has the same osmotic 

 pressure as the standard sugar solution. The method would naturally 

 fail for any substance which acts chemically on the blood or which de- 

 stroys the corpuscles, such as urea or glycerol. 

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