2 $6 PHYSIOLOGICAL CHEMISTRY. 



follow with even very trifling changes in the osmotic pressure 

 of a liquid with which the blood may be mixed. The blood 

 corpuscle may be used then as a kind of indicator to disclose 

 variations in osmotic pressure, and substances may be com- 

 pared as to the osmotic pressure they exert by noting their 

 behavior with the corpuscles. 



It would of course be very difficult to prove anything by 

 measurements on a single corpuscle, but it is possible to make 

 the observation on a large volume. If blood is drawn up into 

 a narrow tube of capillary dimensions, placed in a centrifuge 

 and rapidly rotated the corpuscles are thrown to the outer end 

 of the tube, which must be closed of course. The volume oc- 

 cupied by the corpuscles compared with the original blood 

 volume may be easily seen. 



Koppe's Hematocrit. An instrument in which such an 

 observation may be accurately and easily made was devised 

 by Hedin and called the hematocrit. A special form of this 

 apparatus was constructed by Koppe and is used for the pur- 

 pose of comparison of corpuscle volumes. The essential part 

 of the apparatus, as shown in the figure, is a graduated capil- 

 lary pipette about 7 cm. in length, which may be closed at both 

 ends by small metal plates. At the upper end the capillary 

 bore is widened out so as to form a small mixing vessel. The 

 pipette proper has a graduation of 100 divisions. By means 

 of a syringe attached to the pipette by a bit of rubber tubing 

 blood may be drawn up into the capillary and its volume accu- 

 rately noted. The pipette may be closed and rotated now rap- 

 idly in the centrifugal machine, which throws the corpuscles to 

 the outer end. To prevent coagulation it is best to moisten 

 the pipette first with a layer of cedar oil which does 'not inter- 

 fere with the reading of the blood volume. The relation be- 

 tween blood volume and corpuscle volume may thus be read 

 off on the graduation. A drop of similar fresh blood is next 

 drawn into the capillary and its volume noted; following this 

 some solution is drawn in also, and then with the blood up into 

 the wider mixing part, where by means of a bright, fine wire 



