148 PHYSIOLOGY CHAP. 



test-tubes. In the first series the fluid will be red in some, colourless in 

 others. 



When, e.y., they are red in the test-tubes to which 3*1, 3'0, and 2'9 c.c. of 

 water have been added, and colourless in the rest, the result works out as'/ 

 follows : The mixture of 5 c.c. serum + 2*9 c.c. water shows diffusion of blood 



Slgment, while the mixture of 5 c.c. serum + 2*8 c.c. water remains colourless, 

 n examining the second set of test-tubes, the fluid is seen to be tinted in a 

 .saline solution of 0'58 per cent and in the weaker solutions, while the contents 

 of the tubes with the stronger solutions, 0'59, O60 per cent, etc., remain 

 tintinged. 



The mixture of 5 c.c. .serum + 9 water is therefore isotonic with a 



solution of NaCl at )9 -+-- -0'585 per cent. 



Accordingly in calculating the NaCl solution isotonic with the normal 

 non-diluted serum, the following equation may be employed : 



5: 5 + 2-85 = 0-585 :z, 



5 + 2'85xO'585 

 whence it follows that : x= - O92 per cent. 



In this case the blood .serum is isotonic with a NaCl solution of 0'92 per 

 cent. . ' 



Raoult's Method.- The determination of osmotic pressure by this method 

 is more easily carried out. The apparatus commonly adopted is that of 

 Beckmann (Fig. 43). It consists essentially of a glass vessel u, which is filled 

 with a free/ing mixture (crushed ice and salt), a test-tube B introduced to a 

 certain depth in the vessel (7, and a longer tube A fitted with a lateral tube y 

 which also dips into the tube B. The tube A is closed with a cork, through 

 the centre of which passes the special Beckmann thermometer D (or an 

 ordinary thermometer with a scale divided into hundredths of a degree) and 

 platinum wire F, which is bent into a loop at its lower end. This platinum 

 wire, which is intended to stir the fluid contained in the glass tube A, is 

 automatically set in motion by a little motor driven by water or electricity 

 or other power. In using the apparatus the vessel C is first filled with the 

 freezing mixture, then a few c.c. of serum are poured into the tube A till the 

 bulb of the thermometer is covered, when the stirrer F is set in motion. The 

 mercury column of the thermometer must be watched until, after sinking, it 

 rises again, and then remains for a few seconds at the temperature attained r 

 which is the freezing-point of the liquid. 



In practice, it is usual to assist the freezing and rise of the thermometer 

 by dropping a small crystal of ice into the liquid through the lateral tube E. 



When the freezing-point, which Raoult indicates by A , has thus been 

 obtained, it is easy in the case of blood serum to calculate the solution of 

 ' XaCl with which it is isotonic. If, &//., with ox serum, A = 0'55, when the 

 1 per cent solution is found to freeze at - 0'588, it can easily be calculated 

 that the NaCl solution isotonic with the serum under examination is equal 

 to 0'90 per cent. 



Haematocrite Method. This method (adopted by Hedin, Gartner, 

 Daland, Koppe, Eykman, Gryns, Maiica) is founded on the property possessed 

 by the red corpuscles of varying their volume with the variations of the 

 solutions with which they are in contact. On studying the action of 

 solutions of different concentration of the same substance (provided there is 

 no destructive action on the erythrocytes), these become smaller in more con- 

 centrated solutions, larger in more dilute solutions ; their volume is constant 

 only in a solution which is weaker than that which crenates the corpuscles, 

 and stronger than others which make them swell out On experimenting 

 with various substances these! observer's found for each a solution at which 



