148 PHYSIOLOGY CHAP. 



teM-t ill"--. In lln' firs! series tin' fluid will In- red in .-ome, colourless in 

 others. 



When, ,-.</., they a IT red in the trsi-t ul>r.- lo which JM, 3 - 0, and 2 - 9 c.c. of 

 water have l>een added, and colourless in the rest, the result works out as 

 follows :- The mixture of f> c.c, serum + 2'9 c.e. water shows diffusion of l)lood 

 pigment, while 1 the mixture of 5 c.c. serum+2'8 c.c. water remains colourless. 

 On examining the second set of test-tubes, the Huid is seen to )>e 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 ])er cent, etc., remain 

 untinged. 



_;<) _j. -i-*, 



The mixture of ~> c..c. serum + - ..," \\ater is therefore isotonic with a 



solution of NaCl at =0'585 per ecu!. 



4 



Accordingly in calculating the NaCl solution isotonic with the normal 

 non-diluted serum, the following equation may l>e employed : 



f) : 5 + 2-85 = 0-58.-) : z, 



5 + 2-85x0-585 

 whence il follows that : x =0*92 per cent. 



. 



In this case the blood serum is isotonic with a NaCl solution of O92 per 

 cent. 



Raoultfs Method. Tlie 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 6', which, is filled 

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

 certain depth in the vessel C, and a longer tube A fitted with a lateral tube r 

 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 G is first filled with the 

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

 bull) 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, 

 which is the freezing-point of the liquid. 



In practice, it is usual to assist the free/ing 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 Eaoult indicates by A, has thus been 

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

 NaCl with which it is isotonic. If, /-.(/., with ox serum, A =0'55", when the 

 1 per cent solution is found to free/e 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. 



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

 Daland, Koppe, Eykman, Gryiis, Manca) 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, 

 ami stronger than others which make them swell out. On experimenting 

 with various substances these' observers found for each a solution at which 



