THE BLOOD : PLASMA 



149 



the volume of the corpuscles remains unchanged. These solutions, taken as 

 isotonic, correspond exactly with those found by the methods of Hamburger 

 and Raoult. 



The apparatus employed is practically the same as that described on p. 104, 

 Fig. 31, save that the capillary tube (haematocrite), 7 cm. long, is divided into 

 100 parts, finished at one end by a funnel- 

 shaped swelling. 



To ascertain the solution of NaCl that is 

 isotonic with that of mammalian blood serum, 

 the first step is to aspirate into different 

 haematocrites a quantity as equal as possible, 

 and containing about 0'02 c.c. of blood corpuscles. 

 The haematocrites are placed in the horizontal 

 supports represented in the said figure, and 

 centrifuged till the column of erythrqcytee 

 becomes regular and constant, while the height 

 they reach is simultaneously noted. Next, to 

 the free portion in each haematocrite is added, 

 by means of a Pravaz' syringe or a measuring 

 pipette graduated in hundredths of c.c., a given 

 quantity (0'2 c.c.) of the various solutions of 

 serum, diluted in the same way as those em- 

 ployed in examination of the osmotic pressure 

 of the serum according to Hamburger's method. 

 The erythrocytes are mixed with the solution 

 by means of a fine needle, care being taken to 

 close the capillary end of the haematocrite with 

 the finger, and they are then again centrifuged 

 for an hour and a half, until the level of the 

 stratum of corpuscles remains constant. This is 

 easily ascertained when, on reading the height of 

 the stratum of erythrocytes, at intervals of a few 

 minutes' centrifuging, they show the same figure. 

 On then examining with a lens the several 

 columns of fluid corpuscles, that solution is to 

 be taken as isotonic in which the column in 

 the haematocrite is level with the original. 

 If none of the columns are exactly in this 

 condition, the isotonic solution is intermediate 

 between the tube in which the corpuscles 

 are either just shrunk or just swollen, i.e. 

 the first is just hypertonic, the second is just 

 hypotonic. 



Method of Electrical Conductivity. The 

 cryoscopic method enables us to study the mole- 

 cular concentration of the blood and the serum, 

 that of electrical conductivity permits us to FIG. 43. Beckmann's Cryoscope. 

 study the electrolytes they contain. Accord- 

 ing to Arrhenius, electrical conductivity is due to the dissociated portion 

 of the electrolytes, to the positive and negative ions (kations and anions), 

 their number and their velocity in the fluid. It varies with everything 

 that causes the concentration and mobility of the ions to vary, such as the 

 chemical nature of the electrolytes, their molecular concentration, the 

 presence of anelectrolytes and colloids, the temperature. 



The electrical conductivity (K) of a solution is the reciprocal of the 

 resistance (r), measured in ohms, which it offers; to the passage of the 

 electrical current. 



Resistance is measured by Kohlrausch's method with the apparatus shown 



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