GENERAL PHYSICAL AND CHEMICAL PROPERTIES 27 



is the relative volume of the corpuscles and serum. When the 

 blood is relatively rich in corpuscles and poor in serum, its con- 

 ductivity is low; when it is poor in corpuscles and rich in serum, 

 its conductivity is high. The explanation is that the corpuscle 

 refuses passage to the ions of the dissociated molecules, which, in 

 virtue of their electrical charges, render a liquid like blood a con- 

 ductor (p. 422), or permits them only to pass very slowly, so that 

 the intact red corpuscles have an electrical conductivity so many 

 times less than that of serum, that they may, in comparison, be 

 looked upon as non-conductors (Practical Exercises, p. 69). 



The Relative Volume of Corpuscles and Plasma in Unclotted 

 Blood, or, what can be converted into this by a small correction, 

 the relative volume of corpuscles and serum in defibrinated blood, 

 can be easily determined, with approximate accuracy, by com- 

 paring the electrical conductivity of entire blood with that of its 

 serum.* Another method, more suitable for clinical work, though 

 not so accurate, is the so-called haematocrite method. A small 

 quantity of blood is centrifugalized in a graduated glass tube of 

 narrow bore until the corpuscles have been collected into a solid 

 ' thread ' at the outer extremity of the tube. Their volume and 

 that of the clear plasma which has been separated from them are 

 then read off on the scale. The haematocrite must rotate at such a 

 high speed (10,000 turns a minute) that separation of the corpuscles 

 from the plasma is accomplished before clotting has occurred. 

 Dilution of the blood with liquids which prevent clotting is not 

 permissible for exact work (Practical Exercises, p. 68). By these 

 and other methods too elaborate for description here, it has been 

 shown that the plasma or serum usually makes up rather less than 

 two-thirds, and the corpuscles rather more than onedJaird, of the 

 blood But this proportion is, of course, liable to the same varia- 

 tions as the number of corpuscles in a cubic millimetre of blood. 

 It depends, further, the number of corpuscles being given, on the 

 average volume of each corpuscle. For instance, when the mole- 

 cular concentration, and therefore the osmotic pressure (p. 421), 

 of the plasma is reduced, as by the addition of water or the abstrac- 

 tion of salts, water passes into the corpuscles and they swell ; when 

 the molecular concentration of the plasma is increased, by the 

 abstraction of water or the addition of salts, water passes out of 

 the corpuscles, and they shrink. In human ser^um- the average 



7>r/t\ 



* The formula p =/~r (*74 - K(fy), where p is the number of c.c. of serum 



in 100 c.c. of blood; K(b], K(s), the specific conductivities respectively of the 

 blood and serum (both measured at or reduced to 5 C., and, to obtain whole 

 numbers, multiplied by i.o 4 ), may be used in the calculation. K is the specific 

 conductivity of the liquid i.e., the conductivity of a cube of the liquid of 

 i centimetre side. The conductivity of a similar cube of mercury is 10,630. 

 The number in brackets is the temperature at which the measurement was 

 made. 



