COEFFICIEXT OF VlSCOSUrV 80 



The \aliic of xiscosily is oi' soiiu' iniportancc in the study ol" the 

 circulation ol" the blood, because, if the resistance to the niovenicnt 

 of the blood in the capillary vessels is increased, the heart will have 

 to expel the blood at a greater pressure to force the fluid round the 

 circuit. Several methods have been employed to measure this 

 value. The one most commonly used is to measure the rate of 

 flow of a measured quantity of the fluid under test down a vertically 

 held capillary tube imder standard conditions. This rate, after 

 correcting for density, is usually compared with the rate obtained 

 under the same conditions for an equal quantity of water (Part II., 

 p. 530). Sometimes it is desirable to determine the coefficient of 

 viscosity in C.G.S. units. The coefficient is defined as the tan- 

 gential force per cm.^ on either of two horizontal planes 1 cm. apart, 

 one of which is fixed while the other moves at 1 cm. per second, the 

 space between being filled with the liquid under test. Hatschek 

 used for this determination a piece of apparatus consisting essen- 

 tially of two concentric cylinders, the outer one of which can be 

 rotated at any desired rate while the inner one is suspended from a 

 wire. The liquid fills the space between the cylinders. When the 

 outer cylinder is rotated it carries with it the thin layer of liquid in 

 contact with it. This liquid layer in turn pulls at the layer next 

 to it, and so on till we come to the almost stationary layer in 

 contact with the inner cylinder. That is, we may consider the 

 fluid between the cylinders to be made up of a number of concentric 

 liquid cylinders, each exerting a certain fractional force on the 

 adjacent cylinders. The degree of torsion of the suspending wire 

 gives a measure of the viscosity of the liquid. 



The main factor on which the large value of the viscosity of 

 hydrophilic colloids depends is that the shearing force has to over- 

 come not only the internal resistance of the liquid continuous 

 phase, but the resistance to distortion of the elastic colloid dispersed 

 phase. 



Concentration. — The latter resistance, of course, increases with 

 the number of colloid particles encountered by the distorting force 

 i.e., on concentration. Up to a certain concentration limit, which 

 varies with different colloids, increase of concentration makes very 

 little difference in the value of the viscosity. Above this limit, 

 a very sharp increase of viscosity occurs. One may take this 

 limiting value as a measure of the hydrophilic properties of the 

 colloid, e.g. caseinogen starts to increase its viscosity markedly at 

 about 5 per cent., while glycogen goes up to 25 per cent, before 

 being effectively viscous. 



Temperature. — Alteration of temperature produces marked and 

 regular alterations in the value of the viscosity of pure water, e.g., 



