166 THE BLOOD 



electric charges, from escaping from the plasma. For this reason, the 

 conductivity of the blood must be attributed largely to the plasma. 

 This is proven by the fact that clear plasma possesses a greater con- 

 ducting power than plasma to which corpuscles have been added. 1 

 Viscosity. When a fluid traverses a straight tube of sufficient 

 length, its different constituents arrange themselves eventually paral- 

 lel to the long axis of the tube. It is also to be noted that the fluid 

 does not advance as a uniform whole, but unevenly, so that its central 

 core attains a very great speed, while its more external layers progress 

 with a velocity which steadily decreases from within outward. For 

 this reason, the layer next to the wall must remain perfectly stationary 

 provided, of course, that it moistens the internal surface of the tube. 

 In the second place, it should be noted that the individual molecules 

 of the fluid rub against one another, because as the elements in neigh- 

 boring layers move at different speeds, some of them must be brought 



. , FIG. 96. FRICTION OF BLOOD. 



E, external friction; J, internal friction. 



V 



in contact with one another, while others are separated from one 

 another. Hence, the movement of a fluid is associated first of all 

 with an external friction arising between its outermost layer and the 

 internal surface of the vessel wall, and secondly, with an internal or 

 intermolecular friction, which, as the name indicates, results between 

 the different bodies held in solution or suspension. 



If several different fluids are forced through a narrow tube under 

 a constant pressure and temperature, the quantities obtained of each 

 during a given period of time, varies considerably. For example, if 

 glycerin, water and ether are used, the quantity of ether collected is 

 much larger than that of either water or glycerin. This difference in 

 the readiness with which liquids are capable of traversing a capillary 

 tube, is ascribed to differences in their internal friction. Quite simi- 

 larly, if blood and water are employed, the former displays a much 

 slower movement than the latter. As can readily be surmised, this 

 tardiness is dependent upon its greater content in solids. Fluids are 

 commonly described as "thick" and "thin," and clearly, the thinner the 

 fluid, the less must be its internal friction. But, besides these purely 

 quantitative differences, fluids also possess certain qualitative 

 peculiarities which impart to them either a "sticky" or a "non-sticky" 

 character. The latter kind of fluid, very naturally, possesses a slighter 



1 G. N. Stewart, C. Phys., xi, 1897, 332. 



