274 THE REALITIES OF MODERN SCIENCE 



particles will fall under gravity through a viscous 

 fluid. The emulsion was therefore shaken until thor- 

 oughly mixed and a measurement was made of the rate 

 at which the upper layers cleared of particles. 



The phenomenon of viscosity, sometimes spoken of 

 as molecular friction, is most easily described in the 

 case of the flow of a liquid through a pipe. Suppose 

 the liquid wets the walls, then the layer of molecules 

 immediately adjacent to the walls is at rest and the 

 next layer slides by it. In terms of the kinetics of 

 molecules we say that there is now a "mass motion" 

 in addition to the haphazard molecular motion. The 

 mass motion of the inner streaming layer is, however, 

 interfered with by the absence of such motion on the 

 part of the outer layer. The molecular motion of the 

 streaming molecules causes some of them to collide 

 with those of the fixed layer and they are thus retarded. 

 Layers farther from the stationary layer will be re- 

 tarded less by such collisions. (The effect, being due 

 to collisions and not to attractions, takes place in all 

 aeriform substances, including perfect gases. It is 

 viscosity which retards the motion of bodies through 

 the air and of which we have spoken as air friction.) 

 In terms of force it is measured by the number of dynes 

 which must be applied tangentially to each square 

 centimeter of the surface of one layer of the fluid to 

 cause this layer to move with a velocity of 1 cm. per 

 second with reference to a parallel layer which is 1 cm. 

 away. This ratio is the coefficient of viscosity. This 

 quantitative definition is concerned only with relative 

 velocities and is not limited to the special case which 

 we described where one layer is at rest. 



