2i>0 Prof. J. H. Poynting on Osmotic Pressure. 



points of concentration individual molecules may receive so 

 much energy that they are able to do the work needed to free 

 them from their immediate surroundings. Such molecules 

 will travel off, and as they lose their energy will form new 

 connexions with new surroundings. Thus the solid structure 

 is continually breaking down and renewing itself. If we 

 impose a shear strain on the structure, the strain will of 

 course disappear with the structure in which it is produced. 

 But the breaking down will always lag slightly behind the 

 imposition of the shear, and the still surviving shear strain 

 will be accompanied by a resistance the same in kind as the 

 resistance to shear in a solid, though in a liquid it is only 

 recognized as viscosity. This is the view first set forth by 

 Poisson and developed by Maxwell, and it is to be noted that 

 it gives an explanation of liquid viscosity entirely different 

 from the diffusion explanation which so satisfactorily accounts 

 for gaseous viscosity. 



We may obtain an expression for the coefficient of viscosity 

 by the following method, which is perhaps rather simpler 

 than that of Maxwell. We must assume that a certain frac- 

 tion, say X, of the molecules of the liquid get free per second, 

 and that this fraction remains practically the same when the 

 liquid is sheared. Hence if s is the strain still existing at any 

 instant, it is breaking down at the rate \s per second. If the 

 liquid is moving steadily in parallel planes perpendicular to 

 an axis along which x is measured, and if the velocity is v at 



a distance x from the reference plane, -=- is the rate at which 



shear is being imposed on the liquid. But since the steady 

 state is reached the rate of imposition equals the rate of decay, 

 or 



£=- a> 



If n is the coefficient of rigidity of the structure, the stress 

 due to s is ns, and by our supposition this is the viscous 

 stress, or 



*£="*' (2) 



where rj is the coefficient of viscosity. Dividing (2) by (1) 

 we obtain 



'-£ • • • « 



We may compare the liquid breakdown here imagined with 



