20 Mr. F. T. Trouton. On the Motion under [June 1, 



TABLE VIII. Air Solutions of Glycerine and Water. 



It is of interest, as pointed out by Lord Kelvin,* to consider the 

 increase in velocity accompanying increase in viscosity here exhibited 

 in the light of Professor Osborne Reynolds' * Critical Velocity.' As 

 will be remembered, the ' Critical Velocity,' or the velocity of flow 

 of a fluid just unaccompanied by eddying motions, rises in value with 

 rise in the value of the viscosity. On the diagram, an hypothetical 

 line to indicate the " critical velocity " for the bubble has been drawn. 

 (That is to say, the velocity of the bubble which is accompanied by 

 critical velocity of flow of the fluid past it.) All calculated values of 

 the velocity below this line will stand. Above this line the observed 

 velocity will lie between it and the line given by the calculation. 



The form of the curve was found to be the same with solutions in 

 water of sugar, of calcium chloride, or of sodium hydrate. 



The actual turning down of the curve of velocities at small values 

 of the viscosity, as occurs in these cases, would apparently be a 

 matter of the rate of departure from each other of the critical velocity 

 curve and that calculated from the theory of viscous flow, which 

 latter, it must be remembered, depends on the surface tension and 

 density as well as on the viscosity. These quantities probably have 

 an important bearing in this particular question. By employing, say, 

 gum tragacanth, one can increase the viscosity of water ' without 

 sensible change in density. f Thus, in this respect the driving power 

 is not simultaneously increased as is the case when increase in vis- 

 cosity is produced by addition of glycerine. 



With gum tragacanth the velocity never actually increases on 

 increase of viscosity as in the case of glycerine. But through a con- 

 siderable range in viscosity on the right-hand side of the diagram 



* In the course of a discussion on a note read before the British Association in 

 Edinburgh, 1892. 



f The surface tension of these solutions changes in about the same relative ratio 

 to the viscosity as solutions of glycerine in water do. 



