the Viscosity of Liquids. 461 



allowed to cool, during both of which operations the times of 

 descent of the mercury from one fixed mark to another are 

 carefully noted through the window, the water being stirred 

 constantly. The times of descent are proportional to the 

 viscosities, if a slight correction be made in each case for the 

 variation in the densities of the mercury and the liquid, with 

 change of temperature. 



To illustrate the nature of this correction let us take the 

 case of glycerine, 



a* 



where T is the time of falling, and a and p are the densities 

 of mercury and glycerine respectively. 



If s and r are the corresponding coefficients of expansion. 



This is approximated by taking s= and r = 



5500 1000' 



yu = K(12-34 + -00020T. 



The correction is therefore unnecessary between 0° and 30°, 

 and is only 1 in 1200 from 30° to 70°. For ordinary tern- 

 peratures it may therefore be neglected. 



With a relative viscosity-curve thus obtained, and with one 

 good absolute determination, we have the means of calculating 

 the absolute viscosity at any temperature along the range. 



Conversely, we have the means of estimating small masses 

 of mercury by their speed of flow through glycerine or any 

 other known liquid at a known temperature, or of estimating 

 the mean temperature of the liquid. 



For example, if a column of liquid be heated from above 

 till its state of temperature becomes constant without the aid 

 of convection, we can here determine the temperature-curve 

 along its length, and in fact employ Forbes's method to 

 determine the thermal conductivity of the liquid at various 

 temperatures. 



In concluding this description of the methods now being 

 employed for the determination of absolute and relative 

 viscosities, it may be well to remark on the advantages and 

 disadvantages of these methods that have already manifested 

 themselves. In the first place we are dealing with steady 

 motions, and are able to investigate the phenomena attending 



