NOTE ON THE EFFECT OF CHANGES IN THE VISCOSITY OF 

 WATER ON THE RESULTS OF MECHANICAL ANALYSES 

 CONDUCTED AT VARYING TEMPERATURES. 



By G. W. ROBINSON, B.A. 



Adviser in Agricultural Chemistry, University College of North 

 Wales, Bangor. 



Some discordant results obtained in mechanical analysis during a 

 spell of cold weather led the writer to enquire into the effect of tem- 

 perature on the separation of the various fractions of a soil by mechanical 

 analysis. 



According to Stokes' law, the hmiting velocity of a particle of diameter 



a falling in a liquid of viscosity rj is proportional to — — , p being 



V 

 the density of the solid, and a that of the liquid. 



Now in any given separation, the depth of liquid and time of 

 subsidence are adjusted so that all particles of diameter greater than a 

 certain value shall be left behind on decantation. If, for any reason, 

 7) varies, since v is not changed the diameter, a, of the smallest particles 

 which are left behind on decantation will be proportional to \/tj. 



The value of tj is subject to considerable change when the temperature 

 is varied. For water, Thorpe and Rodger^ give the formula 



•0179 41 



'' ^ (1 + •0231200'*''" 

 where t = temperature in degrees centigrade. 



The densities, p and a, are also subject to a change with the tem- 

 perature, but these are of a smaller order of magnitude and may be 

 ignored in comparison with the changes in rj. 



Calculating — by this formula we get — = V33. 



That is to say that by a fall of 10° from 15° to 5° C, the viscosity of 

 water increases by 33 per cent. Since the diameter a is proportional 

 to V'?, 



0^5 _ /i;oo_ro^ 



a5~V r33~115' 



» Phil. Tram. 1894, A, p. 1. 



