a. W. Robinson 321 



put away in a large cupboard in a room without any burners or other 

 sources of heat. The possibihty of convection currents was thus avoided 

 as far as possible. 



By a suitable choice of depths and times the method could be readily 

 adapted to systems of fractionation other than that in use in this country. 

 In the American systems, where the numbers of fractions separated are 

 large, the amount of work involved and the possibiUties of error are 

 serious. The above method might be of use in such cases. By setting 

 out the results as summation curves the mechanical analyses by any 

 system of grading could be obtained by interjjolation. 



Summary. 



(1) The expression of mechanical composition by means of con- 

 tinuous curves is discussed. It is suggested that a convenient repre- 

 sentation will be obtained by showing summation percentage as a 

 function of the logarithm of settling velocity. 



(2) The effect of a gel coating on the setthng velocity of a particle 

 is examined and it is shown that a reduction in velocity takes place 

 which is a simple function of the thickness of the gel coating. 



(3) A method is outhned by which the mechanical composition of a 

 soil or clay is derived from determinations of the concentration of a 

 settling suspension for different values of depth/time. 



(4) A shortened method for mechanical analysis is described which 

 gives results in good agreement with results obtained by the present 

 standard method. 



(5) The effect of various modifications in conditions of working is 

 discussed. 



(6) The nature of the concentration gradients in a settling column 

 of a suspension is examined. It is shown that below the first few centi- 

 metres the change in concentration with depth is very gradual. 



(Received July 8th, 1922.) 



