a. W. Robinson 143 



where a is- is the diameter of the smallest particles left behind on decan- 

 tation at 15° C. and a^- the diameter of the smallest particles left behind 

 on decantation at 5° C. From this it is seen that the Hmiting diameter 

 at 5° C. is 15 per cent, greater than it is at 15° C. If there is a large 

 proportion of particles at or about this limiting diameter it will mean 

 that considerably smaller values will be obtained for the fraction left 

 behind on decantation if the decantation is performed at a lower 

 temperature. 



An experiment was performed in which some ignited fine sand was 

 well mixed with an approximately equal quantity of ignited silt. 

 Duplicate two gram lots were taken and after stirring well with water 

 in beakers allowed to settle through 7-5 cm. for 75 seconds. The water 

 was at 6° C. The sedimentations were repeated six times by which 

 time no more silt remained in suspension after 75 seconds. The residual 

 fine sand was collected, dried for several hours and weighed. The 

 fine sand was then mixed again with the silt from which it had been 

 separated at 6° C. and a separation was effected in exactly the same 

 way at 11° C. The fine sand obtained at 11° C. was again dried and 

 weighed. A further separation was made at 16° C. in exactly the same 

 way. The results are as follows. 



This shows that a change in temperature, by altering the viscosity of 

 water, does affect the values obtained in mechanical analysis by sedi- 

 mentation. It is recommended therefore that sedimentations be 

 carried out, as far as possible, at a uniform temperature, say 12 to 14° 

 centigrade. So far as the writer is able to find out, this point has not 

 up to the present been emphasized. 



{Received March ird, 1915.) 



