1895.] 



Investigation of the Laws of Attrition. 



29 



ders, it acts as a lubricant, and diminishes the amount attrited.* As 

 explained further on, the pressure must exceed a certain minimum 

 value in each case, or polishiDg will ensue. 



In all cases it is the ultimate constant values which are to be taken 

 as the weight of material attrited ; that is to say, the cylinders should 

 be allowed to rub themselves to the proper surface so as to continue 

 giving a constant value before taking a determination. It is found 

 that there is a surface proper to each pressure. When the pressure 

 is increased the amount at first removed is less than subsequently ; 

 on changing from a higher to a lower pressure the reverse is generally 

 true.f 



With the harder substances used, chipping at the edges of the 

 cylinders was sometimes a source of considerable trouble. It is 

 advantageous to have the edges chamfered off, as this gives a better 

 resistance to crushing. This tendency to chip puts a limit to the 

 pressure which can be used. 



The results given in this paper cannot be considered as the best 

 which can be obtained with the form of apparatus used, for, from 

 the first, the end aimed at was rather to gain a general insight into 

 the phenomenon in question by accumulating a large number of 

 results than to determine accurately the value of the coefficient in 

 individual cases. 



The Critical Pressure of Attrition. 



In the case of each kind of material the pressure must be above a 

 certain value in order that a constant value be finally reached in the 

 weight of material attrited. This particular pressure I have called 

 the critical pressure for attrition. If the pressure be below this the 

 amount attrited gets less and less as time goes on, and ultimately 

 the surface becomes polished on the rubbing being continued long 

 enough. Before this was understood the results obtained at different 

 pressures were often inconsistent, and a great deal of time was thereby 

 lost. For consistent results it is, of course, necessary to operate above 

 the critical pressure. 



Up to the present I have made no determination of the actual value 

 of the critical pressure for any substance ; but the following table, 

 derived from data incidentally obtained in determining attritivity, 

 gives the approximate relative values in the case of the same substance 

 experimented with. The critical pressure necessary to ensure that 

 quartz attrite quartz is assumed to be 1000 : — 



* In the case of the softer substances experimented with it was found advan- 

 tageous to have two extra pads or wisps of cotton wool which could be applied by 

 hand so as to remove the very fine dust. 



f Many of the irregularities in the results given arose probably from not 

 attending sufficiently to this precaution. 



