30 Dr. F. T. Trouton. An Experimental [Nov. 21, 



Table II. 



Quartz 1000 



English plate glass 850 



French plate glass .... 500 



Serpentine 450 



Marble 200 



Alabaster 9 



Plaster of Paris 1 



As alluded to before, the form adopted for the material in the 

 apparatus, namely, cylinders, precludes ascertaining the actual values 

 of the pressures. A method of experiment in which flat surfaces 

 were used would be necessary to do this. 



Attrition of Unlike Substances, 



In the following table are given the results of experiments in which 

 unlike substances were caused to attrite each other. Along the 

 horizontal lines in the table are shown the amounts* attrited by 

 rubbing with a cylinder of the substance given at the left-hand end 

 of the line from each of the substances indicated at the top : — 



Table III, 











© 











artz. 



glish 

 ate. 



ate. 



penti: 



rble. 



baste 



ster c 

 iris. 





& 



I U 





§3 

 m 





03 







4-0 



6-0 



8-2 



23 



87 



161 



427 



English plate glass 



2-4 



5-7 



8 7 



24 



91 



158 



464 



French plate glass. . 



3 1 



5-5 



8-8 













0-2 



0-2 





22 



80 



127 



478 





0-3 



0-4 





15 



100 



175 



533 





o-o 



o-o 





4 



3 



175 



589 



Plaster of Paris. . . . 



o-o 



o-o 













13 



599 



These experiments brought to light the unsuspected fact that the 

 quantity rubbed off a given substance does not increase with the 

 hardness of the substance with which it is rubbed, once the latter is 

 equal to that of itself. In fact, it would look from the table to be 

 rather the other way. This, however, is probably due to secondary 

 causes. So that we may state, as a first approximation, that when 

 rubbed by a harder substance the amount attrited is independent of 

 the other substance. 



* These numbers multiplied by 10~ 12 are the weights per sq. cm., per unit pres- 

 sure in dynes, per cm. displacement. 



