98 If. Z. Kip — Determination of the Hardness of Minerals. 



AVliile I do not claim or consider the values given in the 

 above table as tiiml, I wish to call particular attention to the 

 fact that the hardness value shown for quartz is greater than 

 that for topaz. Similar values for these two minerals were 

 obtained by Rosiwal in 1892; quartz, 149: topaz, 138 (corun- 

 dum being placed at 1000), although his unsupported testimony 

 seems to have failed to shake the traditional faith of mineralo- 

 gists in the infallibility of Mohs's scale. As a result of my 

 experiments I am fully convinced that Rosiwal is correct in 

 stating that quartz is harder than topaz, but that the difference 

 between the two minerals in this regard is comparatively slight. 

 It should be observed that Pfaff, Prof. Jaggar and others who 

 have arrived at the opposite conclusion have failed to eliminate 

 the factor of density in carrying out their tests. In other 

 words, while regarding hardness as resistance to abrasion they 

 have sought to determine its value on the theory that it was 

 to be measured in terms of resistance to excavation. This 

 fact, of course, should also be borne in mind in comparing all 

 the values presented by Rosiwal and the writer with those 

 obtained by investigators who are satisfied to measure only 

 one of the forces employed in producing abrasion and who 

 disregard density as a possible factor in the problem. The 

 idea that the minerals at the upper end of Mohs's scale are 

 almost infinitely harder than those at the lower end is, in my 

 opinion, erroneous. They may appear so when tested by the 

 method usually employed in the laboratory, in which the hard- 

 ness of the constantly changing abrading agent is perhaps not 

 greatly superior to that of the mineral under investigation. 

 The nearer the rigidity and hardness of the instrument of 

 abrasion approaches the ideal, the less will the differences in 

 hardness of the various minerals be found to be. 



Vanderbilt University, Nashville, Term. 



