determining the Hardness of Minerals. 411 



5. Apatite. Basal. 



II 233. 



The value is much affected by surface texture. The filings 

 have a very mealy consistency, and seem to have a lubricating 

 action on the work of the abrader ; when the abrasion was con- 

 tinued for a second 10/* depth, the increment of resistance was 

 enormous. The groove was elliptical, with the longer axis 

 parallel to the next adjacent prism face. 



6. Orthoclase. P Cleavage. 



H. . . . 4665. 



Action very constant, with uniform results for several tests. 

 Ring nearly circular. Oscillation so slight that the microm- 

 eter focus could be observed during the rotation. It was 

 found advisable to check the rotation with the clockwork 

 rather than with the lock for occasional precise observation of 

 the location of the sharp focus, as in this way the diamond was 

 not lifted from the groove, but remained at exactly the depth 

 attained. 



7. Quartz. Basal. 



H 7648. 



Practically no vibration, very constant. 



8. Topaz. Basal. 



H 28867. 



9. Corundum. Rhombohedral cleavage. 



H 188808. 



With so slight a weight the duration of this test with corun- 

 dum was nearly nine hours. Hence the advisability of using 

 greater weight with the harder minerals : nearly all the min- 

 erals of petrographic importance come into this category. 

 Reducing the foregoing values to the standard adopted by 

 Rosiwal, making corundum equal to 1000, we obtain the fol- 

 lowing results ; the values obtained by Rosiwal and Pfaff are 

 appended for comparison : 



Pfaff, 1884. Rosiwal, 1892. Jaggar, 1897. 



9. Corundum 1000 1000 1000 



8. Topaz 459 138 152 



7. Quartz 254 149 40 



6. Orthoclase 191 28'7 25 



5. Apatite 53*5 6'20 T23 



4. Fluorite 373 4*70 -75 



3. Calcite 15-3 2*68 *26 



2. Gypsum 12-03 '34 -04 



In addition to the determination of hardness, the micro- 

 sclerometer may be used for very delicate determinations of 



