30 Kip — Determination of the Hardness of Minerals. 



be traversed on the denser mineral, B, is to the distance to be 

 traversed on the less dense mineral, A, as ^^lenTA ^ s to 

 V den. B. 



In practice, then, we should proceed as follows : 

 Determine the critical pressure, x, and the critical pull, y, 

 for a given mineral by direct observation and measurement. 

 Hardness, then, = ^f^y^ This is Method 1. Establish in 

 this way the hardness of a series of minerals, a, b, c, d. To 

 verify the results. Pass the point over the surface of mineral 



a, with the critical pressure previously determined, at a con- 

 stant rate* until a distance p has been traversed. Determine 

 weight of material abraded. Pass the point over the surface 

 of mineral b, with the critical pressure already established for 



b, at the same rate as on a, a distance 



V den. a 

 p X — 



V den. b 



Weigh the abraded material. This weight should bear the 

 same relation to the weight of substance abraded from a as the 

 molecular weight of b bears to the molecular weight of a. Thus 

 if the molecular weight of a be 75*6 and of b 504 the amount 

 of matter by weight removed from a by 40 movements or trips 

 of the abrader should be 1*5 times the weight of that removed 

 from b in 20 trips, assuming the trips to be all of equal length 

 and assuming the density of a to be 8 and of b to be 64. If 

 the substance to be abraded from a should weigh, let us say, 

 P6 times that removed from b, it is evident that the critical 

 pressure on a as determined by Method 1 has been too great, 

 or that on b too small. These tests should be carried on until 

 the critical pressure for all the minerals under consideration 

 has been established. The critical pull can then be easily 

 determined by direct measurement, and hardness then calcu- 

 lated as in. Method 1. 



The fact should not be concealed that in view of our ina- 

 bility to determine in most cases the weight of the physical 

 molecule as opposed to the chemical molecule, our chief reliance 

 at present will have to be placed on Method 1. 



9. These theoretical considerations make it necessary that 

 an apparatus designed to measure hardness should meet the 

 following recjuirements : 1. It must produce abrasion, not 

 merely penetration. 2. It must provide a means of measure- 

 ment for pull as well as for pressure. 8. It should allow for 

 regulation of rate. 4. The forces producing abrasion should 

 be used solely for that purpose, or if employed otherwise the 

 amount so used should be easily ascertainable. 



* It is desirable that a low rate be adopted, yet not so low as to cause 

 molecular gliding instead of dislocation. 



