302 TECHNOLOGY. . SOS. 



3, Rhomlohcdral Limc-haloide. Any cleavable variety. 

 The minerals called Brown-spar (macro typous Lime-ha- 

 loide), or llhomb-spar (macrotypous and brachytypous 

 Lime-haloide), cannot be employed in its place, the hard- 

 ness of these being considerably higher. 



4, Octahedral Fluor-haloide. Any cleavable variety. 



5, Rhomboliedral Fluor-haloide. The variety from Salzburg 

 called Asparagus-stone, possessing a conchoidal fracture. 

 The Apatite from Saxony or Bohemia, will seldom be 

 found to answer the purpose, though of exactly the same 

 degree of hardness. 



6, Prismatic Feld-spar. A perfectly cleavable variety of 

 Adularia. 



7, Rhombohedral Quartz. Limpid and transparent. 



8, Prismatic Topaz. Any simple variety. 



9, Rhombohedral Corundum. The easily cleavable variety 

 from Bengal, called Corundum-stone. 



10, Octahedral Diamond. 



The minerals, which represent the units of this scale, 

 have been chosen among those species, which may be most 

 easily acquired with the necessary qualifications, excepting 

 perhaps only rhombohedral Fluor-haloide. Yet it has been 

 impossible to find out another which might be as useful in 

 its place. 



The intervals between the members of the scale are not 

 everywhere of the same magnitude. Octahedral Diamond 

 is evidently much harder, if compared with rhombohedral 

 Corundum, than octahedral Fluor-haloide, if compared with 

 rhombohedral Lime-haloide. This, however, is of no con- 

 sequence in the case above mentioned ; for there exists no 

 mineral of a hardness intermediate between the degrees 

 represented by the two first of these species. But the 

 interval between rhombohedral Fluor-haloide and prismatic 

 Feld-spar is likewise greater than it should be. In this case 

 it would be very desirable to have another mineral which 

 might allow of being employed instead of rhombohedral 

 Fluor-haloide. But in general it is very difficult to ascer- 

 tain the perfect equality of the intervals between the differ- 



