MINERALS AND GEOLOGY OF CANADA. 1] 
or cubical cleavage forms ; whilst the cubes of fluor-spar break off 
most readily at the corners or angles, and yield regular octahedrons, 
fig. 3. 
Hardness.—Vhe hardness of a mineral is its relative power of 
resisting abrasion, not that of resisting blows: many of the hardest 
minerals being exceedingly brittle. Practically, the character is of 
great importance. By its aid gypsum may be distinguished in a 
moment from calc-spar or limestone, calc-spar from feldspar, and 
copper pyrites from iron pyrites, not to mention other examples.* 
The degree of hardness in minerals is conventionally assumed to'vary 
from 1 to 10 (1 being the lowest) as in the following scale drawn up 
by a German mineralogist, Mohs, and now generally adopted: 
Scale of Hardness—Mohs’ Scale. 
1. Foliated Tate. 
2. Rocx Satr, a transparent cleavable variety. 
3. CancaREous Spar, a transparent variety. 
4. Fruuorn Spar. 
5. APATITE. 
6. FELDSPAR. 
7. Rock CRYSTAL. 
8. Topaz. 
9. CoRUNDUM. 
10. THz Diamonp. 
In order to ascertain the hardness of a mineral by means of this 
scale, we attempt to scratch the substance under examination, by the 
different specimens belonging to the scale; beginning with the hard- 
est, in order not to expose the specimens to unnecessary wear. Or, 
we take a fine file, and compare the hardness of the mineral with 
that of the individual members of the scale, by drawing the file 
briskly across them. The comparative hardness is estimated by the 
resistance offered to the file; by the noise produced by the file in 
passing across the specimens; and by the amount of powder so 
* Gypsum may be scratched by the finger nail. Calc-spar and copper pyrites may be 
scratched easily by a knife; whilst feldspar and iron pyrites are hard enough to scratch 
window-glass. Not long ago, as mentioned by Sir William Logan, a farmer in the Ottawa 
district was put to much expense and annoyance by mistaking feldspar for crystalline lime- 
stone,and attempting to burn it into lime. 
