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_,/ The Mohst scale includes 10 terms, represented by mineral typos, arranged in order of increasing hardness. They are: 1) talc, 2) rock salt or gypsum, 3) calcite, 4) fluorite, 5) apatite, 6) feldspar, 7) quartz, 8) topaz, 9) corundum, 30) diamond. Gypsum and rock salt can be scratched by the finger nail; soft steel will scratch feldspar which, in turn, will be scratched by quartz. Most of these mineral types have cleavage planes, and their hardness varies on the different planes and in different directions. Hardness is not a constant, real characteristic of minerals, however. Some observations seem to prove that, like refractive power, hardness varies according to the medium in which the test is carried on, although those variations seemed so slight that little heed was taken of them until recently. Tests mark recently at the Sediment—Petrographisches Institut of Gottingen show that, in the case of minerals, the variations may be large enough to justify the adoption of new methods of drilling since these methods would result in appreciable savings in work, time, and material.

Use of sulphitic liquid to reduce the resistance of quartzose rocks to drilling