A manual of mineralogy / By Arthur Aikin.

  • Aikin, Arthur, 1773-1854.
Date:
1815
Catalogue details

Licence: Public Domain Mark

Credit: A manual of mineralogy / By Arthur Aikin. Source: Wellcome Collection.

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    gree of force required to effect a permanent separa- tion, from the mass, of thosé integrant molecules te which the external impulse is immediately applied. In common language hardness and refractoriness are often confounded. A stone that endures many heavy blows before it gives way is considered as harder than another which requires fewer blows for its fracture. The most unexceptionable method of ascertaining the hardness of a mineral is the greater or less ease with which it yields to the point or edge of a knife of hardened steel. The whole range of hardness obtained by the use of this instrument may be con- veniently divided into three portions; the first of which will comprehend the higher degrees, and may be called hard, of which common Felspar is an ex- ample: the next includes the middling, degrees, and _may be called moderately hard, of which common Hornblende, Apatite, and Fluor are examples: 2 lower degree includes such as yield with ease to the knife, and may therefore be called soft, of which Calcare- ous Spar, Heavy Spar, and Witherite, are examples. Two other degrees of hardness however, the highest and.the lowest, yet remain, for the determination of which the knife cannot conveniently be applied. The lowest or the very soft, are such as yield not only to the knife but to the nail, of which Chalk and common Steatite are examples. The highest or the very hard, are those upon which the knife makes no impression, but on the contrary, when drawn strongly over their surface leaves a greyish black line of its own substance » in the same manner as plumbago does on paper. B2