A treatise on the external characters of minerals / [Robert Jameson].

  • Jameson, Robert, 1774-1854.
Date:
[1805]
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on the external characters of minerals / [Robert Jameson]. Source: Wellcome Collection.

    23/154 (page 17)
    A mineral is said to be tarnished wfien it shews on its sur- face fixed colours different from those in its interior or fracture. These colours are distinguished, according to their origin, some minerals shewing them, a. In the bosom of the earth, others b. On the exposure of the receat fracture to the action of the ai,r. In those minerals where the tarnish has taken place in the bosom of the earth, no new tarnish takes place on exposure of a fresh fracture to the air, which intimates that this kind of tarnish is produced by a thin covering of some foreign matter, or by the action of percolating fluids. Where a change is induced on a ffesh fracture by exposure to the air, it is evidently owing to some chemical change, produced by the agency of the atmosphere. Examples of the first class we have in lead glance, iron glance, and blende; of the second, in native arsenic, magnetic pyrites, and variegated copper ore. Some minerals are not only changed in their natural re- pository, but also undergo alteration on exposure to the air, thus combining the two kinds of tarnish. Copper pyrites is an example of this kind. They are further divided according to their kind. Fossils in whatever manner they receive their tarnish are, a. simple, or b. variegated. When we say a colour is simple, we mean that one colour predominates over the whole surface ; varie- gated, when the surface shews many different colours that a/e distinct or run into each other. Of the merely simple tarnished colours we may mention 51s examples the following: a. Grey—glance cobalt. /3. Black—native arsenic. 4. Brown—magnetic pyrites. c