Several silver ores are distinguished by peculiar characteristics, and are accordingly denoted by particular names. That which is called the _Vitreous Silver Ore_, is scarce any thing else but a combination of silver and sulphur. Another is known by the name of the _Horny Silver Ore_, because when in thin plates it is semi-transparent: in this ore the silver is mineralized by sulphur and a little arsenic. The _Red Silver Ore_ is of the colour which its name imports, sometimes more, sometimes less vivid; and is chiefly composed of silver, arsenic, and sulphur: it also contains a little iron. These three ores are very rich in silver: the first contains nearly three fourths of its weight, and the others about two thirds of theirs. There is a fourth, called the _White Silver Ore_, which, though it be heavier, is not so rich in silver, because it contains much copper. Many other minerals contain silver, yet are not, properly speaking, silver ores; because a much greater quantity of other metals than of silver is found in them. When a silver ore is to be decomposed, in order to have the silver pure, or when silver is to be extracted out of any ore that contains it, the first thing to be done is to roast the ore, in order to clear it of the volatile minerals: and as silver cannot be had pure without the operation of the cupel, which requires more or less lead to be joined with it, it is usual to mix with the torrified silver ore a quantity of lead, proportioned to that of the heterogeneous matters combined with the silver, and to melt the whole together. Part of the added lead vitrifies during the fusion, and at the same time converts some of the heterogeneous matters also into glass, with which it forms a scoria that rises to the surface of the matter. The other part of the lead, with which the silver is mixed, falls to the bottom in the form of a regulus, which must be cupelled in order to have the silver pure. _Of Copper Ores._ Copper is much seldomer found in a metalline form than gold or silver: it is commonly in a mineral state: it is mineralized by sulphur and arsenic: almost all its ores contain also more or less of iron; sometimes a little silver, or even gold, together with unmetallic earths and stones, as all ores do. Most copper ores are of a beautiful green or blue, or else in shades blended of these two colours. The minerals called _mountain green_, and _mountain blue_, are true copper ores; not in the form of hard stones, like other ores, but crumbly and friable like earth. Nevertheless, there are several copper ores of different colours, as ash-coloured, whitish, and shaded with yellow or orange; which colours arise from the different proportions of arsenic, sulphur, and iron, which these ores contain.