48. _Carbon_ is a name given to the pure inflammable part of charcoal. It is abundantly diffused throughout nature, for it enters into the composition of several minerals, and of all vegetable and animal bodies. The purest form under which carbon is known to exist is in the diamond (50). It may, however, be obtained sufficiently pure, for all common purposes, by burning a piece of wood, covered with sand, in a vessel called a crucible. In combination with oxygen (21) it forms carbonic acid (26). Carbon is a chief component part of pit─coal (217), petroleum (213), and other bituminous substances. ──────────────────────────────────────────────────────────────────────── CLASS I.—STONES. ORDER I.—EARTHY STONES. ─────── _I. HARD: those which scratch Glass._ ─────── 49. OF GEMS IN GENERAL. Gems, or precious stones, as they are frequently called, are, for the most part, transparent, and have a vitreous or glassy appearance. Their different colours are occasioned by metallic oxides (21) of various kinds, with which they are impregnated. Some writers have classed them by their colours, but this is a very uncertain mode, as different gems have not unfrequently the same colour; and, in many cases, the same gems are of different colours. The usual distinction of gems into Oriental and Occidental is also liable to error, as the best gems, from whatever part of the world they are brought, are always called Oriental. The most estimable of all the kinds are the diamond (50), ruby (54), emerald (67), and sapphire (53); and stones a grain in weight, and equal in quality, are valued in the following proportions, at 8_l._ per carat for diamonds, 4_l._ for rubies, and 3_l._ for each of the others. The amethyst (79), topaz (61), and aqua─marine (61), are considered of nearly equal value with each other; and the garnet (70) is the cheapest of precious stones. The ancients engraved upon several kinds of gems; but they appear to have been ignorant of the art of cutting the diamond, the ruby, and the sapphire, which were too hard for them to operate upon. The emerald and the noble opal (102) were too highly esteemed as precious stones to have often found their way into the hands of engravers. It has been asserted that the ancients did not use the topaz for engraving; but there is extant a beautiful _intaglio_, representing an Indian Bacchus, which is said to be a topaz. The garnet was often engraved upon: and there are many master─pieces of the art in calcedony (91) and carnelian (93.) Onyx and sardonyx (92) were employed for that species of engraving in relief called _cameos_; and, in many instances, it is pleasing to observe with what dexterity the ancient artists availed themselves of the different colours in the alternate zones to express the different parts and shades of their figures.