Diamond is the purest form of carbon. It is combustible, but requires great heat to burn it ; and, being the most valuable of our gems, as well as the hardest substance in nature, is more properly considered in this book with the gems or stones harder than quartz.
The diamond, graphite, anthracite, the coals, lignites, and, lastly, wood and other vegetable matter all form carbonic acid when burned ; but from some of them hydrogen and its compounds, especially hydrocarbons, are also evolved, a regular series thus existing through the bituminous and cannel coals to the true hydrocarbon minerals, such as petroleum and mineral wax.
Next to the diamond in purity comes graphite or plumbago, which is found in the earliest and most highly metamorphosed formations, where it represents the vegetation of those times, which has, under pressure, lost all it volatile constituents, and been also rendered schistose by metamorphic action. It is very valuable when pure and massive, and its wide application in the manufacture of lead pencils and crucibles is well known. It will be readily recognised by comparing it with the lead of pencils. Inferior qualities have to be thoroughly washed, prepared, and pressed, while pure varieties can be sawn in their natural state.
Coals. Coal seams, as already pointed out, are formed from vegetable matter ; but it was while the deposition of the various sandstones, &c., which overlie them, was going on that the character of the carbonaceous deposits first began to change ; great weight was put upon them, in the first instance, by the overlying rocks and thus they became solidified, and, by means of this pressure, and the heat induced by pressure, chemical action set in, which had the effect of slowly driving off the more volatile constituents of the coals. Water and various hydrocarbons were driven off, and the carbonaceous beds, which at first very nearly approximated to the composition and character of wood, were- by degrees changed into coal.
Those coals which contain over 10 per cent, of water have suffered less change than the others, and they have many disadvantages as compared with the anhydrous varieties. These hydrous coals are sub-divided, by their physical characters, into lignites, brovm coals, and pitch coals ; but it is very hard to draw a clear and distinct line between them. Thick deposits of brown coal are found in various localities ; at Lai Lai, in Victoria, the beds are 150 feet thick and are covered with basalt ; and in New Zealand extensive deposits are mined both in the north and south, the seam at the Miranda colliery being 55 feet thick.
When the better class of these hydrous coals are first taken from a mine, they would frequently puzzle any but an experienced observer to distinguish them from the true coals. They have a compact structure, are black and shining, and in many other ways bear a strong resemblance to the true coals. If, however, they be left exposed to the air for some time one has no difficulty in distinguishing them, for they begin to lose their water, and, in doing so, crack in all directions and then fall to pieces. This being the case long transport is impossible, and the employment of the coal must be purely local ; moreover, it must be burned as soon as it is raised from the mine as stacking on the ground will reduce its value.
These hydrous coals, however, are of considerable value where true coals are not obtainable, and will even compete very favourably with them when the true coals have to be brought from a distance; but they have another disadvantage from the occurrence of water in their composition viz., that the water is nob only unable to supply any heat itself, but requires a certain amount of heat to convert it into steam ; and for this reason, where both classes of coal are readily obtainable, it is frequently preferable to employ an inferior class of anhydrous coal rather than the best lignite, brown or pitch coal.
The anhydrous coals, as before stated, may be divided into Anthracite or non-bituminous coal, Cannel or highly bituminous coal, Steam or household or less bituminous coal, and many other sub-divisions are also made to which it is not necessary to call attention.
Anthracite is coal in which the process of carbonisation has been pushed to its greatest extent. It never contains less than 80 per cent, of carbon, and is frequently almost entirely composed of it. Anthracite does not soil the fingers, and is of a glossy black appearance ; it is difficult to kindle, but in burning gives off an intense beat with little or no smoke. From the difficulty in burning it is not so well adapted for household consumption as the free burning coals, although it is largely used in America for that purpose. It is principally employed in smelting metals and raising steam.
Cannel Coal, again, does not soil the fingers, but in other respects differs materially from anthracite. It has received the name of cannel from the property it possesses of burning readily with a flame like a candle. It is highly bituminous or contains a large proportion of volatile matter, and is principally employed in the manufacture of gas. Although other coals are also employed for gasmaking, the quantity of gas obtained from them is generally less than, and the quality always inferior to, that made from cannel.
The ordinary or household coals may be variously subdivided according to the properties which each possess, but the only one of importance is between the caking and non-caking coals. Caking Coals are those from which, in burning, there exudes a black bituminous substance which cements the coal together, in the tire, into a pasty mass. This class of coal is the one from which coke is chiefly made, and is also used both for domestic purposes and for raising steam. The other, or non-caking coals, do not run together when heated, and are of a more free burning character.
Jet is a variety of coal, is black, and takes a good polish. It is of value for the manufacture of ornaments, such as crosses, earrings, &c. The most important deposit known is that of the Jurassic coal measures, near Whitby, in Yorkshire, where two qualities are found, one very hard and valuable, another softer and of less value.
The bog oak of Ireland must not be confounded with jet ; it is simply wood impregnated with iron, and occurs in swamps where iron ore is forming at the present day.
The name of jet is commonly given to black glass beads and glass jewellery, but these are not likely to be mistaken for the genuine article. Jet is much lighter and not so brittle as glass, but its origin does not appear to be well understood. It is described by some authors as a variety of lignite, but it is anhydrous, and is generally associated with cannel coal ; it is very probably a fossil gum. Jet occurs in the Hartley Vale and Joadja Creek shale mines in New South Wales, as thin seams which have no great lateral extension.