Coal, in mineralogy, a solid, inflammable, and bituminous sub-stance, commonly used for fuel: it consists of various species; the principal of which are :

1. The Luthantrax, or Pit-coal ; a black, solid, compact, but brittle mass, and moderately hard, which retains its solidity, when heated. Its component parts, according to Mr. KirwaN, are petrol, or asphal-tum, mixed with a small portion of argillaceous earth, and frequently blended with pyrites, or fire -stone. A red tincture is extracted from this species of coal, by means of spirit of wine.

2. Culm-coal, which, together with a moderate quantity of petrol, has a larger proportion of argillaceous earth, and vitriolic acid, than the pit-coal, to which it bears a strong resemblance. Its texture is not so bright as that of the former species ; and it burns with a flame, without being consumed, leaving a slate nearly of the same size as the original volume of the coal.

3. Slate-coal, which contains so large a quantity of argillaceous earth, that it has the appearance of common slate. It, nevertheless, burns by itself, with a flame, and is found principally in the quarries hear Purbeck ; and in such abundance, that the poorer class of inhabitants in that neighbourhood are wholly supplied with it, for their common fuel.

4. The Ampelites, or Canal-coal, is of a dull black colour, and easily breaks in every direction. It burns with a bright flame, but frequently flies to pieces in the fire : it may, however, be divested of this property, by being immersed in water for several hours, previ-ously to its being used. As this coal is of an uniform, hard texture, it is readily turned on a lath, and takes a good polish. Hence, it is used for making various toys, which greatly resemble those manufactured from the finest jet.

5. Kilkenny-coal is the lightest of the various species of this fossil. Although containing the largest proportion of asphaltum, it emits less smoke and flame, produces a more intense degree of heat, and is more slowly consumed than the canal-coal. This valuable coal is chiefly found in the county of Kilkenny, in Ireland.

These are the principal varieties of coal most commonly known; but they are not uniformly of the same kind or nature, in the different places where they are found. On the contrary, the various proportions and qualities of their ingredients, produce a great number of other varieties, which are calculated for different purposes, according to the quantity arid quality of their contents. Hence it happens, that various kinds of coal are often found intermixed in one stratum, and some of the finer sorts frequently run like veins among the coarser species.

Coals are applied to various purposes, and are eminently useful in the smelting of ores, especially when burnt into coke (to which we refer); but, by these processes, considerable quantities of tar and pitch have hitherto been, inattentively, wasted. To obviate these losses, the ingenious Lord Dun-donald ere6ted ovens of a pecu-liar construction, for burning pit-coal into coke, and, at the same time, for collecting, in separate vessels, the volatile alkali, pitch, oil, and tar, which would otherwise have been dissipated. For this invention he obtained a patent, on the 30th of April, 1781, for 14 years; which term was afterwards, by an act of parliament, extended to 20 years, to commence from the 1st day of June, 1785. His ovens are so contrived, as to admit the external air to pass through the vessels, or buildings containing the coal, from which any of the above-mentioned substances are to be ex-tracted. After being kindled, the coals are decomposed by a slow, but imperfect combustion, without dissipating the ingredients. The residuum in the oven, forms excellent cinders, or coke ; while the volatile particles are condensed in reservoirs., placed at proper distances.

It is a circumstance worthy of notice, that not less than 70 kinds of coal are brought to the London market; the value and prices of which differ, in general, from 15. to 10s. and sometimes even 15s. in the chaldron, according to their qualities. About 45 of these various sorts are imported from Newcastle, and the remainder from Sunderland ; the whole of which may be divided into four classes :

The first class contains only six kinds of coal; called Wall's-end, Bigg's-main, Walker's, Heaton-main, Willington, and Hebburn-wain. The prices of these sorts vary, according to their abundance in the market, from 1s. to 3s. per chaldron ; but they are generally upon a par, except the Wall's-end, which is mostly 6d. or 1s. dealer than the others.

The second class includes three sorts; all of which run large. They light and burn like a candle, and produce white ashes. These are usually mixed either with some of the first class, or with any of the strong sorts of the second, third, and fourth classes , because they run large, and make them burn in a more lively manner. These three sorts are, Hartley, Coupen-main, and Blythe ; and their price is generally from 2s. to 4s., more or less, below that of Wall's-end, according to their scarcity or abundance in the market. Next to these are twelve sorts, which possess nearly the same qualities as the beat coals, but are in general smaller, and seldom vary more than 2s. in the chaldron, though they are usually from 3s. to 4s. in price under the Wall's-end.

The third class consists nearly of the same number as the second, and is likewise divided into two sorts : the first of which burns quickly, and produces white ashes ; the other is very strong and good, but, at the same time, very small, and is used by smiths and manu-facturers. The prices of this class of coals are generally from 4s. to 6s. per chaldron, more or less, under that of the Wall's-end, according to their abundance or scarcity.

Lastly, the fourth class contains all the remaining kinds of coal: they differ also in quality; some burn light, produce white ashes, are slaty, and very indifferent; others are small and strong, but not good enough for smiths. The price of these varies greatly, especially of the lighter kind. It is, in general, from 8s. to 10s. and even 15s. lower than the Wall's-end. These different classes, and particularly some of the inferior sorts, are frequently mixed together, and thus afford an opportunity of changing the prices of coals ; this, however, is always to the loss of the consumer, who loses 10s. or more in the quality, in the hope of saving 4s. or 6s. m the price.

The following is a striking instance of the great variation to be found in the quality of coal: in weighing different kinds of that fossil, there was the surprizing dif-ference of 30lbs. in the weight of two sacks, which were equally filled.

All the coals brought to the London market are publicly sold, only by the whole, half, or quarter ship. Those who have neither craft nor wharfs to unload, at the rate of 40 chaldrons per day, purchase from some of the greatest coal-merchants: this is called loading on account; and the former pay 1s. per chaldron for commission.

Pool-measure is one-fourth of a chaldron extra, on any five chaldrons ; and a room of coals of 5 1/4 chaldrons, contains about 68 sacks of three bushels each, or somewhat less; but this quantity may be divided into from JO to 90 sacks, if they are filled up, and not measured by the bushel, under the inspection of a sworn meter. The pool measure, therefore, being larger than the bushel measure, the profit of a coal-merchant may be estimated, upon an average, at five sacks upon five chaldrons, that is, at about 8 per cent.

Coals constitute one of the chief articles of domestic convenience, especially during the severity of winter. Hence, in that season, they frequently become extremely scarce, and are sold at an extravagant price. To remedy this evil, in some measure, a preparation of clay and coal-dust has been successfully employed ; of which we shall communicate the following particulars :

Coal-Balls : Take two-thirds of soft, mellow clay (for instance, a ton), which is free from stones, and work into it three or four bushels of small sea-coal previously sifted; form this composition into balls, or cakes, about three • or four inches in diameter, and let them be thoroughly dried. When the fire burns clear, place four or five of these balls in the front of the grate, where they will soon become red, and yield a clear and strong heat, till they are totally consumed. The expence of a ton of this composition is but trifling, when compared with that of a chaldron of coals, as it may be prepared at one-fourth of the cost, and will be of greater service than a chaldron and a half of the latter.

A similar kind of fuel is prepared in the Bishopric of Liege, and is a source of considerable emolument to the inhabitants, who sell great quantities of it annually. A correspondent in the second volume of the "Museum Rusticum, etc." mentions this preparation, and adds, that he has seen several fires of it burning in the house at that time occupied by the Royal Society, in Crane-court, Fleet-street. We therefore seriously recommend this article to the attention of those, who, together with the ability, possess the means of alleviating the wants of others.

A patent was granted, in the year 1800, to Mr. Frederic, of Wellbeck-street, for his invention of a fuel, which burns longer than the common coal. As the patentee has published part of the process, in a separate treatise, we shall ex-tract from it the following particulars : The principal ingredient is clay, or where that cannot be procured, cow-dung, road or street mud, saw-dust, turf, horse-dung, straw, and especially tanners' waste; to which may be added, broken glass pulverized, or pitch, tar, oil-cakes, or any other combustible matter, that is not too expensive. These are to be mixed with coal-dust, in circular pits, five or six feet in diameter, and paved at the bottom with bricks. In one of these pits, some clay should be previously softened with water, and well worked with an iron rake ; after which operation, any other of the ingredients may be added in the following manner : Two men provided with a pail should first fill one of the pits a foot deep with clay, and throw in the small coal, together with the other ingredients, according to the quantity and proportion required. The whole should then be stirred repeatedly with a large rake, and the pit progressively be filled up, till the clay becomes so thoroughly incorporated with the other substances, and acquires such a degree of consistence, that it can no longer be stirred. More clay should be added ; and the same operation repeated till the pit is full; when the mixture should remain in it, till the water is in a great measure evaporated, and the composition becomes fit for use ; during which time another pit may be filled in a similar manner.

When the mixture has acquired a sufficient degree of consistence, and is ready to be formed into cakes, a mould made of deal, about four cubic inches square, should be prepared, and previously wetted, to prevent the mass from adhering to it ; but, before this composition be put into the mould, Mr. Frede-ric recommends saw-dust to be spread over it, by means of which the cakes will dry more quickly, and burn much better. The last operation is that of drying, which should be effected in a shed, about seven feet high, and as long as may be necessary. The cakes may also be dried on the ground, in the open air, but as they are liable to be wetted by rain, the labour already bestowed upon them would be useless. A shed, therefore, if it can be procured, is most eligible, and should be divided into upright rows six or seven feet high, about three inches thick, and three feet distant; being intersected every six inches by a cross bar twelve inches in length, for receiving, on both sides, laths of about three quarters of an inch thick ; and which should be about two inches and a half apart. On these laths, the cakes are to be laid for drying, which, during the summer, will take place in leas than a week.

This invention, we conceive, is of considerable utility, and reflects great credit on the patentee, who has voluntarily consented to relinquish his privilege, and offered to explain his process to any public establishment, or charity, that may be inclined to prepare these cakes, upon a large scale, so as to sell them at a reduced price, and thus furnish the poor with that most necessary article of domestic comfort, fuel.

Use of Coals as manure. - The first experiments for ascertaining the effect of pounded coals, or their ashes, on the fertility of meadows and corn-fields, we believe, were made in Germany, by Counsellor StumpF, about the year 1791. On account of the vitriolic acid contained in coals, they are, for this purpose, superior to gypsum, especially on cold, calcareous soils. According to his directions, the coal-dust, or powder, ought to be scattered on the fields, late in autumn, about the thickness of the back of an ordinary knife, so that he employed about four cwt. of coal to manure a German acre of 180 square roods, Rhenish measure. But, as there is a great difference between those coals, the residue of' which, after burning, consists of Calcareous earth, or stone, and others, which leave an aluminous slate; he advises the agriculturist to make vise of the former kind for every species of clover and grasses, as well as for wheat, rye, barley, oats, or similar grain ; and to avail himself of the latter in the culture of spelt, buck-wheat, as likewise of clover, and the different species of grain, bat particularly of all the leguminous fruit, such as peas, beans, etc.

Coal-mine, a coal-work, or place from which coals are dug and raised. The maliciously setting coal-mines on fire, is felony without benefit of clergy, by stat. 10 Geo. II. c. 32, sett. 6.

Small-coal, is a kind of charcoal, prepared from the spray, and brushwood, stripped off the branches of coppice-wood, which are sometimes tied up in bundles for that purpose, and sometimes charred, without being tied ; which operation is called coming it together.