The process of combustion is but an oxyda-tion of the substance being burnt, and the heat evolved is the result of this chemical combination. Any material that is capable of oxydation may be used as a fuel; but as an economical question, we can only usefully employ those which contain a great quantity of heat-making matter in a small space, so we have adopted carbon and its compounds as our every-day warmth-givers. These compounds of carbon form a wonderful series of bodies—sparkling in the diamond, glistening in graphite, shining in anthracite, transparent as air in carbonic acid, and a fine black powder in soot. The most really economical, in point of heat, is charcoal, which is nearly pure carbon, and is capable of giving out a most powerful heat; it is prepared by heating wood in close vessels, when all the gases, resins and tars distil over, and pure charcoal remains behind. It should retain perfectly the shape and run of the fibers of the wood, and should be hard, compact and rather brittle, to be good. In wood there is generally from 24 to 26 per cent of pure charcoal. On the average, a given weight of charcoal will give out more heat than the same weight of any other fuel, with the exception of two varieties of soft coal from South Wales. Next in order comes anthracite, which is the oldest of all kinds of fossil fuel. Its structure is perfectly homogenous, or precisely the same throughout the mass ; it breaks with a shell-like fracture, has a jet black color, and a glassy luster, on which is often seen a beautiful play of colors. Of this there is plenty in the world, only requiring to be raised. Pennsylvania alone has sufficient to supply us for an definite period ; and in this country it is, practically, the best fuel that can be obtained. The average of American anthracite contains about 90 per cent of carbon, while that of France (of which however, there is very little) contains 94 per cent of the same. After anthracite is coke, which is, so to speak, coal charcoal. When brown or soft coal is heated in close vessels or in heaps, to which very little or no air has access, the tar and gases are driven off, and coke remains. The gases are now generally collected, and with them we light our houses and streets, and the tar serves many a useful purpose. Good coke should possess sufficient solidity to bear the weight of a smelting f urn ace without crushing, as smelting is one of its principal uses. It ought to be hard and coal-like in form, and should have no soft, damp, black dust on its surface, and must not be exposed too long to the action of the atmosphere and weather, or it will soon perish, and become valueless. It is sometimes made by the coal being carbonized in heaps, other times in brick mound-shaped fireplaces, and the best is carbonized in ovens, while the worst varieties are those which come from the gas-house, and have been carbonized in retorts. It should be almost pure carbon, having in addition only the mineral constituents or ashes of the coal from which it was made. Brown, or soft coal, is well known by everybody ; it breaks in layers, is shiny when broken, but quickly loses that appearance, blacks the fingers when touched, and contains a great quantity of tar ; the various varieties contain from 60 to 90 per cent of carbon, and all give forth, in burning, a dense, black smoke, owing to its want of compactness, and imperfect combustion. The great beds of soft coal are in Great Britain, of which a great quantity, nearly one quarter of her whole area, is coal. Peat is semi-fossilized vegetable matter, or rather woody fiber in a state of semi-decay ; it occurs in bogs, of which those of Ireland are notorious. It is cut in square blocks and dried, when it forms a good and pleasant fuel, and has a peculiar odor, that is considered agreeable by those who use it. A very good quality of charcoal can be made from it, that is of great value in certain smelting operations ; and we are inclined to think that peat is destined to enjoy greater respect, as an object of economical use, than it yet has done. Wood is the oldest used of all fuels, because the most easily attainable. Of wood, ash, fir, lime and elm are the best, and next to these come poplar, sycamore, beech and oak, in point of economy. But in all cases, the choice of fuel must depend more on locality than on philosophical principles; and we have but given the result of much patient investigation and actual experiment.