The following table, by Mr. Tredgold, shows the comparative and real effect of the principal varieties of solid fuel in converting water into steam.
Mr. S. F. Gray is of opinion that fire-balls, of the size of goose eggs, composed of coal and charcoal in powder, mixed with a due proportion of wet clay, and well dried, would make a much more cleanly and in all respects a pleasanter fire, than can be made with crude coals, and not more expensive. He states, that in Flanders and Germany the practice of making equal weights of clay and coals together, and forming them into cakes, is common, and that the labour of the preparation is amply repaid by the improvement of the fuel, the coals thus mixed burning much longer, and giving more heat, than when they are burnt in their crude state ; that although clay is an incombustible body, the fact is certain that coals so mixed afford more heat. For the purpose of lighting a fire speedily, Mr. Gray recommends the formation of " kindling balls, composed of equal parts of coal, charcoal, and clay, the two former reduced to a fine powder, well mixed and kneaded with clay moistened with water, and then formed into balls of the size of hens' eggs, and thoroughly dried, which, he says, may be used with great advantage, instead of wood. These kindling balls, he flirther observes, may be made so inflammable as to take fire in an instant, and with the smallest spark, by dipping them in a solution of nitre, and then drying them again ; if made of pure charcoal mixed with a solution of nitre, they would be still more inflammable. In situations where coals are scarce or dear we think that the mixtures recommended by Mr. Gray might be found convenient and economical; but when it is considered that the average price of coals in England is not more than a shilling for a hundred weight, we can hardly conceive it possible that the same weight of fire-balls, of the size of hens' eggs, could be manufactured for the sum mentioned. It would appear, from Mr. Gray's remarks, that he
was not aware that several patents had previously been taken out for the very objects mentioned by that gentleman ; and although the advantages of them may not be very apparent in most situations, there are doubtless many localities where it may be otherwise ; for the latter reason we shall, therefore, insert a brief notice of some of them. Mr. Sunderland's patent dated 1825, is for a fuel, in which gas-tar, clay, and refuse woody matter, are combined in various proportions, according to the degree of inflammability required. One part of gas-tar, one of clay, and two parts of any convenient woody matter, such as saw-dust, tanners' spent bark, dyers' refuse wood, or peat, burn extremely well If equal parts of the tar, clay, and saw-dust be employed, they make a com position which burns vividly and with a brilliant flame. The materials are, of course, to be thoroughly mixed, made up into lumps, and dried either artificially or in the open air, preparatory to their being used as fuel. Messrs. Christie and Harper s patent, dated 1824, was for various mixtures of culm and stone coal (or anthracite) with bituminous or caking coal, depending upon the nature of the heat required ; for the boiler furnaces of steam engines, where the bars are half an inch apart, the patentees state that one-fourth of the bituminous coal answers well for invigorating the other three-fourths. In 1800 Mr. Peter Devey had a patent for an improved artificial fuel, and the same gentleman, in 1821, had another patent for fuel balls, the particulars of which will be found in the specifications of their patents in the lnrolment offices in Chancery.