With respect to the quantity of gas obtainable from a given weight of coal, it depends greatly upon the quality of the coal, and also upon the construction of the retorts, and the method of working them. Mr. Peckston, in his valuable treatise on gas lighting, whilst writing on this branch of the subject, observes that pit coal may be divided into three classes, according to the proportions of its component parts. Such coals as are chiefly composed of bitumen are to be considered as belonging to the first class. These coals burn with a bright yellowish blase during the whole process of combustion; they do not cake, neither do they produce cinders, but are reduced to white ashes. At the head of this class is to be placed Cannel coal; and most of the varieties of Scotch coal, as well as some of those found in Durham and Northumberland, belong to it, likewise the coals from Lancashire and the north-western coasts of When ellipsoidal retorts are used, (which is the form which Mr. Pe decidedly prefers,) and charged with li bushel, or about 1261bs of coal, the fol lowing quantities of gas may be obtained in the manufactory, or on the large scale. From a ton of Lancashire Cannel 11,600 cubic feet of gas.
Newcastle (Hartley's) 9,600 PP Staffordshire (best kind) 6,400 IP The coke obtained from coals of this class is in small quantity, and of very inferior quality. The second class of coals comprehends those varieties which cake in burning. These contain lees bitumen and more charcoal than the first class. They produce less ashes, but afford hard grey cinders, which when burnt over again with fresh coals, produce a very strong heat. The gas obtained from these coals is not of so rich a quality as that from the first class, butthe coke is extremely well adapted for domestic and culinary purposes. When ellipsoidal retorts are used, charged as before, with about a bushel and a half, from a ton of Wallsend, may be obtained . . 10,300 cubic feet of gas. Temple Main 8,100 fi Primrose Main 6,200 Pembry 4,200 PP The third class of coal consists of such as are chiefly composed of charcoal, chemically combined with different earths, and containing little or no bitumen. Amongst the varieties of this coal are the Kilkenny coal, the Welch coal, and the stone coal. None of the coals comprised in this class can be profitably used for making gas.
Mr. Peckston gives the following table, exhibitln the comparative quantity of gas obtainable from the following different species of coals comprehended in the first and second classes, the Cannel coal being considered the standard, and estimated at 1000.
With respect to the best form of retorts, and the mode of working them, so as to produce the largest quantity of gas, we give the following summary of three sets of experiments detailed in Mr. Peckston's work; the coals in each balance were of the same quality. 103 chaldtun 12 bushels, distilled in circular retorts, charged with 2 bushels of coals, and each charge worked off in 6 hours, afforded 8300 cubic feet of gas per chaldron, and required 43 chaldron 14 bushels of coals for heating the retorts, = 42 per cent. on the quantity employed for making gas. By cylindrical retorts charged with 2 bushels at each charge, and which was worked off in 8 hours, 85 chaldron, 27 bushels of coals yielded 10,000 cubic feet of gas per chaldron, and required for car bonization or heating of the retorts 21 chaldrons 16 bushels of coals, or about 25 per cent. of the quantity carbonized. With ellipsoidal retorts, the two diameters of which were 20 inches and 10 inches respectively, charged with II bushel, and each charge worked off in hours, 61 chaldron 8 bushels of coals yielded 14,000 cubic feet of gas per chaldron, and required for carbonization 19 chaldrons 27 bushels of coals, or 32 per cent. of the quantity carbonized. Mr. Peckston likewise states that five elliptical retorts are capable of carbonizing 45 bushels, or 33 cwt of coals, in 24 hours, but their average work may be taken at 1 chaldron, or 27 cwt. in that time.
Mr. Anderson, of Perth, made a great number of experiments, to determine the comparative quantity of light afforded by candles and coil gas; the size of the candles which he employed, was short sixes. The following are some of the results: A 3-let burner consumed per hour 2,074 cub. in. = 6 candles. An Argand of 5 holes 2,592 8 Ditto 10 Pt 3,798 12 Ditto 14 5,940 Ill Hi If Ditto 18 6,840 21 The mean of these results is, that 324 cubic inches of coal gas yield light equal to that of one candle for an hour; but this is the coal gas of Perth, the specific gravity of which, Mr. Anderson says, is 650.