GAS, COAL . 575 



6 to 9 feet, fig. 1030 ; third, the ear-shape, jig. 1031, now little used, 2 feet 9 inches, 

 and of the same length as before; fourth, the D-shaped retort, /#. 1032/20 inches 

 wide and 14 inches high. This form of retort is at present far more extensively used 

 than any of the others. 



Fig. 1033 shows a bed of 5 D-shaped iron retorts. The length is 7 feet, and the 

 transverse area, from one foot to a foot and a half square. The arrows show the 

 direction of the flame and draught. 



The charge of coals is most conveniently introduced in a tray of sheet-iron, made 

 somewhat like a grocer's scoop, adapted to the size of the retort, which is pushed home 

 to its further end, inverted so as to turn out the contents, and then immediately 

 withdrawn. 



All these retorts are set horizontally in the furnace, and they have a flanch cast upon 

 their open end, to which a mouthpiece, A A,^. 1034, can be securely bolted. The 

 mouthpiece is provided with a socket, B, for the reception of the standpipe, and also 

 with an arrangement by which a lid, c c, can be screwed gas-tight upon the front of 

 the mouthpiece as soon as the charge of coal has been introduced. By applying a 

 luting of lime mortar to that part of the lid which comes into contact with the mouth- 

 piece, a perfectly tight joint is obtained. 



Sometimes iron retorts are made of double the above length, passing completely 

 through the furnace, and being furnished with a lid and standpipe at each end. 

 Such is the construction of Mr. Croll's and of Lowe's reciprocating retorts. These 

 retorts are charged from each end alternately, and there is an arrangement of valves 

 by means of which the gas evolved from the coal recently introduced is made to pass 

 over the incandescent coke of the previous charge, at the opposite end of the retort. 

 It is highly probable that some advantage is derived from this arrangement during 

 the very early stage of the distillation of the fresh coal ; but on the whole, for reasons 

 stated above, the principle is undoubtedly bad, for although it enables the manufacturer 

 to produce a large volume of gas, the quality is so much inferior as to reduce the total 

 illuminating effect obtainable from a given weight of coal. 



Wrought-iron Eetorts. Mr. King, the eminent engineer 'of the Liverpool Gas 

 Works, has for many years successfully used retorts of wrought iron. They are 

 made of thick boiler plates, riveted together, and are of the D shape, 5 feet wide, 6 

 feet long, and 18 inches high at the crown of the arch. About 1 ton of coal can be 

 worked off in these retorts in 24 hours. Occasionally the bottoms are of cast iron, 

 which materially prevents the great amount of warping to which wrought iron is sub- 

 ject when exposed to high temperatures. 



Earthenware, or Clay Eetorts. These are usually of the D shape, although they 

 are occasionally made circular or elliptical. Their dimensions are about the same as 

 those of the cast-iron retorts commonly used, but their walls are necessarily thicker, 

 varying from 2 to 4 inches in thickness ; this, added to the circumstance that clay 

 is a very bad conductor of heat, undoubtedly causes the expenditure of a larger 

 amount of fuel in heating these retorts ; nevertheless, this disadvantage is, perhaps, 

 less than might be supposed, since iron retorts soon become coated outside with a 

 thick layer of oxide of iron, which also greatly hinders the free communication of 

 heat to the iron beneath. Moreover, the lower price and much greater durability of 

 clay retorts are causing their almost universal adoption in gas-works, especially since 

 the removal of pressure by exhausters greatly reduces the amount of leakage to which 

 clay retorts are liable. 



The following is an extract relating to clay retorts, from the ' Reports of Juries ' of 

 the Great Exhibition of 1851 : 



' The use of fire-clay is not of very ancient date, and has greatly increased within 

 the last few years, It is found in England almost exclusively in the coal-measures, 

 and from different districts the quality is found to differ considerably. The so-called 

 ' Stourbridge clay,' is the best known, and will be alluded to presently ; but other 

 kinds are almost, if not quite, as well adapted for the higher purposes of manufacture, 

 being equally free from alkaline earths and iron, the presence of which renders the 

 clay fusible when the heat is intense. The proportion of silica and alumina in these 

 clays vary considerably, the former amounting sometimes to little more than 50 per 

 cent., while in others it reaches beyond 70, the miscellaneous ingredients ranging from 

 less than to upwards of 7 per cent. 



' The works of Messrs. Cowen and Co. are among the most extensive in England, 

 and they obtain their raw material from no less than nine different seams, admitting 

 of great and useful mixture of clay for various purposes. 



' After being removed from the mine, the clay is tempered by exposure to the 

 weather, in some cases for years, and is then prepared with extreme care. The objects 

 chiefly made are fire-bricks and gas-retorts ; the latter being now much used, and 

 preferred to iron for durability. 



