MANITTACTUM Or GAS.] 



UNDULATORY FORCES. LIGHT. 



117 



condensers and purifiers. The details of the process 

 vary with each particular gas, and, consequently, it is 

 necessary to describe them under different heads. 



Coal Gas. The apparatus which is employed for the 

 manufacture of coal gas consists, first, of a retort, made 

 of iron or clay, set in a furnace in such a manner that 

 it may be heated throughout to a tolerably uniform 

 temperature; an outlet pipe ascends from the end of 

 the retort, and terminates in another pipe, called the 

 hydraulic main. This part of the apparatus is so con- 

 structed, that the pipe which delivers the gas from the 

 retort dips down, to the extent of three or four inches, 

 into the liquid contained in the hydraulic main. By 

 this means the end of it is sealed or closed by a water- 

 valve, and no gas can return from the other parts of the 

 apparatus into the retort during the time that the latter 

 is being charged with coal. The hydraulic main is a 

 large horizontal pipe, which runs from one end of the 

 building to the other ; it receives all the exit-pipes from 

 the retort, and discharges its contents into a series of 

 smaller pipes, which are so arranged in water and air, as 

 to form a large cooling surface for the condensation of 

 the liquid matters contained in the raw gas. Here it is 

 that water and tar are deposited ; and the apparatus is 

 go constructed that the fluid matters run off, as fast as 

 they are condensed, into a tank which is conveniently 

 placed to receive them. This part of the apparatus is 

 called the condenser. From the condenser the gas passes 

 to the purifiert, which are vessels charged with lime and 

 other substances that have the power of absorbing the 

 various impurities of coal gas. To assist the flow of gas 

 through these vessels, an instrument called an eshauster 

 is sometimes employed : it is a kind of air-pump worked 

 by a steam-engine, which draws the gas away from the 

 retorts, and so relieves them from that enormous amount 

 of pressure that they would otherwise have to encounter 

 while their gaseous contents are being forced on through 

 the various obstructions that intervene between them 

 and the gasometer. Leaving the purifiers, the gas passes 

 into the large receiver or gasometer, where it is stored. 



Material* employed in the Manufacture of Coal Oat. 

 Of the three varieties of coal, known to chemists, the 

 black or bituminous is the only one which is employed 

 to any extent for the manufacture of gas. Lignite, or 

 brown coal, is not sufficiently plentiful for this purpose ; 

 and glance-coal, or anthracite, is not rich enough in 

 hydrogen to be of any use to the gas manufacturer. A 

 small quantity of a peculiar bituminous shale, named 

 Boghead coal, has of late been employed in London and 

 elsewhere ; but its nature is not sufficiently well deter- 

 mined to enable us to say whether it is to be regarded 

 as a coal or not.* 



The varieties of black coal are exceedingly numerous : 

 in a general way, however, they may be divided into 

 four kinds namely, caking coal, which has the property 

 of melting when it is heated, and of running together; 

 c^-lint coal, which is so named from its splintery frac- 

 ture ; cherry coal, which burns without caking at all ; 

 and cannel coal, which is exceedingly hard, compact, 

 and bituminous. The first of these occurs abundantly 

 in the nri^lilx.urhood of Newcastle, Northumberland, 

 and Durham ; the second in South Wales; and the last 

 in So it.land and in Lancashire. Although these varieties 

 of coal differ very considerably in their value for gas- 

 producing purposes, yet regard must always be paia to 

 the convenience or facility with which they are obtained ; 

 and hence we find that, in London, the coals of New- 

 castle, with certain cannels of Lancashire and Scotland, 

 are employed. In Bristol and its neighbourhood, the 

 coals of (Gloucestershire and Wales are used ; in Bir- 

 mingham, those of Staffordshire and Wigan ; in Leicester, 

 iM>,'ham, and Derby, the coals of Derbyshire ; in 

 Leeds and Sheffield, those of Yorkshire ; in Liverpool, 

 Salford, and Manchester, those of Wigan ; in Edinburgh 

 and the north of Scotland, the coals are chiefly derived 

 from the Lothians and from Fifushire ; in Glasgow they 



At the promt tim* 'July, 11), an notion is pending in the Scotch 

 law courts to deci<l thi qunilon, bctwttn tne lesson and lcirc of the 

 Torbanehill property. JUitor. 



are obtained from Lesmahago, Kelvinside, Wilson- 

 town, <tc. ; and in Greenock they are procured from 

 Monkland and Skaterig. 



The temperature at which the carbonisation of the 

 coal is effected, and the manner in which the heat is 

 applied, have an important influence on the quality of 

 the gas and other products obtained. If the heat is too 

 low, the quantity of gas produced is small, while that of 

 the tar is large : on the contrary, if it is too high, the 

 latter is sacrificed for the generation of the former. In 

 the one case the gas is too rich in hydro-carbons, and in 

 the other it is too poor. In practice, therefore, it is 

 necessary to hit the happy medium ; and it is generally 

 thought that Newcastle coal requires a temperature of 

 a dull red heat (1300 Fah.) for its distillation, and that 

 the richer cannels will bear a temperature of 1800" Fah., 

 or a bright cherry red. 



Some idea may be formed of the relative value of the 

 different kinds of coal, by reference to the following 

 table, which exhibits the proportions of volatile matter, 

 coke, and ash, produced by each description of coal, as 

 well as the per-centage of sulphur contained in th-i coal, 

 the coke, and the gas. The coals are arranged in the 

 order of their gas-producing properties ; and it will be 

 seen that the amount of volatile matter ranges between 

 23 and 68 per cent., and that the quantity of sulphur 

 contained in the volatile matter is from 1 to 6 per 

 cent. : 



With regard to the manner in which heat is to be 

 applied, it may be said, that, within certain limits, the 

 more quickly it is so, the greater the quantity, and the 

 better the quality of the gas obtained ; for too slow a 

 heat generates volatile matter, which condenses in tar; 

 and too quick a heat decomposes the gas, and destroys 

 its illuminating powers. Lastly, it may be remarked, 

 that the duration of the heat ought not to exceed five 

 hours ; for at the expiration of that time the gases which 

 are evolved are of little use for illuminating purposes, 

 and the sulphur which is contained in the coke begins to 

 distil over as bisulphuret of carbon, which is a most ob- 

 jectionable impurity. 



Purification of Gat. This is effected in various ways; 

 some of the contrivances being mechanical in theii 

 action, and others chemical. When the volatile matten 



