472 
Coal Gas. 
Reference to Mr. S. Clegg’s Apparatus for extracting Carbu . - 
retted Hydrogen Gas from Pit Coal. See Plate, fgs. 1, 2,3, 
4, 5, and 6. 
In fig. 1, A shows the cast iron retort, into which are put the* 
-coals intended to be decomposed by means of a fire underneath 
it, the heat of which surrounds every part of it, excepting the 
mouth or part by which the coals are introduced. The lid or iron 
plate B, which covers the mouth of the retort, is ground on, air 
tight, and fastened by means of a screw in the centre ; C is a 
shield or saddle of cast iron, to preserve the retort from being 
injured by the intensity of the fire underneath it, and to cause it 
to be heated more uniformly. D D D represents the cast iron 
pipe which conveys all the volatile products of the coal to the 
refrigeratory of cast iron E, in which the tar, See. extracted from 
the coal is deposited, and whence they can be pumped out by 
means of the copper pipe F. G is the pipe which conveys the 
gas to the top of the cylindrical vessel or receiver H ; this re- 
ceiver is air tight at the top, and consequently the gas displaces, 
the water in the vessel H, to a level with the small holes, where 
the gas is suffered to escape and rise through the water of the 
well I, into the large gasometer K. The use of the vessel H is 
pointed out as follows, viz. If the pipe G reached all through 
the water, without passing into the vessel H, the gas would 
not be rendered pure or washed : and if part of the pipe did 
not rise above the water, the water would have free communi- 
cation with the tar, besides exposing the retort A to a very great 
pressure, so as to endanger its bursting when red hot. This 
vessel or receiver H, in a large apparatus, is about eighteen 
inches diameter, and two feet long : the quantity of gas therefore 
which it contains, is sufficient to fill the pipes and retort when 
cool, prevent the pipe G from acting as a siphon, and expose the 
gas to the water without endangering the retort. 
When the operation begins, the upper part of the cylindrical 
gasometer K, fig. 1, made of wrought iron plates, is sunk down 
nearly to a level with the top of the circular well I, and is con- 
sequently nearly filled with water, but it rises gradually as the 
gas enters it and displaces the water ; the two weights L L sus- 
pended over pullies by chains keep it steady and prevent its turn- 
ing round, otherwise the lower stays M of the gasometer would 
