1^8 
and of subsequent moisture, produces such a change on the iron, 
that it is no longer capable of being held in combination with 
the acid, which therefore unites with the clay. An addition of pot- 
ash, and of ammonia, that contained in putrified urine being ge- 
nerally employed, being now made, a deposition of earthy and fer- 
ruginous matter takes place, and then the remaining liquor being 
drawn off into another vat, the crystals of this triple salt are soon 
formed. 
The whole of that part of the territory of Liege which is dug for 
coal, is also worked for the alum earth ; considerable quantities of 
alum are said to be found there, formed in crystals on the slate, and 
adhering to the clefts of the free-stone. 
By a little alteration of the above described process, the sulphate 
of iron, or, as it is improperly called, green vitriol, or copperas, is 
obtained from the pyritous strata which accompany coal. 
The melting of the bituminous matter of coal is very often suf- 
ficiently apparent in the burning of coal in our ordinary fires ; and 
by the action of heat on this substance, in close vessels, or in a proper 
apparatus for distillation, its constituent parts are ascertained with a 
tolerable degree of certainty. The products thus obtained, are, a 
water impregnated with ammonia, concrete carbonate of ammonia, 
an acid, and an oil, which becomes more heavy, and of a darker 
colour, in proportion as the operation advances. At the same time, 
a considerable quantity of elastic and inflammable gas passes over, 
which has been considered as volatile oil in a state of vapour, 
but which really is hydrogen gas mixed with nitrogen gas, carbon 
in a state of solution, and carbonic acid gas. There remains in the 
retort a scorified cindery matter, which is yet capable of burning, 
and is exactly the same as the coke, which is frequently made on 
a larger scale, for particular purposes, where its mode of burning 
is more desirable than that of coals. The ashes of coals yield the 
