60 
POPULAR SCIENCE REVIEW. 
ammonia are well known ; not only is it largely employed in 
medicine, lint, when mixed with some aromatic substance, it is 
used in scent-bottles, thus affording a striking instance of the 
transmutations effected by scientific agency, the foetid liquid of 
the gas-works being transformed into a scent used by ladies as a 
cherished luxury. Besides sal-ammoniac, the manufacture of 
which from gas-water exceeds 4,000 tons annually, about 5,000 
tons of sulphate of ammonia are also produced by adding sul- 
phuric acid to the gas liquor. This is of great value as a 
manure, and is also one of the principal component parts of 
alum, so largely used in dyeing and calico printing. 
This compound alum consists of the earth alumina in com- 
bination with ammonia and sulphuric acid. Chemically speaking 
it is a double salt of sulphate of alumina and sulphate of am- 
monia, and is prepared among other ways by acting upon a 
mineral containing alumina with sulphuric acid, and then mixing 
the resulting liquid with sulphate of ammonia. For the pur- 
poses to which alum is applied the sulphate of ammonia is 
superfluous, as all that the dyer wants is a solution containing 
the earth alumina ; but it must have no impurities in it. Now the 
only practicable way of purifying a salt is by crystallization, 
and it so happens that alumina salts are about the most un- 
crystallizable in the whole range of chemistry. Fortunately, 
however, the double salts which alumina forms with alkalies 
possess, like most double salts, very strong crystalline ten- 
dencies, and therefore manufacturers add sulphate of ammonia 
to the sulphate of alumina in order to obtain a compound which 
is capable of ready purification owing to its eminently crystal- 
lizable properties. 
But the alum manufacturer is not only indebted to gas 
makers’ by-products for his ammonia, but likewise for the sul- 
phuric acid. We mentioned that one of the chief impurities in 
gas was sulphur, and alluded to a method of removing- this body 
by a mixture of sawdust and iron. The action of the sulphur 
compound upon this is to produce water and sulphide of non ; 
when it has in this manner absorbed as much sulphur as it can, 
air is passed through the mixture ; the effect of this is to con- 
vert the sulphide of iron back again into oxide of iron, the 
sulphur being separated in the form of powder. The mixture 
being in this way revivified is ready to absorb a fresh quantity 
of sulphur-impurity from the gas, and thus the processes go on 
alternately until the pores of the mixture are completely choked 
up with the deposited sulphur. 
This spent material is now used for the manufacture of sul- 
phuric acid by burning it in properly constructed furnaces, and 
allowing the products of combustion to mix with nitrous vapours 
and aqueous vapour in enormous leaden chambers. The sulphur 
