ALUM. 
tus, because it is slaty. Its colour is black- 
ish, because it contains some bitumen. In 
most cases it is necessary to burn it before 
it can be employed : this is done by means 
of a slow smothered fire. Sometimes long 
exposure to the weather is sufficient to pro- 
duce an efflorescence of alum on the surface. 
It is then lixiviated, and the water concen- 
trated by evaporation, and mixed with pu- 
trid urine, or muriate of potash ; crystals of 
alum and of sulphate of iron usually form to- 
gether. The composition of alum has been 
but lately understood with accuracy. It has 
been long known, indeed, that one of its in- 
gredients is sulphuric acid; and the experi- 
ments of Pott and Margraff proved incontes- 
tibly that alumina is another ingredient. But 
sulphuric acid and alumina are incapable of 
forming alum. Manufacturers knew that the 
addition of a quantity of potash or of ammo- 
nia, or of some substance containing these al- 
kalies, is almost always necessary, and it was 
proved, that in every case in which such ad- 
ditions are unnecessary-, the earth from 
which the alum is obtained contains already 
a quantity of potash. Various conjectures 
were made about the part which potash 
acts in this case ; but Vauquelin and Chap- 
tal appear to have been the first chemists 
that ascertained by decisive experiments 
that alum is a triple salt, composed of sul- 
phuric acid, alumina, and potash or ammo- 
nia united together. Alum crystallizes in 
regular octahedrons, consisting of two four- 
sided pyramids applied base to base. The 
sides are equilateral triangles. The form of 
its integrant particles, according to Hauy, 
is the regular tetrahedron. Its taste is, 
as we have observed, astringent. It always 
reddens vegetable blues. Its specific gra- 
vity is 1.7109. At the temperature of 
60° it is soluble in from 15 to 20 parts of 
water and in |ths of its weight of boiling wa- 
ter. When exposed to the air, it effloresces 
slightly. When exposed to a gentle heat 
it undergoes the watery fusion. A strong 
heat causes it to swell and foam, and 
to lose about 44 per cent, of its weight, 
consisting chiefly of water of crystallization. 
What remains is called calcined or burnt 
alum, and is sometimes used as a corrosive. 
By a violent heat, the greater part of the 
acid may be driven off. Though the pro- 
perties of alum are in all cases pretty nearly 
the same, it has been demonstrated by Vau- 
quelin that three varieties of it occur in com- 
merce. The first is, super-sulphate of alu- 
mina and potash; the second, super-sul- 
phate of alumina and ammonia; the third, 
is a mixture or combination of these two, 
and contains both potash and ammonia. It 
is the most common of all; doubtless, be- 
cause the alum-makers use both urine and 
muriate of potash to crystallize their alum. 
Vauquelin has lately analysed a number of 
specimens of alum manufactured in different 
countries. The result was, that they all con- 
tain very nearly the same proportion of in- 
gredients. Tlie mean of ali his trials was as 
follows : 
Acid 30.52 
Alumina 10.50 
4 Potash 10.40 
Water 48.58 
100.00 
When an unusual quantity of potash is 
added to alum liquor, the salt loses its usual 
form and crystallizes in cubes. This con- 
stitutes a fourth variety of alum, usually dis- 
tinguished by the name of cubic alum. It 
contains an excess of alkali. When the pot- 
ash is still further increased, Chaptal Isas 
observed, the salt loses the property of cry- 
stallizing altogether, and falls down in flakes. 
This constitutes a fifth variety of alum, con- 
sisting of sulphate of potash combined with 
a small proportion of alumina. If three 
parts of alum and one of flour or sugar be 
melted together in an iron ladle, and the 
mixture dried till it becomes blackish and 
ceases to swell; if it be then pounded ynall, 
put into a glass phial, and placed in a sand- 
bath till a blue flame issues from the month 
of the phial, and after burning for a minute 
or two be allowed to cool, a substance is ob- 
tained known by the name of Homberg’s 
pyrophorus, whicli has the property of catch- 
ing fire whenever it is exposed to the open 
air, especially if the air be moist. This sub- 
stance was accidentally discovered by Hom- 
berg about the beginning of the eighteenth 
century, while he whs engaged in his expe- 
riments on the human fseces. He had dis- 
tilled a mixture of human feces and alum 
till he could obtain nothing more from it by- 
means of heat ; and four or five days after 
while he was taking the residuum out of the 
retort, he was surprised to see it take fire 
spontaneously. Soon after, Lemery the 
younger discovered that honey, sugar, flour, 
or almost any animal or vegetable mat- 
ter, could be substituted for human feces ; 
and afterwards, Mr. Lejoy de Suvigny 
shewed that several other salts containing 
sulphuric acid may be substituted for alum. 
Scheele proved that alum deprived of pot- 
ash is incapable of forming pyrophorus, and 
that sulphate of potash may be substituted 
K 2 
