ALU 
for alum. And Mr. Proust has shewn that 
a number of neutral salts, composed of ve- 
getable acids and earths, when distilled by 
a strong fire in a retort, leave a residuum 
which takes fire spontaneously on exposure 
to tile air. These facts have thrown a great 
deal of light on the nature of Homberg’s py- 
rophorus, and enabled us in some measure 
to account for its spontaneous inflammation. 
It has been ascertained, that part of the sul- 
phuric acid is decomposed during the for- 
mation of the pyrophorus, and of course a 
part of the alkaline base becomes uncom- 
bined with acid; and the charcoal, which 
gives it its black colour, is evidently divided 
into very minute particles. It has been as- 
certained, that during the combustion of 
the pyrophorus a quantity of oxygen is ab- 
sorbed. The inflammation is probably oc- 
casioned by the charcoal ; the sulphuret of 
potash also acts an essential part. Perhaps 
it produces a sudden increase of tempera- 
ture by the absorption and solidification of 
water from the atmosphere. 
A new process for making alum is used at 
some Works, for which we are indebted to 
Mr. Sadler, which is as follows : The boilers 
are filled with prepared liquor of 10 penny- 
weights, to which sulphate of potash is ad- 
ded, and boiled together, until it weighs 16 
pennyweights, by which time the whole of 
the superfluous alumina and the oxyde of 
iron is precipitated. The fluid is then run 
into a settler, where it remains until clear, 
after which it is pumped into a second boiler, 
and evaporated up to 26 pennyweights, let 
into the coolers, and left to crystallize. By 
this process, it is said, he gains the whole of 
the alum at one evaporation, and from the 
mother liquor remaining there is a product, 
the sulphate of iron. 
ALUMINA, in chemistry, one of the five 
proper earths. It was discovered by the 
alchemists that alum was composed of sul- 
phuric acid and an earth, the nature of 
which was long unknown; but Geoffroy, 
and afterwards Margraff, found that the 
earth of alum is an essential ingredient in 
clays, and gives them their properties, hence 
it was called argil-, but Morveau gave it 
the name of alumina, because it is obtained 
in a state of the greatest purity from alum 
by the following process. Dissolve alum in 
water, and add to the solution ammonia as 
long as any precipitate is formed. Decant 
off the fluid part, and wash the precipitate 
in a large quantity of water, and then allow 
it to dry. The substance thus obtained is 
alumina ; not however in a state of absolute 
ALU 
purity, for it still retains a portion of the 
sulphuric acid with which it was combined 
in the alum. But it may be rendered toler- 
ably pure, by dissolving the newly precipi- 
tated earth in muriatic acid, evaporating 
the solution till a drop of it in cooling depo- 
sits small crystals, setting it by to crystallize, 
separating the crystals, concentrating the 
liquid a second time, and separating the 
crystals which are again deposited. By 
this process, most of the alum which the 
earth retained will be separated in crystals. 
If the liquid be now mixed with ammonia as 
long as any precipitate appears, this precipi- 
tate, washed and dried, will be alumina 
nearly pure. Alumina has little taste : when 
pure, it has no smell ; but if it contains oxyde 
of iron, which it often does, it emits a pecu- 
liar smell when breathed upon, known by 
the name of earthy smell. This smell is 
very perceptible in common clays. The 
specific gravity of alumina is 2.00. When 
heat is applied to alumina, it gradually loses 
weight, in consequence of the evaporation 
Of a quantity of water with which, in its 
usual state, it is combined; at the same 
time its bulk is considerably diminished. 
The spongy alumina parts with its moisture 
very readily ; but the gelatinous retains it 
very strongly. Spongy alumina, when ex- 
posed to a red heat, loses 0.58 parts of its 
weight ; gelatinous, only 0.43 : spongy alu- 
mina loses no more than 0.58 when exposed 
to a heat of 130° Wedgewood ; gelatinous in 
the same temperature loses but 0.4825. Yet 
Saussure has shown that both species, after 
being dried in the temperature of 60°, con- 
tain equal proportions of water. Alumina 
Undergoes a diminution of bulk proportional 
to the heat to which it is exposed. This 
contraction seems owing, in low tempera- 
tures, to the loss of moisture ; but in high 
temperatures it must be owing to a more in- 
timate combination of the earthy particles 
with each other ; for it loses no perceptible 
weight in any temperature, however high, 
after being exposed to a heat of 130° Wedga- 
Wood. 
Mr. Wedgewood took advantage of this 
property of alumina, and by means of it 
constructed an instrument for measuring 
high degrees of heat. It consists of pieces 
of clay of a determinate size, and an appa- 
ratus for measuring their bulk with accu- 
racy: one of these pieces is put into the 
fire, and the temperature is estimated by 
the contraction of the piece. The contrac- 
tion of the clay-pieces is measured by means 
of two brass rules, fixed upon a plate, tire 
