Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 19, Scripture-SUG

ANALYSIS OF STONES, See. 7*5
Preliminary ance, tiie oilier eartlis predominate ; but if it remain in
precedes. a p0Wdery form, the aluminous earth is in greateft pro-
portion. The oxides of different metals are indicated by
the colour of the mafs •, when it is of a dark or brown-
ilh red, the metallic oxide is that of iron a grafs green
colour denotes manganefe j and yellovvilh green the
oxide of chromium.
But there are fome ftones on which potalh has a very
feeble a£lion, and in this cafe borax has been fubftituted
for the alkali. This is the method which was followed
by Mr Chenevix in analyfing aluminous ftones. A hun¬
dred grains of fapphire in powder were mixed with 250
grains of calcined borax, and fubjefted to a ftrong heat
in a crucible of platina for two hours. When the mafs
was cold, it exhibited the appearance of a greenilh blue
glafs, which adhered ftrongly to the crucible ; but the
whole being boiled for fome hours in muriatic acid, it
was completely diffolved ; the earthy matter was then
precipitated by means of fub-carbonate of ammonia, and
the precipitate, after being well waftied, was again dif¬
folved in muriatic acid •, and in this way the borax was
feparated. The remaining part of the analyfis was nearly
fimilar to that dire&ed for other ftones, excepting only
that the alumina was feparated from the potalh by means
of muriate of ammonia.
But to return to the examination and farther treat¬
ment of the mafs in the filver crucible, which after be¬
ing removed from the furnace, and wiped on the out-
fide, is to be placed in a porcelain capfule it is then
filled with water, and this water is renewed oceafional-
ly, till the whole matter is feparated from the crucible.
By this means a part of the compound of the alkali
with the fdiceous and aluminous earths, is diffolved,
and with a fufficient quantity of water, the whole may
be diffolved. Muriatic acid is now to be added till
the whole of the mafs is brought to a ftate of folution.
This, however, will not be the cafe, if the ftone be
compofed chiefly of filica. On the firft addition of the
acid, a flakey precipitate is produced, becaufe the acid
unites with the alkali, which held the mafs in folution.
An effervefcence afterwards takes place, which arifes
from the decompofition of a portion of carbonate of pot-
afh, formed during the fufion •, and the flakey precipi¬
tate is again diflblved, as well as the matter which re¬
mained in the form of powder at the bottom of the veffel.
If the powder be filica and alumina, there is no effer¬
vefcence ; but if it contain lime, an effervefcence is pro¬
duced. The folution in the muriatic acid being formed,
if it fliall appear colourlefs, it may be inferred that it con¬
tains no metallic oxide, or at lead a very fmall portion.
An orange red colour (hews that it contains iron, a purp-
Hfli red indicates manganefe, and a golden yellow, chro¬
mium.
The folution is now to be introduced into an evapo¬
rating difb of porcelain, and being covered with paper,
is to be placed on a fand bath, and evaporated to dry-
nefs. Towards the end of the evaporation, as the li¬
quid affumes the form of a jelly, it muft be conftantly
ftirred with a rod of lilver or porcelain, to permit the
acid and water to pafs off, and to allow the whole mafs
to be equally dried } for it is in this way that the filica
and alumina are feparated from each other. I he mat¬
ter being reduced to a dry powder, add to it a large
quantity of pure water, expole it to a moderate heat,
and pour it on a filter. This folution may be denomi¬
nated A. Wafti repeatedly the powder which remains Preliminary
upon the filver, till the water with which it is walhed PraceiTes^
no longer precipitates filver from its folutions. The v~
powder remaining is filiceous earth, which is firft to be
dried between folds of blotting paper, and then made red
hot in a crucible of platina or filver ; and when it is cold
it is to be accurately weighed. If it be pure filiceous earth,
it is in the form of a white powder, is of a white co-(
lour, does not adhere to the fingers, and is infoluble in
acids. If it be at all coloured, it (hews that it contains
fome metallic oxide, and is a proof that the evaporation
has been carried on with too great a heat. To feparatc
the oxide, boil the filica with an acid, and then walh
and dry it as before. This acid folution is to be added
to the folution A, and the whole is to be evaporated to
about the quantity of an Englifh pint; then add to it a
folution of carbonate of potalh, till the precipitation
ceafes j and it may be neceffary to boil it a few mo¬
ments, to allow the whole of the precipitate to fall to
the bottom. The whole of the precipitate being colledl-
ed at the bottom, the fupernatant liquid is decanted oft',
and the water being put in its place, the precipitate and
water are thrown on a filter ; and when the water has
run off, the filter with the precipitate upon it is placed
on the folds of blotting paper. After the precipitate has
acquired fome degree of confiftence, colledt it carefully
with an ivory knife, mix it with a folution of pure pot-
afli, and boil it in a capfule of porcelain. The potalh
diffolves the alumina or glucina, and the other fub-
ftances remain in the form of a powder. This powder
may be called B.
Add to the folution of potalli as much acid as will
faturate the potalh, and aifo rediffolve any precipitate
which at firft appeared ; and then add carbonate of am¬
monia till the talle of it be perceptible in the liquid.
The whole of the alumina is now precipitated in the
form of white flakes, while the glucina remains diffol¬
ved, if a fufficient quantity of carbonate of ammonia
had been employed. Filter the liquid ; and the alumina
remaining on the filter being waftied and dried, and af¬
ter being made red hot, and allowed to cool, is weigh¬
ed. To prove its being alumina, diffolve it in fulphu-
ric acid, and a fufficient quantity of fulphate or acetate
of potalh being added, the whole of it will be converted
into alum cryftals, if the earth employed be aluminous
earth.
To fe par ate the glucina, the liquid which paffed
through the filter is to be boiled for fome time, and if
the folution contain any of this earth it will be precipi¬
tated in the form of a light powder, which may be dried
in the ufual manner, and weighed. It is a fine, foft,
light, tallelefs powder, when in a ftate of purity ; and
the application of heat does not make it concrete, as
happens to alumina.
We now return to the reliduum B, in which may be
expefted lime, magnefia, and fome of the metallic ox¬
ides. But if it be fufpefted that this refiduum contains
any yttria, it is to be treated with carbonate of ammo¬
nia, which diffolves the yttria, and leaves the other bo¬
dies untouched. The yttria being feparated, the reli¬
duum B is to be diffolved in weak fulphuric acid, and
the folution evaporated to drynefs. Add a fmall quan¬
tity of water, which will diffolve the fulphate of mag¬
nefia, as well as the metallic fulphates •, but the fulphate
of lime remains undiffolved, or if any part of it Ihould
4X2 diffiftve*