Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 5, BUR-CHI

Cobalt,
&c.
Ann. de
'Jhitn. xlii.
jiiS-
1607
^epara-
' isn.
160S
feympathe-
;ic ink.
1609
Theories.
leio
Another
irocefs.
CHEMISTRY.
is difengaged j the oxide of cobalt is precipitated, and bines with the oxide, by mixing a Join lion of nitrate
a nitrate of potalh is formed *. of cobalt with a folution of borax.
627
Cobalt,
8tc.
4. Muriate of Cobalt.
1. Muriatic acid has no effeft on cobalt in the cold $
but a fmall quantity is diffolved with the afliftance of
heat. But the black oxide of cobalt is readily diflblv-
ed in muriatic acid. The folution is accompanied with
effervefcenci->i and the difengagement of oxymuriatic
acid gas. When this folution is concentrated by eva¬
poration, it becomes of a fine green colour, which
changes to red when it is diluted with rvater. By
farther evaporation it is cryftallized, and affords fmall
deliquefcent cryftals of muriate of cobalt in the form
of needles.
2. When thefe cryftals are diffolved in water, and
fo diluted that the folution is nearly colourlefs, cha¬
racters marked with it on paper difappear entirely :
but when heated, affume a fine green colour. This
folution was one of the firft known fympathetic inks.
In making experiments with this folution, the charac¬
ters are written on paper, or, that the experiment may
be more amufing, a landfcape is drawn with a pen¬
cil, reprefenting the verdure of fummer on a winter
fcene. Thofe parts of the picture in which the fym¬
pathetic ink has been ufed, are invifible in the cold $
but when it is moderately heated, they become of a
fine green colour, changing from the winter to the
fummer fcene. When it is removed to the cold, the
colour again difappears, and if too much heat be not
applied, the fame change may be frequently repeated.
When too much heated, the blue colour is converted
to a brown, which becomps permanent.
3. Various theories have been prepofed to account
for this remarkable change. According to fome, it
is owing to the moifture of the atmofphere being ab-
forbed that the colour difappears ; and when this is
driven off by heat, it is reftored. But to this opinion
it has been objeCled, that the fame effeCl is produced,
when paper, on which characters have been written
with this folution, is entirely excluded from the at-
mofphere, by being introduced into clofe veffels. Ac¬
cording to others, the fympathetic effeCt of this folu¬
tion depends on the iron which is combined with the
cobalt. Some fuppofe that the concentration of the
folution, which takes place by the aCHon of heat, is
the caufe of the appearance of the colour ; and its di¬
lution, by abforbing moifture from the atmofphere,
the caufe of its difappearance ; while others are of
opinion that it is partially deprived of its oxygen by
being heated, and abforbs it again in the cold, when
the colour vanifhes.
This fympathetic ink may be eafily prepared, by
diffolving the zaffre of commerce in nitro-muriatic
acid.
5. Fluate of Cobalt.
Fluoric acid diffolves the oxide of cobalt, and forms
with it a yellow-coloured gelatinous folution 5 or, by
careful evaporation, it affords cryftals, which are fluate
of cobalt.
6. Borate of Cobalt.
Boracic acid has no aftion on cobalt} but it com-
7. Phofphate of Cobalt.
Phofphoric acid diflblves the oxide of cobalt, and
forms with it a reddifti-coloured turbid folution,
which affords a precipitate when the acid is fatu-
rated.
8. Carbonate of Cobalt.
This fait is formed by precipitating cobalt from it^
folutions in acids, by means of alkaline carbonates.
One hundred parts of cobalt, which afford only 143
of precipitate by means of the pure alkalies, yield
160 parts, when the precipitate is effefted by carbo¬
nate of foda.
9. Arfeniate of Cobalt.
This fait is formed by combining the nitrate of co¬
balt w ith the arfeniate of potafh or of foda. It is fome-
times found native, and it exhibits the deepeft and moft
beautiful red of all the falts of cobalt.
10. Tungftate of Cobalt. '
I t. Molybdate of Cobalt. Tt .
12. Chromate of Cobalt. Unknown.
13. Columbate of Cobalt. 1
14. Acetate of Cobalt.
This fait is readily formed, by diffolving the oxide
of cobalt in acetic acid. It does not yield cryftals by
evaporating, but is deliquefcent in the air. It affumes
a blue colour when it is heated, but is red in the
cold, fo that it forms a fympathetic ink.
15. Oxalate of Cobalt.
This fait may be formed by precipitating the oxide
of cobalt from its folution in acids, by means of oxalic
acid. This precipitate, when it is dried, is in the
form of a red powder, which is infoluble in water, but
may be diffolved in excefs of oxalic acid, and crvftal-
lized.
16. Tartrate of Cobalt.
The oxide of cobalt is foluble in tartaric acid, and
forms a red-coloured folution, which affords cryftals
by evaporation.
II. Aflion of Alkalies, Earths, and Salts.
1. The alkalies have no a&ion whatever on cobalt; alkalies,
but w'hen the oxides are fufpended in water, they fe-
parate them from other matters. ^
2. Some of the earths, but particularly filica, enter Earti1S4
into combination with the oxide of cobalt and the fix¬
ed alkalies, and form a beautiful blue-coloured glafs.
The quantity of oxide mult be fmall, otherwife the
glafs will appear nearly black and opaque, on account
of the intenfity of the colour.
3. Some of the neutral falts expofed to a high tem-Salts,
perature along with csbalt burn with a perceptible
flame. It is by this means that the oxide is prepared
for the purpofe of enamels and colouring porcelain.
The hyperoxymuriate of potalh, with one-third of
its weight of cobalt in powder, detonates by percuf-
fion.
4 K 2
Cobalt