CHEMISTRY, 
with the black flux, alkali; carbon, or the other faline 
folvents ; but, with the addition of oxyd of lead or cop¬ 
per, the metals that afil'e are mixed with a portion of 
molybdena, which may be feparated. Let oxyd of mo- 
lybdena, made into a palle with oil, and dried by lire, be 
put into a clofe crucible, and urged with a Ifrong heat 
for two hours; the fubllance will be found glutinous, 
black, breaking eafily in the fingers, and having a me¬ 
tallic brilliancy; feen through a magnifier, little round 
grains of a greyilh metallic colour may be difcerned. 
This is true molybdena in the metallic ftate. 
When calcined, it changes into an oxyd more or 
lefs white. It detonates with nitre 5 and the refidue 
is oxyd of molybdena mixed with alkali. The nitric 
acid converts it into a white acid oxyd. Treated with 
alkalis, in the dry way, hydrogen gas is difengaged, and 
the refidue is only the molybdena in the oxyd hate, com¬ 
bined with the alkali. It mixes with metals in various 
ways : combined with iron, copper, and filver, it is very 
friable. Laftly, treated with fulphur, it re-produces the 
mineral molybdena, or fulphure of molybdena. 
Molyedic Acid.- —This acid is of a light yellow co¬ 
lour, leaving on the tongue an acidical metallic talfe: 
its lpecific gravity, according to Bergman, is to that of 
pure water, as 3-46 to i'oo. The air occafions no change 
in it; but it cannot be fublimed without air. It tinges 
the native phofphat with a fine green. Under the blow¬ 
pipe, it is volatilized in a white fume,'which is condenf- 
ed in needle-formed cryftais, at the fame time that part 
is melted on the fides of the crucible. Diftilled with 
three parts of fulphur, fulphure of molybdena is repro. 
duced. It diflolves in 576 parts of water at a mean tem¬ 
perature. It decompoles the folutions of foap, and pre¬ 
cipitates alkaline fulphures. 
The molybdic acid diflolves in great quantities in the 
concentrated fulphuric acid, by the afliflance of heat. 
This folution affumes a fine blue colour, and becomes 
thick by cooling. Both phenomena may be made to dif- 
appear by heat, and re-appear again in proportion as the 
fluid cools. If the combination be ftrongly heated in a 
retort, the fulphuric acid is volatilized, and the molyb¬ 
dic acid remains in a dry ftate at the bottom of the veffel. 
The nitric acid has no action on the molybdic acid. The 
common muriaticacid diflolves a large quantity by ebul¬ 
lition. This folution affords a refidue of a fine deep blue 
colour, when diftilled to drynefs. If the fire be more 
ftrongly urged, the refidue affords a white fublimate, and 
another bluifh ; a grey refidue remaining in the retort. 
The fublimate is deliquefeent, and colours metals blue : 
the muriatic acid paffes oxygenated into the receiver. It 
is eafy to underftand, that, in this operation, the muriatic 
acid deprives the molybdi c acid of a portion of oxygen, and 
that a portion of this acid paffes to the ftate of molybdena. 
The molybdic acid, by the afliftance of heat, decompofes 
die alkaline nitrats and muriats, by difengaging their 
acids, and forms with their bafes neutral falts, whofe 
properties Scheele did not examine. This acid likewife 
difengages the carbonic acid from the three alkalis, and 
forms neutral falts with their bafes. 
Though Scheele has not made us acquainted with all 
the properties of the neutral falts which we defign by the 
names of molybdats of potafh, of foda, of ammoniac, 
&c. he has neverthelefs pointed out three, which are fuf- 
ficient to characterize their ftate of neutralization. He 
has dilcovered, 1. That fixed alkali renders the acid of 
molybdena more foluble in water. 2. That this fait pre¬ 
vents the acid of molybdena from volatilization by heat. 
3- That the molybdat of potafh falls down by cooling in 
final 1 cryftalline grains, and that it may likewife be fepa¬ 
rated from this folvent by the fulphuricand muriatic acids. 
The acid of molybdena decompofes the barytic nitrat 
and muriat. The barytic molybdat, formed in thefe 
operations, is foluble in water. The acid of molybdena 
appears partly to decompofe the fulphat of potafh by a 
267 
ftrong heat. The molybdic acid diflolves feveral metals, 
and affumes a blue colour in proportion as this acid 
abandons to them part of its oxygen. 
The affinity of molybdena for oxygen feems to be very- 
weak, for confulerable changes are produced in it by 
only a finall difference in the proportion of the acids or 
alkalis with which it is combined, or the temperature to 
which it is expofed. In confirmation of this it may be 
remarked, that all metals, gold and platina excepted, de¬ 
prive the molybdic acid of a portion of its oxygen, fo as 
to caufe it to become blue. Pelletier obferved, that a.. 
folution of molybdic acid became blue, when hydroge¬ 
nous gas was paffed through it. Klaproth remarked, that 
light, in certain circuinltances, changed the colour of 
molybdic acid to blue. The fame phenomena were ob¬ 
ferved by Mr. Hatchett. Molybdic acid yields its oxygen 
in diftillation to arfenic, by which the arfenic is convert¬ 
ed into the white oxyd. The molybdic is alfo the only 
metallic oxyd which is deprived of its oxygen by diftil¬ 
lation with fulphat of ammoniac. 
The blue carmine is prepared by precipitating tin fr»m 
its folution in muriatic acid by the molybdat of potafh. 
The muriatic acid unites with the alkali, and the mo¬ 
lybdic with the tin, to form the blue precipitate. 
Of CHROME. 
This nietal, newly difeovered by Vauquelin, is found 
in the acid ftate in the red lead of Siberia, and in the 
oxyd ftate in the emerald, and in the green lead which 
is found among red lead. It was thought proper to give 
a name to this metal, from its principal diftinguifhing 
properties. For, the combinations of this metal with 
oxygen, give a green oxyd, or a red oxyd, according to 
the proportions of each principle, and thefe fubftances 
communicate the fame colour to all. the combinations 
they enter into; the name chrome, from therefore, 
which fignifies colour, is very properly applied to this 
fubftance. 
To obtain this acid, boil the red-lead, reduced to pow¬ 
der, with two parts of carbonat of potafh. The lead 
combines with the carbonic acid of the potafh; the al¬ 
kali unites with a particular acid, which gives it an orange 
colour, and the property of furni thing cryftais of the fame 
colour. This new combination is afterwards decompofed 
by the mineral acids, and by evaporating the liquor ufed 
in the operation, we obtain, 1. The fait produced by the 
mineral acid which was formed. 2. Acid of red lead, 
in the form of lengthened prifms of a ruby-colour. 
Another method, not lefs eafy than the foregoing, 
confifts in pouring upon one part of red-le;;d in powder, 
the fame quantity of muriatic acid, mixed with an equal 
part of water : a muriat of lead is thus formed, which 
is depofited in the form of white cryftais, and the liquor 
affumes a beautiful orange colour. In this operation the 
native acid of the red lead is feparated, and remains dif- 
folved in the water of the muriatic acid. Bv evaporat¬ 
ing the liquor llowly in the dark, cryftais of the acid of 
lead are formed, which are of a ruby-red colour. But 
if the muriatic acid be in too great quantity, or too high¬ 
ly concentrated, or if the operation be conduced in a 
ftrong heat, inftead of a red acid, a deep green liquor is 
formed by the combination of the oxyd of the new me¬ 
tal with the muriatic acid. Sulphuric acid alfo decom¬ 
pofes the red lead, but it is difficult to feparate the pro¬ 
ducts which are formed. Nitric acid produces no change 
in the nature of red lead. 
Chromic Acid.— This is of an orange-red colour, 
with a pungent metallic fmell; it is very foluble in wa¬ 
ter ; and the folution, evaporated with a gentle heat, or 
fpontaneoufly in the air, cryltallizes in finall lengthened, 
prifms, of a ruby-red colour. If paper be wet with this 
acid, and expofed for forne days to the rays of the fun, 
it affumes a green colour, which does not alter in the 
dark. A thin piece of iron or pewter, put into the folu- 
