128 On some Compounds of Chrome. 
gas and air through chromate of potash mixed with an alkaline’ 
carbonate. Carbonate of chrome then falls down on boiling the 
mixture; but if it contains too much nitrous acid, the whole will 
pass to the state of nitrate of chrome. This method, however, 
often fails: it is evidently the nitrous acid which reduces the 
chromic, and the oxide thus produced attracts to itself the car- 
bonic acid driven from the carbonate by an excess of acid. A 
better method of obtaining this carbonate is, to evaporate to 
dryness, a mixture of nitrate of ammonia, chromate, and car- 
bonate of potash; or of muriate of ammonia, with: a nitrate, 
carbonate, and chromate of alkali, This mixture, when gently 
dried, blackens ; it is then to be re-dissolved in water, and a drop 
or two of ammonia, which has the effect, I believe, of separating. 
a small quantity of carbonate of chrome which the nitrate of 
ammonia had retained in solution. 
If too high a degree of heat be applied, the excess of nitrate 
will re-produce the chromate. Here it is the protoxide of azote 
(nitrous oxide) in its nascent state which decomposes the chro- 
mic acid; for, when once become gaseous, it has no longer this 
property. If, on the other hand, the chromate of potash and 
the nitrate of ammonia are acidified with nitric acid, and dried 
and heated in a tube protected from the contact of air, no car- 
bonate of chrome whatever is obtained. 
A mixture of nitre and muriate of ammonia acts in the same 
mauner as nitrate of ammonia, because a double decomposition 
takes place, on account of the facility with which the nitrate of 
ammonia assumes the gaseous form. This dauble decomposi- 
tion always occurs when these salts are heated with the nitrate 
of any metal capable of forming a fixed chloruret with muriate 
of ammonia. 
Therefore, to obtain nitrous oxide, instead of employing caustic 
nitrate of ammonia, we may use nitrate of potash and muriate 
of ammonia, in the proportions suited to complete decomposi- 
tion, leaving, however, an excess of nitrate to avoid any subli- 
mation of the sal-ammoniac. The proportions may be about 
three parts of nitre to one of sal-ammoniac. 
Chromites. 
The existence of these salts is still doubtful, and Berzelius has 
not yet ventured to admit them positively in his System of Mi- 
neralogy. However, Vauquelin obtained a precipitate by pour- 
ing chromate of potash into proto-sulphate of iron, which he has 
found to be composed of oxide of iron and oxide of chrome, and 
is analogous to the chromic ore of the Var, particularly when 
the latter is calcined. Other chromites: tay be obtained with 
the muriates of manganese and of tin with oxide of chrome. 
That 
