163 
the compounds of chromium^ 
the solution.* The same change takes place if we boil a 
mixture of chromate of potash and muriatic acid together. 
The addition of a little alcohol greatly promotes the rapidity 
of this process. The method proposed by Berzelius is one 
of the most tedious and unproductive. He passes a current of 
sulphuretted hydrogen gas through a solution of chromate 
of potash, previously acidulated with muriatic acid. 
In whatever way the green oxide of chromium is procured, 
it is always at first in the state of a hydrate. If we dry the 
precipitate in the open air, it is a greenish blue tasteless 
light substance, which may or may not contain carbonic acid, 
according to the re-agent employed in throwing it down. If 
we dry it on the filter by artificial heat, its colour becomes a 
good deal darker, and it retains almost exactly half its weight 
of water. When it is dried in the open air without artificial 
heat, Y^ths of its weight are water. In this state of hydrate 
it dissolves readily in almost all acids. A moderate heat 
expels the water, and leaves the oxide in the state of an ex¬ 
ceedingly beautiful green powder, scarcely soluble in any acid 
whatever. When this green oxide is heated nearly to redness 
in an open vessel, it generally glowSy or becomes intensely 
red hot, so as to resemble the appearance of burning tinder. 
This glowing does not always take place, though it is a pretty 
common phenomenon. It has not yet been determined, upon 
what this curious property depends. 
• To prevent misapprehension, it will be proper to state, that the oxide thrown 
down by sulphurous acid has a brown colour; but if we dissolve it in muriatic 
acid, and throw it down by ammonia, the precipitate obtained will be green oxide. 
It will be shown hereafter, that brown oxide differs from green oxide, merely by 
retaining a small quantity of chromic acid, with which the green oxide is combined. 
