200 METHOD OF PREPARING CHROMIC ACID, ETC. 
two effects, and from the peculiar property which will sub- 
sequently be described, I adopted the following process as 
the best: 
A boiling saturated solution of the bichromate of pot- 
ash is formed, and during ebullition a weighed quantity of 
sulphuric acid added to it sufficient, to form with the pot- 
ash bisulphate. The mixture is allowed to cool, when it 
solidifies for the greater part to a granular red mass. This 
is not chromic acid, but sulphate of potash, with adherent 
chromic acid, together with which is formed a concentrated 
solution of chromic acid, which likewise contains some sul- 
phate of potash. The mass is stirred with a rod to cause 
the granular part to subside, and the liquid portion decant- 
ed. The residuous mass is agitated wit h several small quan- 
tities of cold water, aud what dissolves poured off. In this 
way there is at last contained in the dish an orange-colour- 
ed sulphate of potash with very little chromic acid. Most 
of the chromic acid is contained in the united solution. 
This process depends on the circumstance, .that bisulphate 
of potash, which is very soluble at a boiling temperature (1 
part in | water,) is dissolved with difficulty at the ordinary 
temperature, and that cold water mostly removes from it 
sulphuric acid with scarcely any potash, leaving behind 
neutral sulphate of potash, while the chromic acid is ex- 
tremely soluble in the cold water. The concentrated solu- 
tion of the chromic acid, containing a little sulphate of pot- 
ash and sulphuric acid, may now be somewhat evaporated, 
and the chromic acid precipitated from it by the addition 
of sulphuric acid, without any perceptible traces of the sul- 
phate of potash being thrown down with it; for this salt is 
readily soluble in the monohydrated sulphuric acid, and 
still more readily in acid more diluted. The chromic acid 
is separated from the liquid by draining on a funnel, the 
neck of which is loosely stopped with fragments of glass, 
and then dried on porous tiles; by re-solution in water 
and slow evaporation, it may be obtained perfectly pure in 
