PRECIPITATION BY ANIMAL CHARCOAL. 205 
certain metallic acids are precipitated by charcoal ; oxide of 
lead dissolved in caustic potash was precipitated by charcoal ; 
also the acids from antimoniate of potash and tungslate of am- 
monia. On the other hand, no effect was obtained on arseniate 
and arsenite of soda ; and arsenious acid dissolved in water 
could not be entirely precipitated by animal charcoal. Bichro- 
mate of potash and chromic acid were reduced by the char- 
coal, in the cold slowly but yet completely. The chromate of 
potash became converted into carbonate of potash. Further- 
more, the charcoal precipitated iodide of mercury from the am- 
moniacal iodide of mercury ; and sulphuret of antimony from 
the ammoniacal sulphuret arsenic. 
A solution of iodine in water or in iodide of potassium is quickly 
decolourized by charcoal ; but it is impossible to precipitate 
sulphur from its solutions in alcohol or oil of turpentine ; for 
even after a large addition of charcoal, the filtered fluid heated 
over a silver plate still yielded traces of sulphuret of silver. 
Salts having an alkaline base, as cream of tartar, ferrocyanide 
of potassium, gypsum, and alum, and also lime-water, appeared 
to be unaffected by charcoal ; but a reaction takes place on 
chloride of barium, particularly if a few drops of caustic am- 
monia be added to the solution. 
In the precipitation of metallic salts by charcoal, three cir- 
cumstances may happen : — 1. The salt may be absorbed with- 
out decomposition. 2. The oxide contained in the salt may 
be reduced ; or 3. The salts may be precipitated in a basic 
state. With some of the salts (sulphate of the protoxide of iron 
and corrosive sublimate) the latter takes place. As soon as 
the charcoal begins to act, the solution of the salt becomes dis- 
tinctly acid, and by quantitative examination, the greatest part 
of the acid will be again found in the fluid. This separation 
of the salts into acid and basic compounds is the reason why 
the last traces of the bases are so difficult to be removed by 
charcoal ; for the acid which has become free prevents the per- 
fect precipitation. Hence also, a salt to which some free acid 
has been added, is but little or not at all affected by charcoal. 
