Calomel (muriate, mild muriate, chloride, protochloride of mercury), is
commonly met with in the shops in the form of a heavy powder, having a
faint yellowish-white colour, and no taste or smell. In mass it forms
compact, fibrous, translucent, shining cakes of great density. It is
insoluble in water.
It is distinguished by the effects of heat, and those of the solution of
caustic potass. Heated in a tube it sublimes unchanged, and condenses in
a crystalline or crumbly mass. The solution of caustic potass or soda
turns it at once black, disengaging protoxide of mercury and acquiring
hydrochloric acid, the presence of which is proved by neutralizing the
solution with nitric acid, and adding nitrate of silver, when a heavy
white precipitate is formed, the chloride of silver. In applying this
process, care must be taken to employ potass quite free of muriates, and
nitric acid free of muriatic acid. Ammonia also renders calomel powder
black, but the action and product are much more complex in their nature.