mercury subchloride,
Hg2Cl2, which occurs in nature as the mineral horn-quicksilver, found as
translucent crystals belonging to the tetragonal system, with an
adamantine lustre, and a dirty white grey or brownish colour. The chief
localities are Idria, Obermoschel, Horowitz in Bavaria and Almaden in
Spain. It was used in medicine as early as the 16th century under the
names _Draco mitigatus, Manna metallorum, Aquila alba, Mercurius
dulcis_; later it became known as calomel, a name probably derived from
the Greek [Greek: kalos], beautiful, and [Greek: melas], black, in
allusion to its blackening by ammonia, or from [Greek: kalos] and
[Greek: meli], honey, from its sweet taste. It may be obtained by
heating mercury in chlorine, or by reducing mercuric chloride (corrosive
sublimate) with mercury or sulphurous acid. It is manufactured by
heating a mixture of mercurous sulphate and common salt in iron retorts,
and condensing the sublimed calomel in brick chambers. In the wet way it
is obtained by precipitating a mercurous salt with hydrochloric acid.
Calomel is a white powder which sublimes at a low red heat; it is
insoluble in water, alcohol and ether. Boiling with stannous chloride
solution reduces it to the metal; digestion with potassium iodide gives
mercurous iodide. Nitric acid oxidizes it to mercuric nitrate, while
potash or soda decomposes it into mercury and oxygen. Long continued
boiling with water gives mercury and mercuric chloride; dilute
hydrochloric acid or solutions of alkaline chlorides convert it into
mercuric chloride on long boiling.

The molecular weight of mercurous chloride has given occasion for much
discussion. E. Mitscherlich determined the vapour density to be 8.3 (air
= 1), corresponding to HgCl. The supporters of the formula Hg2Cl2
pointed out that dissociation into mercury and mercuric chloride would
give this value, since mercury is a monatomic element. After
contradictory evidence as to whether dissociation did or did not occur,
it was finally shown by Victor Meyer and W. Harris (1894) that a rod
moistened with potash and inserted in the vapour was coloured yellow,
and so conclusively proved dissociation. A. Werner determined the
molecular weights of mercurous, cuprous and silver bromides, iodides and
chlorides in pyridine solution, and obtained results pointing to the
formula HgCl, etc. However, the double formula, Hg2Cl2, has been
completely established by H.B. Baker (_Journ. Chem. Soc._, 1