a specific gravity of 1.212. Perfectly dry
hydrochloric acid gas has no action on metals, but in aqueous solution
it dissolves many of them with evolution of hydrogen and formation of
chlorides.

The salts of hydrochloric acid, known as _chlorides_, can, in most
cases, be prepared by dissolving either the metal, its hydroxide,
oxide, or carbonate in the acid; or by heating the metal in a current
of chlorine, or by precipitation. The majority of the metallic
chlorides are solids (stannic chloride, titanic chloride and antimony
pentachloride are liquids) which readily volatilize on heating. Many
are readily soluble in water, the chief exceptions being silver
chloride, mercurous chloride, cuprous chloride and palladious chloride
which are insoluble in water, and thallous chloride and lead chloride
which are only slightly soluble in cold water, but are readily soluble
in hot water. Bismuth and antimony chlorides are decomposed by water
with production of oxychlorides, whilst titanium tetrachloride yields
titanic acid under the same conditions. All the metallic chlorides,
with the exception of those of the alkali and alkaline earth metals,
are reduced either to the metallic condition or to that of a lower
chloride on heating in a current of hydrogen; most are decomposed by
concentrated sulphuric acid. They can be distinguished from the
corresponding bromides and iodides by the fact that on distillation
with a mixture of potassium bichromate and concentrated sulphuric acid
they yield chromium oxychloride, whereas bromides and iodides by the
same treatment give bromine and iodine respectively. Some metallic
chlorides readily form double chlorides, the most important of these
double salts being the platinochlorides of the alkali metals. The
chlorides of the non-metallic elements are usually volatile fuming
liquids of low boiling-point, which can be distilled without
decomposition and are decomposed by water. Hydrochloric acid and its
metallic salts can be recognized by the formation of insoluble silver
chloride, on adding silver nitrate to their nitric acid solution, and
also by the formation of chromium oxychloride (see above). Chlorides
can be estimated quantitatively by conversion into silver chloride, or
if in the form of alkaline chlorides (in the absence of other metals,
and of any free acids) by titration with standard silver nitrate
solution, using