m._, 1905, 47, p. 446). Its
critical temperature is 146 deg. C. Liquid and solid chlorine are both
yellow in colour. The gas must be collected either by downward
displacement, since it is soluble in water and also attacks mercury; or
over a saturated salt solution, in which it is only slightly soluble. At
ordinary temperatures it unites directly with many other elements; thus
with hydrogen, combination takes place in direct sunlight with explosive
violence; arsenic, antimony, thin copper foil and phosphorus take fire
in an atmosphere of chlorine, forming the corresponding chlorides. Many
compounds containing hydrogen are readily decomposed by the gas; for
example, a piece of paper dipped in turpentine inflames in an atmosphere
of chlorine, producing hydrochloric acid and a copious deposit of soot;
a lighted taper burns in chlorine with a dull smoky flame. The solution
of chlorine in water, when freshly prepared, possesses a yellow colour,
but on keeping becomes colourless, on account of its decomposition into
hydrochloric acid and oxygen. It is on this property that its bleaching
and disinfecting power depends (see BLEACHING). Water saturated with
chlorine at 0 deg. C. deposits crystals of a hydrate Cl2.8H2O, which is
readily decomposed at a higher temperature into its constituents.
Chlorine hydrate has an historical importance, as by sealing it up in a
bent tube, and heating the end containing the hydrate, whilst the other
limb of the tube was enclosed in a freezing mixture, M. Faraday was
first able to obtain liquid chlorine.

Chlorine is used commercially for the extraction of gold (q.v.) and
for the manufacture of "bleaching powder" and of chlorates. It also
finds an extensive use in organic chemistry as a substituting and
oxidizing agent, as well as for the preparation of addition compounds.
For purposes of substitution, the free element as a rule only works
slowly on saturated compounds, but the reaction may be accelerated by
the action of sunlight or on warming, or by using a "carrier." In
these latter cases the reaction may proceed in different directions;
thus, with the aromatic hydrocarbons, chlorine in the cold or in the
presence of a carrier substitutes in the benzene nucleus, but in the
presence of sunlight or on warming, substitution takes place in the
side chain. Iodine, antimony trichloride, molybdenum pentachloride,
ferric chloride, ferric oxide, antimony, tin, stannic oxide and