ferrous sulphate have all been used as chlorine carriers.

The atomic weight of chlorine was determined by J. Berzelius and by
F. Penny (_Phil, Trans._, 1839, 13). J.S. Stas, from the synthesis of
silver chloride, obtained the value 35.457 (O = 16), and C. Marignac
found the value 34.462. More recent determinations are: H.B. Dixon and
E.C. Edgar (_Phil. Trans._, 1905); T.W. Richards and G. Jones (_Abst.
J.C.S._, 1907); W.A. Noyes and H.C. Weber (ibid., 1908), and Edgar
(ibid., 1908).

_Hydrochloric Acid._--Chlorine combines with hydrogen to form
hydrochloric acid, HCl, the only known compound of these two elements.
The acid itself was first obtained by J.R. Glauber in about 1648, but J.
Priestley in 1772 was the first to isolate it in the gaseous condition,
and Sir H. Davy in 1810 showed that it contained hydrogen and chlorine
only, as up to that time it was considered to contain oxygen. It may be
prepared by the direct union of its constituents (see Burgess and
Chapman, _J.C.S._, 1906, 89, p. 1399), but on the large scale and also
for the preparation of small quantities it is made by the decomposition
of salt by means of concentrated sulphuric acid, NaCl+H2SO4=NaHSO4+HCl.
It is chiefly obtained as a by-product in the manufacture of soda-ash by
the Leblanc process (see ALKALI MANUFACTURE). The commercial acid is
usually yellow in colour and contains many impurities, such as traces of
arsenic, sulphuric acid, chlorine, ferric chloride and sulphurous acid;
but these do not interfere with its application to the preparation of
bleaching powder, in which it is chiefly consumed. Without further
purification it is also used for "souring" in bleaching, and in tin and
lead soldering.

It is a colourless gas, which can be condensed by cold and pressure to
a liquid boiling at -83.7 deg. C., and can also be solidified, the solid
melting at -112.5 deg. C. (K. Olszewski). Its critical temperature is
52.3 deg. C., and its critical pressure is 86 atmos. The gas fumes
strongly in moist air, and it is rapidly dissolved by water, one
volume of water at 0 deg. C. absorbing 503 volumes of the gas. The gas
does not obey Henry's law, that is, its solubility in water is not
proportional to its pressure. It is one of the "strong" acids, being
ionized to the extent of about 91.4% in decinormal solution. The
strongest aqueous solution of hydrochloric acid at 15 deg. C. contains
42.9% of the acid, and has