e form of alkaline chlorides it is
found in sea-water and various spring waters, and in the tissues of
animals and plants; while, as hydrochloric acid it is found in volcanic
gases.

The preparation of chlorine, both on the small scale and commercially,
depends on the oxidation of hydrochloric acid; the usual oxidizing agent
is manganese dioxide, which, when heated with concentrated hydrochloric
acid, forms manganese chloride, water and chlorine:--MnO2 + 4HCl = MnCl2
+ 2H2O + Cl2. The manganese dioxide may be replaced by various other
substances, such as red lead, lead dioxide, potassium bichromate, and
potassium permanganate. Instead of heating hydrochloric acid with
manganese dioxide, use is frequently made of a mixture of common salt
and manganese dioxide, to which concentrated sulphuric acid is added and
the mixture is then heated:--MnO2 + 2NaCl + 3H2SO4 = MnSO4 + 2NaHSO4 +
2H2O + Cl2. Chlorine may also be obtained by the action of dilute
sulphuric acid on bleaching powder.

Owing to the enormous quantities of chlorine required for various
industrial purposes, many processes have been devised, either for the
recovery of the manganese from the crude manganese chloride of the
chlorine stills, so that it can be again utilized, or for the purpose of
preparing chlorine without the necessity of using manganese in any form
(see ALKALI MANUFACTURE).

Owing to the reduction in the supply of available hydrochloric acid
(on account of the increasing use of the "ammonia-soda" process in
place of the "Leblanc" process for the manufacture of soda) Weldon
tried to adapt the former to the production of chlorine or
hydrochloric acid. His method consisted in using magnesia instead of
lime for the recovery of the ammonia (which occurs in the form of
ammonium chloride in the ammonia-soda process), and then by
evaporating the magnesium chloride solution and heating the residue in
steam, to condense the acid vapours and so obtain hydrochloric acid.
One day before him E. Solvay had patented the same process, but
neither of them was able to make the method a commercial success.
However, in conjunction with Pechiney, of Salindres (near Alais,
France), the Weldon-Pechiney process was worked out. The residual
magnesium chloride of the ammonia-soda process is evaporated until it
ceases to give off hydrochloric acid, and is then mixed with more
magnesia: the magnesium oxychloride formed is broken into small pi