n their properties. Chlorine acts upon both bromides and
free hydrobromic acid, liberating bromine from them:

KBr + Cl = KCl + Br,

HBr + Cl = HCl + Br.

Silver bromide is extensively used in photography, and the bromides of
sodium and potassium are used as drugs.

~Oxygen compounds.~ No oxides of bromine are surely known, and
bromine does not form so many oxygen acids as chlorine does.
Salts of hypobromous acid (HBrO) and bromic acid (HBrO_{3}) are
known.

IODINE

~Historical.~ Iodine was discovered in 1812 by Courtois in the ashes of
certain sea plants. Its presence was revealed by its beautiful violet
vapor, and this suggested the name iodine (from the Greek for violet
appearance).

~Occurrence.~ In the combined state iodine occurs in very small quantities
in sea water, from which it is absorbed by certain sea plants, so that
it is found in their ashes. It occurs along with bromine in salt springs
and beds, and is also found in Chili saltpeter.

~Preparation.~ Iodine may be prepared in a number of ways, the principal
methods being the following:

1. _Laboratory method._ Iodine can readily be prepared in the laboratory
from an iodide by the method used in preparing bromine, except that
sodium iodide is substituted for sodium bromide. It can also be made by
passing chlorine into a solution of an iodide.

[Illustration: Fig. 57]

2. _Commercial method._ Commercially iodine was formerly prepared from
seaweed (kelp), but is now obtained almost entirely from the deposits of
Chili saltpeter. The crude saltpeter is dissolved in water and the
solution evaporated until the saltpeter crystallizes. The remaining
liquors, known as the "mother liquors," contain sodium iodate
(NaIO_{3}), in which form the iodine is present in the saltpeter. The
chemical reaction by which the iodine is liberated from this compound is
a complicated one, depending on the fact that sulphurous acid acts upon
iodic acid, setting iodine free. This reaction is shown as follows:

2HIO_{3} + 5H_{2}SO_{3} = 5H_{2}SO_{4} + H_{2}O + 2I.

~Purification of iodine.~ Iodine can be purified very
conveniently in the following way. The crude iodine is placed
in an evaporating dish E (Fig. 57), and the dish is set upon
the sand bath S. The iodine is covered with the inverted
funnel F, and the sand bath is gently heated with a Bunsen
burner. As the dish becomes warm the iodine rapidly evap