is, however,
more unstable than either hydrochloric or hydrobromic acids, and on
exposure to the air it gradually decomposes in accordance with the
equation

2HI + O = H_{2}O + 2I.

Owing to the slight affinity between iodine and hydrogen the acid easily
gives up its hydrogen and is therefore a strong reducing agent. This is
seen in its action on sulphuric acid.

The salts of hydriodic acid, the iodides, are, in general, similar to
the chlorides and bromides. Potassium iodide (KI) is the most familiar
of the iodides and is largely used in medicine.

~Oxygen compounds.~ Iodine has a much greater affinity for oxygen
than has either chlorine or bromine. When heated with nitric
acid it forms a stable oxide (I_{2}O_{5}). Salts of iodic acid
(HIO_{3}) and periodic acid (HIO_{4}) are easily prepared, and
the free acids are much more stable than the corresponding
acids of the other members of this family.

GAY-LUSSAC'S LAW OF VOLUMES

In the discussion of the composition of hydrochloric acid it was stated
that one volume of hydrogen combines with one volume of chlorine to form
two volumes of hydrochloric acid. With bromine and iodine similar
combining ratios hold good. These facts recall the simple volume
relations already noted in the study of the composition of steam and
ammonia. These relations may be represented graphically in the following
way:

+---+ +----+ +------+ +------+
| H | + | Cl | = | H Cl | + | H Cl |
+---+ +----+ +------+ +------+

+---+ +---+ +---+ +--------+ +--------+
| H | | H | + | O | = | H_{2}O | + | H_{2}O |
+---+ +---+ +---+ +--------+ +--------+

+---+ +---+ +---+ +---+ +--------+ +--------+
| H | | H | | H | + | N | = | NH_{3} | + | NH_{3} |
+---+ +---+ +---+ +---+ +--------+ +--------+

In the early part of the past century Gay-Lussac, a distinguished French
chemist, studied the volume relations of many combining gases, and
concluded that similar relations always hold. His observations are
summed up in the following law: _When two gases combine chemically there
is always a simple ratio between their volumes, and between the volume
of either one of them and that of the product, provided it is a gas._ By
a simple ratio is meant of course the ratio of small whole numbers, as
1 : 2, 2 : 3.

EXERCISES

1. How do we account for the