orates
and condenses again on the cold surface of the funnel in
shining crystals.

This process, in which a solid is converted into a vapor and is
again condensed into a solid without passing through the liquid
state, is called _sublimation_.

~Physical properties.~ Iodine is a purplish-black, shining, heavy solid
which crystallizes in brilliant plates. Even at ordinary temperatures it
gives off a beautiful violet vapor, which increases in amount as heat is
applied. It melts at 107 deg. and boils at 175 deg.. It is slightly soluble in
water, but readily dissolves in alcohol, forming a brown solution
(tincture of iodine), and in carbon disulphide, forming a violet
solution. The element has a strong, unpleasant odor, though by no means
as irritating as that of chlorine and bromine.

~Chemical properties.~ Chemically iodine is quite similar to chlorine and
bromine, but is still less active than bromine. It combines directly
with many elements at ordinary temperatures. At elevated temperatures it
combines with hydrogen, but the reaction is reversible and the compound
formed is quite easily decomposed. Both chlorine and bromine displace it
from its salts:

KI + Br = KBr + I,

KI + Cl = KCl + I.

When even minute traces of iodine are added to thin starch paste a very
intense blue color develops, and this reaction forms a delicate test for
iodine. Iodine is extensively used in medicine, especially in the form
of a tincture. It is also largely used in the preparation of dyes and
organic drugs, iodoform, a substance used as an antiseptic, has the
formula CHI_{3}.

~Hydriodic acid (HI).~ This acid cannot be prepared in pure condition by
the action of sulphuric acid upon an iodide, since the hydriodic acid
set free is oxidized by the sulphuric acid just as in the case of
hydrobromic acid, but to a much greater extent. It can be prepared in
exactly the same way as hydrobromic acid, iodine being substituted for
bromine. It can also be prepared by passing hydrosulphuric acid into
water in which iodine is suspended. The equation is

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

The hydriodic acid formed in this way dissolves in the water.

~Properties and uses.~ Hydriodic acid resembles the corresponding acids of
chlorine and bromine in physical properties, being a strongly fuming,
colorless gas, readily soluble in water. Under standard conditions 1
volume of water dissolves about 460 volumes of the gas. It