readily
dissolves in acids, being decomposed by them and converted into soluble
acid phosphates, as explained in connection with the acids of
phosphorus.

STRONTIUM

~Occurrence.~ Strontium occurs sparingly in nature, usually as
strontianite (SrCO_{3}) and as celestite (SrSO_{4}). Both minerals form
beautiful colorless crystals, though celestite is sometimes colored a
faint blue. Only a few of the compounds of strontium have any commercial
applications.

~Strontium hydroxide~ (Sr(OH)_{2}.8H_{2}O). The method of preparation of
strontium hydroxide is analogous to that of calcium hydroxide. The
substance has the property of forming an insoluble compound with sugar,
which can easily be separated again into its constituents. It is
therefore sometimes used in the sugar refineries to extract sugar from
impure mother liquors from which the sugar will not crystallize.

~Strontium nitrate~ (Sr(NO_{3})_{2}.4H_{2}O). This salt is prepared by
treating the native carbonate with nitric acid. When ignited with
combustible materials it imparts a brilliant crimson color to the flame,
and because of this property it is used in the manufacture of red
lights.

BARIUM

Barium is somewhat more abundant than strontium, occurring in nature
largely as barytes, or heavy spar (BaSO_{4}), and witherite (BaCO_{3}).
Like strontium, it closely resembles calcium both in the properties of
the metal and in the compounds which it forms.

~Oxides of barium.~ Barium oxide (BaO) can be obtained by strongly heating
the nitrate:

Ba(NO_{3})_{2} = BaO + 2NO_{2} + O.

Heated to a low red heat in the air, the oxide combines with oxygen,
forming the peroxide (BaO_{2}). If the temperature is raised still
higher, or the pressure is reduced, oxygen is given off and the oxide is
once more formed. The reaction

BaO_{2} <--> BaO + O

is reversible and has been used as a means of separating oxygen from the
air. Treated with acids, barium peroxide yields hydrogen peroxide:

BaO_{2} + 2HCl = BaCl_{2} + H_{2}O_{2}.

~Barium chloride~ (BaCl_{2}.2H_{2}O). Barium chloride is a white
well-crystallized substance which is easily prepared from the native
carbonate. It is largely used in the laboratory as a reagent to detect
the presence of sulphuric acid or soluble sulphates.

~Barium sulphate~ _(barytes)_ (BaSO_{4}). Barium sulphate occurs in nature
in the form of heavy white crystals. It is precipitated as a crystalline
powder when a barium s