for the orthophosphates (see page 244) that in a primary phosphate only
one hydrogen atom of phosphoric acid is replaced by a metal. Since the
calcium atom always replaces two hydrogen atoms, it might be thought
that there could be no primary calcium phosphate; but if the calcium
atom replaces one hydrogen atom from each of two molecules of phosphoric
acid, the salt Ca(H_{2}PO_{4})_{2} will result, and this is a primary
phosphate. It can be made by treatment of the normal phosphate with the
necessary amount of sulphuric acid, calcium sulphate being formed at the
same time, thus:

Ca_{3}(PO_{4})_{2} + 2H_{2}SO_{4} = Ca(H_{2}PO_{4})_{2} + 2CaSO_{4}.

The resulting mixture is a powder, which is sold as a fertilizer under
the name of "superphosphate of lime."

ARSENIC

~Occurrence.~ Arsenic occurs in considerable quantities in nature as the
native element, as the sulphides realgar (As_{2}S_{2}) and orpiment
(As_{2}S_{3}), as oxide (As_{2}O_{3}), and as a constituent of many
metallic sulphides, such as arsenopyrite (FeAsS).

~Preparation.~ The element is prepared by purifying the native arsenic, or
by heating the arsenopyrite in iron tubes, out of contact with air,
when the reaction expressed by the following equation occurs:

FeAsS = FeS + As.

The arsenic, being volatile, condenses in chambers connected with the
heated tubes. It is also made from the oxide by reduction with carbon:

2As_{2}O_{3} + 3C = 4As + 3CO_{2}.

~Properties.~ Arsenic is a steel-gray, metallic-looking substance of
density 5.73. Though resembling metals in appearance, it is quite
brittle, being easily powdered in a mortar. When strongly heated it
sublimes, that is, it passes into a vapor without melting, and condenses
again to a crystalline solid when the vapor is cooled. Like phosphorus
it can be obtained in several allotropic forms. It alloys readily with
some of the metals, and finds its chief use as an alloy with lead, which
is used for making shot, the alloy being harder than pure lead. When
heated on charcoal with the blowpipe it is converted into an oxide which
volatilizes, leaving the charcoal unstained by any oxide coating. It
burns readily in chlorine gas, forming arsenic trichloride,--

As + 3Cl = AsCl_{3}.

Unlike most of its compounds, the element itself is not poisonous.

~Arsine~ (AsH_{3}). When any compound containing arsenic is brought into
the presence of nascent hydrogen, arsine (AsH_{3}), corresponding