64 HYDRIDES.Certain products which are formed by the occlusion of hydrogen on
the part of some of the heavy metals, and are to be regarded as solid
solutions, should not be confused with the true hydrides; e.g., palladium-
hydrogen.- Cerium Hydride.
If cerium dioxide is reduced by means of metallic magnesium in an atmos-
phere of hydrogen, the metallic cerium, as fast as it is set free, combines with
the gas to form cerium hydride.
Ignite a few grams of eerie ammonium nitrate (No. 171), or
cerous ammonium nitrate, at first gently, and finally with the blast
lamp, and mix the pure, yellow cerium dioxide thus obtained
with powdered magnesium in the proportion of 172 parts CeO 2
to 64 parts Mg. Place the mixture in a boat and introduce it
into a short combustion tube, one end of which is connected
through a sulphuric acid wash-bottle with a hydrogen generator,
while the other end can be closed when required. After the
air in the apparatus has been completely replaced by hydrogen,
heat the tube in a short combustion furnace, at first gently, until
all moisture is removed; then close the exit end of the combus-
tion tube and open the cock of the generator wide so that the
reaction mixture stands under a slight pressure of hydrogen.
On now heating the tube with the full flame, the mixture glows,
the walls of the tube above it become blackened, and gas bubbles
pass through the wash-bottle, at first rapidly and then less and
less frequently, corresponding to the rate of consumption of the
hydrogen. Heat the mixture five or ten minutes longer, andthen allow it to cool under hydrogen pressure. The reddish-brown,
fairly compact reaction-product consists of a mixture of cerium
hydride and magnesium oxide. Break pieces of it from the boat
and set fire to them with a match; the material burns with a
hydrogen flame to a nearly white ash. While the hydrogen is
burning, occasional flashes occur from the ignition of particles of
unoxidized magnesium. It is possible to keep cerium hydride
for a long time in sealed vessels.
By heating calcium turnings to a dull red heat in an atmos-
phere of hydrogen, calcium hydride, CaH 2 , may be prepared in
an analogous manner.