Selenitic Plaster. X plaster of this
description, which is said to be success
fully employed in Edinburgh, is pre
pared from a mixture of hydraulic lime,
gypsnm, and siliceous sand. The meth
od of preparation is as follows. Any
fair hydraulic limestone is calcined aud
mixed with about five per cent of de
hydrated gypsnm. After grinding to
a fine powder, this mixture is made in
to a mortar with alwut five or six parts
nf clear, sharp sand and without hair.
When laid on as a plaster, onr account
affirms, this cement sets, dries, and
hardens within twenty-four hours, so
that a second coat can he laid on on
the second day, and houses thus plas
tered are tit for immediate. habitation.
This preparatiotnfurthermore, is rep
resented to be impervious to moisture,
and not subject to breaking or blister
ing, as is frequently the case with im
perfectly slaked lime. Its cost iu Ed
inburgh is said to be no greater than
that of ordinary mortar.
Ammonium Sitrate. This extraordi
nary substance has been reinvestigated
by Berthellot, who obtained it by
double decomposition from barium ni
trate aud ammonium sulphate. The
filtered solution, evapo rated overquick
lime in vacuo, yields crystals of the
substance desired. It is also, formed by
the simultaneous reaction upon each
other of nitrogen textroxide, dry am
monia, and oxvren. It forms a wliir.
crystalline, deliquescent salt, having
peculiar elastic properties, it decom
poses slowly at ordinary temperatures,
at 60 degrees to 70 degrees centigrade
it explodes violently; under a severe
blow it detonates. In solution the salt,
anomalously, decomposes more rapidly
than when dry ; heated torrents of ni
trogen are evolved. It seems to be one
of the least stable bodies known.
