Pkvtottereotxpy. A sheet of ordinary
Clate-elaM larger than the picture to
e reproduced is coated ia the dark
room with a solution made by dissolv
ing 1 ounce of potassium bichromate iu
IS ounces of water, warming gradually,
then adding 2 ouuees of flue gelatine
and filtering through linen at the boil
ing beat. A diapositive is taken from
au ordiuary negative, and laid with the
collodiou side to the gelatine face of
the prepared plate in diffused light for
10 io 30 miuutes The plate is then ta
ken from tbe frame in tbe dark room
and washed with water for five or ten
minutes, till tbe relief is fully devel
oped, after which it is dried with filt
ered paper and coated with glycerine
by means of a camel's hair pencil, and
the excess of liquid is removed with
filter-paper. From this plate a cast is
made in plaster of parts of the consis
tency of oil, and from tbe plaster cast
a metal one may be taken.
The EUutieity of Purified Air. Some
reseat cues of Mendelett and Kirpit
schoff show that tbe product of any
volume of air by its pressure, which
according to the law of Marriott e
should be constant for all pressure, ia
not so for the air, varying considerably
when the pressures diminish to so
small a quantity as half a millimeter.
This product in fact, which for perfect
gases is constant, varies rapidly in tbe
case of sir. The deviations from the
law of Marriotte for rarefied air are
contrary to those observed by Regnault
for compressed air. In fact, the pro
duct increases when the pressure in
creases, and diminishes when the pres
sure diminishes. These discordances
are far larger than the possible errors
of observation.
