n and is kept in a dark bottle.

Let the volume of silvering liquor required in any operation be
denoted by 4 v. The liquor is prepared as follows:

I. Measure out a volume v of the stock solution of silver nitrate,
and calculate the weight of salt which it contains; let this be w.
In another vessel dissolve pure Rochelle salt to the amount of 2.6 w,
and make up the solution to the volume v. These two solutions are to
be mixed together at a temperature of 55 deg. C, the vessels with their
contents being heated to this temperature on the water bath. After
mixing the liquids the temperature is to be kept approximately
constant for five minutes, after which the liquor may be cooled. The
white precipitate which first forms will become gray or black and very
dense as the liquid cools. If it does not, the liquor must be
reheated to 55 deg. C, and kept at that temperature for a few minutes and
then again allowed to cool. The solution is in good order when all
the precipitate is dense and gray or black and the liquor clear. The
blacker and denser the precipitate the better is the solution. The
liquor is decanted and filtered from the precipitate and brought up to
the volume 2 v by addition of some of the wash water.

II. Measure out a volume 0.118 v of the stock solution into a
separate vessel, and add to it a 5 per cent solution of ammonium
hydrate, with proper precautions, so that the precipitate at first
formed is all but redissolved after vigorous shaking. It is very
important that this condition should be exactly attained. Therefore
add the latter part of the ammonia very carefully. Make up the volume
to 2 v.

Mix the solutions I. and II. in a separate vessel and pour the
mixture into the depositing vessel. The surface to be silvered should
face downwards, and lie just beneath the free surface of the liquid.
Bubbles must of course be removed.

The silver deposit obtained in this manner is exceedingly white and,
bright on the surface next to the glass, but the back is mat and
requires polishing.

The detail of the process described above was worked out in my
laboratory by Mr. A. Pollock, to whom my thanks are due.

This process gives good deposits when the solutions are freshly
prepared, but the ammonia solution will not keep; The surfaces to be
silvered require to be absolutely clean. The process is assisted by a
summer temperature, say 70 deg. Fahr, and possibly by the action of
light. Six or s