A Substitute for the Hyposulphite Solution.--M. DAGUERRE recommends the use
of a solution of salt water for removing the coating off the plate.
I found this of some service at one time during my travels.
My hyposulphite bottle got broke and its contents lost, so as only to leave
enough for preparing gilding. I resorted to the use of salt solution,
and found it to answer well. Make a saturated solution of salt in water.
First wash the plate with clear water; then immerse it in the saline solution,
when it should be agitated, and the coating will soon disappear.
Another process with a salt solution of half the strength of the above
is very interesting and effectual. The plate having been dipped into
cold water, is placed in a solution of common salt, of moderate strength;
it lies without being acted upon at all; but if it be now touched
on one corner with a piece of zinc, which has been scraped bright,
the yellow coat of iodine moves off like a wave and disappears.
It is a very pretty process. The zinc and silver forming together
a voltaic pair, with the salt water intervening, oxidation of the zinc
takes place, and the silver surface commences to evolve hydrogen gas;
while this is in a nascent condition it decomposes the film of iodide
of silver, giving rise to the production of hydriodic acid, which is
very soluble in water, and hence instantly removed.

This process, therefore, differs from that with hyposulphite.
The latter acts by dissolving the iodide of silver, the former
by decomposing it. It is necessary not to leave the zinc
in contact too long, or it deposits stains, and in large plates
the contact should be made at the four corners successively,
to avoid this accident.

Gilding Solution.--To one pint of pure rain or distilled
water add fifteen grains of pure chloride of gold,