The first yellow surface appears to be an oxide of silver with, possibly, a minute quantity of copper in combination, which being removed leaves a surface chemically pure.
Another Method .-- The best and simplest mode with which we are acquainted is to divide an earthenware vessel with a diaphragm: one side should be filled with a very dilute solution of sulphuric acid, and the other with either a solution of ferroprussiate of potash, or muriate of soda, saturated with chloride of silver.

The copper plate, varnished on one side, is united, by means of a copper wire, with a plate of zinc.

The zinc plate being immersed in the acid, and the copper in the salt, a weak electric current is generated, which precipitates the silver in a very uniform manner over the entire surface.
Another Method .-- A piece of brass or of polished copper, brass is preferred, is perfectly planished and its surface made perfectly clean.
A solution of nitrate of silver, so weak that the silver is precipitated slowly, and a brownish color, on the brass, is laid uniformly over it, "at least three times," with a camel's hair pencil.
After each application of the nitrate, the plate should be rubbed gently in one direction, with moistened bitartrate of potassa, applied with buff.

This coat of silver receives a fine polish from peroxide of iron and buff.

Proofs are said to have been taken on it, comparable with those obtained on French plates.
M.SOLIEL'S PROCESS FOR DETERMINING THE TIME OF EXPOSURE IN THE CAMERA.
M.Soliel has proposed the use of the chloride of silver to determine the time required to produce a good impression on the iodated plate in the camera.