This indicates that the solution is impure, or that the plate have not been thoroughly washed and are still contaminated with the soluble chlorides which are contained in the solution.
"From the fact that the plate if prepared with positive electricity gives a positive picture, while it prepared otherwise it gives a negative, it is evident that electricity plays an important part in this process.

The same is true to some extent with the compounds formed with iodine, bromine, and fluorine.
"On heating the plate, the brown coating of chloride melts into a translucent enamel, and the heat should be withdrawn when a cherry-red color is produced.

It the heat is continued longer, the plate assumes a lighter color, and becomes less sensitive; and the enamel will finally scale off.

To produce a picture by the ordinary process of M.
Neipce, unaccelerated, it should be exposed for from three to five hours to sunlight in the camera, though pictures may be procured by contact, in from fifteen to thirty minutes." MULTIPLYING DAGUERREOTYPES ON ONE PLATE.
I have produced some interesting specimens of the Daguerreotypic art, by exposing in the camera only a portion of the sensitive plate to the action of light.

When on the exposed portion an image is formed, then taking the tablet into the dark room, change ends and expose the sensitive portion, and produce another image, developing as usual.
This plan is adapted for taking likenesses for lockets.