The water used should be rain or distilled water.
"About one part of the mixed chlorides should be used to three or four of water.
"The battery may be either Smee's, Daniell's, or Grove's; if of either of the former, it should be of two series; if of the latter, one cup is sufficient.
"The plate on being immersed in the liquid, almost instantly takes a violet color.

It should be allowed to remain from two to five minutes, according to the strength of the battery, and until it becomes nearly black.

It should now be carefully washed, and afterwards heated over a spirit lamp until it takes a cherry-red color, and it is then ready for exposure in the camera.

Before speaking of exposing the plate, it may be well to speak of some difficulties which the inexperienced operator may find in preparing it.

If the battery is not in good order, and a sufficient current is not passed through the solution, the plate will become coated--and apparently almost as well as when the battery is working well--but on exposure it will give a negative picture, and but little colored; while if the battery is in good order, the impression is invariably positive.
"Sometimes on heating the plate after washing, the surface is covered with spots or assumes a variegated appearance.