should be repeated several times until the water
which passes off is tasteless. The carbonate of the metal rapidly
changes to an oxide by contact with the air, and it will generally be
found, when it is sufficiently washed, that it is at least half oxide.
On adding hydrochloric acid cautiously to the mixture, a chloric of the
metal will be formed, and carbonic acid will be evolved from the
remaining carbonate. The chloride formed is soluble; but as there are
two chlorides of these metals, and we wish to produce the one which
contains the most chlorine, it is best to add the acid cautiously until
the solution is decidedly acid. After filtering the solution, it is
fit for use; and it should be preserved in well-stoppered bottles. 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. 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 wi