a cheaper, slower, and not quite so effective
form of this method, whilst the thio processes represent sulphide toning
at its best); copper toning; toning by re-development. These methods
differ, not only in the results which they give, but also as regards the
perfection with which each attains its particular effect; on the principle
of the lady in the play who spoke the "absolute truth under the
circumstances," each may claim to be included among the really serviceable
processes.

In the sulphide process, the image which, in a black-developed print,
consists of metallic silver in fine division, is converted into silver
sulphide, a substance which in the ordinary way is also black, but when
produced in a fine condition on a photographic print is brown to sepia
color. Silver sulphide is a most permanent substance. Therefore a
sulphide-toned print should be permanent, too, a conclusion which is fully
borne out in practice. A sulphide-toned print is at least as permanent as
the bromide from which it is made. The image of the latter is susceptible
to practically only one agent likely to come in contact with it, namely,
sulphur fumes from burning gas, which partially sulphurize it and give
rise to iridescent markings resembling those due to stale paper. Now, as
the sulphide-toned print is the result of this sulphurizing process
carried out with intention to a state of completeness, the result should
be--and proves to be in practice--immune to this one cause of defacement.

In converting the silver image into one of silver sulphide, the method is
to first act on (bleach) the silver image with some reagent which will
change it into a compound of silver susceptible to the action of sulphide.
Iodine has been used for this, giving an image of silver iodide. Bromine
gives one of silver bromide. A mixture of potass bichromate and
hydrochloric acid gives silver chloride, as does also a solution of
chlorine, though in the former case the presence of the chromium compounds
affects the color obtained. But the best of the lot is a solution of the
two substances potassium ferricyanide and potassium bromide, which forms
an image of silver ferrocyanide and silver bromide. Both of these are
converted into silver sulphide when treated with a solution of sodium
sulphide. In the case of the hypo-alum process, in which the prints are
toned direct (without bleaching) in a mixture of hypo and alum, the image
is also changed into silver sulphide,