s totally unfits the sulphide for use. Sulphide which has gone
liquid will at all times be found to work perfectly, but it is of course
open to suspicion, and, in any case, it is not possible to know what is
the strength of a solution made up with such a supply. For this reason, it
is best to make up the sulphide into solution of 20 per cent strength, and
add this to water to make the toning bath. And it is here that a caution
must be noted. The weak working solution, which is only about 1 to 2 per
cent strength, keeps very badly indeed, and should be made up fresh from
the stock solution at the time of toning each batch of prints. This is one
of the most necessary items to bear in mind in using the sulphide process.

Sodium sulphide is sold in various degrees of purity, and the label on the
bottle is not always in exact correspondence with the condition of the
substance inside, but the two forms which must be adhered to for sulphide
toning are the ordinary "pure" and the "pure for analysis." The former can
be obtained from any reliable drug store or photographic dealer. It comes
in small lumps, yellowish to greenish in color; when dissolved in water
the solution will be yellow, and will usually show a deposit which must be
filtered off. This sulphide will give tones which are sepia brown with
most papers. In the case of the "pure-for-analysis" sulphide, which is the
recrystallized variety, the salt will be pure white and will form a quite
colorless and clear solution in water. The tone given by this kind of
sulphide is usually of a more purplish color. The distinct difference
between the two commercial varieties of sulphide should not be overlooked,
as it allows the worker to modify the process usefully when dealing with
papers differing (as all papers do) to a slight extent in their
adaptability to sulphide toning. The purer form has certainly much better
keeping properties than the other, but either, if made up in 20 per cent
solution, keeps for a month or two at least--which is enough for all
purposes. The chief difference between the two is noticed in the diluted
or working solutions. That of the purest sulphide _may_ be kept and used
again, though it is not really good policy to do so.

The supply of sulphide should therefore be dissolved as soon as purchased,
as follows: _Stock sulphide solution_--20 per cent; sodium sulphite 4
ounces; water to make 20 ounces. The actual toning solution is made up at
the time of t