Fakenham, Norfolk.
_Improvement in Positives._--I have great pleasure in communicating to you
an improvement in the process of taking positives, which may not be
uninteresting to some of your readers, and which ensures by far the most
beautiful tints I have yet seen. I take three ounces of the hyposulphite of
soda, and dissolve it in one pint of distilled or rain water; and to this I
add about one or one and a half grains of pyrogallic acid, and seventy
grains {534} of chloride of silver; which must be squeezed up between the
finders facilitate its solution and separate the lumps, which, in their dry
state, are tough, and not easily pulverised. The whole is then to be set
aside for a week or two in a warm place. The solution, at first colourless,
becomes brown, and ultimately quite opaque; in this state it is fit for
use, and the longer kept the better it becomes. I generally use French
paper for this process, and, according to the time of immersion, obtain
fine sepia or black tints; the latter requiring long over-exposure to the
light, and proportionately long exposure to the action of the liquid; which
however will be found, particularly when old, to have a more rapid action
than most other setting liquids, and has the merit of always affording fine
tints, whatever the paper used. I imagine the pyrogallic acid to possess a
reducing influence on the salts of silver employed; but this effect is only
produced by its combination with the hyposulphite of soda and chloride of
silver. I may add, that in any case the pictures should be much overdone
before immersion, as the liquid exerts a rapid bleaching action on them;
and when the liquid becomes saturated, a few crystals of fresh hyposulphite
will renew its action.