_Rev. Mr. Sisson's New Developing Fluid_ (Vol. vii., p. 462.).--The REV.
MR. SISSON's developing fluid for collodion positives, the formula for
which was published in the last Number of "N. & Q.," is merely a weak
solution of the protonitrate and protosulphate of iron. It does not, as he
seems to think, contain any lead; for the whole of the latter is
precipitated as sulphate, which the acetic acid does not dissolve even to
the smallest extent: and MR. SISSON will find that an equivalent proportion
of the nitrate of baryta will answer equally as well as the nitrate of
lead.
I have myself for a long time been in the habit of using a weak solution of
the protonitrate of iron in conjunction with acetic acid for positive
pictures; for, although I do not consider it so good a developer as that
made according to the formula of DR. DIAMOND, it produces very good
pictures; occupies very little time in preparing, and will moreover keep
good for a much longer time than a more concentrated solution would.