_Mr. Weld Taylor's Iodizing Process._--The process I sent to your columns
last month, for iodizing paper, is applicable _only_ to the paper of Canson
Frères; and I may further explain, that if the solution does not answer
well, it may be washed over again with a solution of iodide of potassa only
of the usual strength, and then set on a dish of slightly-acidulated water,
to assist the separation and set free the potash. To make the matter clear
to MR. SHADBOLT, I may observe, to one who is in the habit of iodizing
paper, a considerable amount of the passage relating to cyanide of potassa
could not be misunderstood; the nitrate of silver being added to the iodide
of potassa, forms at once a precipitate which it is required to take up.
The old double iodide says, add iodide of potassa till it does so, and it
will do so; but the cyanide of potassa does it much better, and the
cyanogen is lost as the paper dries, otherwise it would take no image at
all. In the process I gave it merely requires an equivalent, "and cyanide
of potassa is always of use in many of these processes." That equivalent is
of course best arrived at by a solution, as, if the cyanide of potassa were
added in the lump, it would be lost or be in excess.
Further, I may enlighten MR. SHADBOLT by assuring him that the iodizing
paper with the ammonio-nitrate of silver, which I never saw published yet,
is the best way; and I may confidently assert that the better ways of
iodizing papers are not published at all. It is a tedious process to do,
but it is as certain as taking a positive from a negative. At present I
have not space to give my way of doing it. I may also add, that it will not
answer with all papers. In fact, all samples of paper require some
modification of the process, as the chemicals are different in the various
modes of bleaching paper by different manufacturers. The ammonio-nitrate is
perfect with Whatman's paper; indeed it is a subject of much regret, that
this maker has not turned out a paper as thin and hard as the Canson
Frères. The latter gentlemen have added some chemical, probably iodine, to
their paper, which renders it almost impossible to iodize it at all. I
believe it to be iodine, because the paper becomes perfectly black over
_free_ iodine, which no English paper will do. At all events, this paper is
very uncertain, although it has a quality in appearance that is unsurpassed
by any other.