[We are obliged to our correspondent not only for the compliment he has
paid to our services to photography, but also for his suggestion. There
are many reasons, and some sufficiently obvious, why _we_ should not
undertake the task proposed; and there are as obvious reasons why it
should be undertaken by the Photographic Society. That body has not
only the means of securing the best judges of such matters, but an
invitation from such a body would probably call into the field of
competition all the best photographers, whether professional or
amateur.]
_Double Iodide of Silver and Potassium._--I shall feel greatly indebted to
you, or to any correspondent of "N. & Q.," for information as to the
proportion of iodide of silver to the ounce of water, to be afterwards
taken up by a _saturated solution_ of iodide of potassium, and converted
into the double iodide of silver and potassium.
I generally pour all waste solution of silver into a jar of iodide of
potassium solution; and last year, having washed some of the precipitated
iodide of silver, I redissolved it in a solution of iodide of potassium of
an unknown strength. Paper prepared with this solution answered very
satisfactorily, kept well after excitation, and was very clear and intense;
but this was purely accidental: and if you can tell me how to insure like
success this summer, without a series of experiments, for which I have but
little time just now, the information will be very acceptable to me, and
probably to many others.
I excite my paper with equal proportions of saturated solution of gallic
acid and aceto-nitrate of silver, one or two drops of each to the drachm of
distilled water. I always plunge the bottle of gallic acid solution into
hot water when first made, which enables it to take up more of the acid; on
cooling, the excess crystallises at the bottom. This ensures an even
strength of solution: it will keep any length of time, if a small piece of
camphor be allowed to float in it.