Hôtel de l'Europe,
à Pau, Basses Pyrénées.
P.S.--Since writing the former part of this letter, I see in _La Lumière_ a
paper on the subject of printing positives, in part of which the addition
of nitric acid is recommended to the bath; but as my experiments have been
quite independent of theirs, and my process one of a different nature, I
still send it to you. When I have an opportunity, I will send a couple of
specimens of my workmanship. I had prepared some for the Exhibition, but
could not get them off in time. I may add that the developing agent I use
is the same in every way as that I have before indicated through the medium
of your pages; but where formic acid cannot be got, the best developer is
made as follows:
Pyrogallic acid 27 grs.
Acetic acid 6 drs.
Water 9 oz.
_On Sensitive Collodion._--As I have lately received many requests from
friends upon the subject of the most sensitive collodion, I am induced to
send you a few words upon it.
Since my former communication, I believe a greater certainty of manufacture
has been attained, whereby the operator may more safely rely upon
uniformity of success.
I have not only tried every purchasable collodion, but my experiments have
been innumerable, especially in respect to the ammoniated salts, and I may,
I think, safely affirm that all preparations containing ammonia ought to be
rejected. Often, certainly, great rapidity of action is obtained; but that
collodion which acted so well on one day may, on the following, become
comparatively useless, from the change which appears so frequently to take
place in the ammoniacal compounds. That blackening and fogging, of which so
much has been said, I much think is one of the results of ammonia; but not
having, in my own manipulations, met with the difficulty, I have little
personal experience upon the subject.
The more simple a collodion is the better; and the following, from its
little varying and active qualities, I believe to be equal to any now in
use.
A great deal has also been said upon the preparation of the simple
collodion, and that some samples, however good _apparently_, never
sensitize in a satisfactory manner. I have not experienced this difficulty
myself, or any appreciable variation.
The collodion made from the Swedish filtering paper, or the papier Joseph,
is preferable, from the much greater care with which it is used.
If slips of either of these papers be carefully and completely immersed for
four hours in a mixture of an equal part (by weight) of strong nitric acid
or nitrous acid (the aqua fortis of commerce) and strong sulphuric acid,
then _perfectly_ washed, so as to get entirely rid of the acids, the result
will be an entirely soluble material. About 100 grains of dry paper to a
pint (twenty ounces) of ether will form a collodion of the desired
consistence for photographic purposes. If too thick, it may be reduced by
pure ether or alcohol. However carefully this soluble paper or the gun
cotton is prepared, it is liable to decompose even when kept with care. I
would therefore advise it to be mixed with the ether soon after
preparation, as the simple collodion keeps exceedingly well. Excellent
simple collodion is to be procured now at the reasonable price of eight
shillings the pint, which will to many be more satisfactory than trusting
to their own operations.
_To make the sensitizing Fluid._--Put into a clean stoppered bottle,
holding more than the quantity required so as to allow of free shaking, six
drachms of iodide of potassium and one drachm of bromide of potassium; wet
them with one drachm of distilled water first, then pour into the bottle
ten ounces of spirits of wine (not alcohol); shake frequently until
dissolved. After some hours, if the solution has not taken place, add a few
more drops of water, the salts being highly soluble in water, though
sparingly so in rectified spirits; but care must be taken not to add too
much, as it prevents the subsequent adhesion of the collodion film to the
glass.
A drachm and a half to two drachms, according to the degree of intensity
desired, added to the ounce of the above collodion, which should have
remained a few days to settle before sensitizing, I find to act most
satisfactorily; in fine weather it is instantaneous, being, after a good
shake, fit for immediate use. If the sensitive collodion soon assumes a
reddish colour, it is improved by the addition of one or two drops of a
saturated solution of cyanide of potassium; but great care must be used, as
this salt is very active.