[The aceto-nitrate _may_ be used, but in our own practice we do not do
so. It is apt to give an unpleasant brownish colour. The solutions of
silver, {650} whether used for albumenising or otherwise, being reduced
to a state of chloride by the addition of common salt so long as any
precipitate is formed: fine silver may then be readily obtained by
heating a crucible, the chloride consisting of three-fourths of pure
metal. It is a false economy to use dirty or doubtful solutions, and by
adopting the above course the pecuniary loss is very trifling. Our
ordinary stoves will not always give a sufficient heat, but any working
jeweller or chemist having the ordinary furnace would accomplish it.]
_On the Restoration of old Collodion._--Many plans have been suggested for
the restoration of collodion when it has lost its sensitiveness by age. In
the last Number of the _Photographic Journal_, p. 147., MR. CROOKES
proposes "to remove the free iodine from the collodion by means of a piece
of pure silver. For two ounces of liquid I should recommend a sheet of
stout silver foil, about two inches long and half an inch broad. It will
require to remain in contact with the collodion for about two days, or even
longer if the latter be very dark-coloured; and in this case it will
sometimes be found advantageous to clean the surface of the silver, as it
becomes protected with a coating of iodide, by means of cyanide of
potassium or hyposulphite of soda.
"When thus renovated, the collodion will be found as sensitive and good as
it was originally."
This plan is certainly more simple than any that has yet been recommended.
The action of the silver being its mere combination with the free iodine,
thereby producing the reduction of the collodion to its original colourless
condition, I would venture to put this question to MR. CROOKES (to whom the
readers of "N. & Q." are already under great obligations): Does he consider
that it is the mere presence of free iodine which causes the want of
sensitiveness in the collodion? This is all which appears to be
accomplished by the process which Mr. Crookes recommends.
Now, as one who has had some experience, both in the manufacture and uses
of collodion, such a view does not agree with my practice and observation.
Occasionally, upon sensitising collodion, I have found it assume a deep
sherry colour a few hours after being made. This must have depended upon
the free iodide it contained, and yet such collodion has worked most
admirably. I have now before me a large body of collodion almost red, and
which has been made some three or four months; yet the last time I used
this, about a week since, it was just as good as when it was first made.
Undoubtedly collodion does more or less deteriorate with age; but here I
would observe, that there is an immense difference in the different
manufactures of collodion, and which can be ascertained by use only, and
not by appearance.
But Mr. Hennah, who has had much practical experience, recommends the
collodion to be made sensitive merely by the iodide of potassium; and he
said, "if it did not work quite clearly and well, a little tincture of
iodine brought it right." Here, then, is added the very thing which MR.
CROOKES proposes to abstract.
Again, MR. CROOKES considers the free iodine to be the cause of the
colouring of the collodion; will he then kindly explain its _modus
operandi_?
As he has on several occasions given your readers the benefit of his great
chemical knowledge, I trust they may be favoured by him with a solution of
these difficulties, which have puzzled