to know whether it can be used
again for the same purpose.

JOHN LEACHMAN.

[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 d