l superfluous
nitrate of silver, by standing it for a minute or so on edge upon a
piece of blotting-paper. Lay it flat upon a board, collodion side
upwards, and the negative prepared above upon it, collodion side
downwards. Expose the whole to daylight for a single second, or to
gas-light for about a minute, and develope as usual. The result will be
a _transmitted positive_, but with reversed sides; and from this, when
varnished and treated as the original negative, any number of negatives
similar to the first may be produced.

The paper at the angles is to prevent the _absolute_ contact and
consequent injury by the solution of nitrate of silver; and, for the
same reason, it is advisable not to attempt to print until the primary
negative is varnished, as, with all one's care, sometimes the nitrate
will come in contact and produce spots, if the varnishing has been
omitted. Should the negative become moistened, it should be _at once_
washed with a gentle stream of water and dried.

I have repeatedly performed the operation above described so far as the
production of the positive, and so perfect is the impression that I see
no reason why the second negative should be at all distinguishable from
the original.

I am, indeed, at present engaged upon a _similar_ attempt; but there are
several other difficulties in my way: I, however, entertain no doubts of
perfect success.

GEO. SHADBOLT.

_Towgood's Paper._--A. B. (Vol. ix., p. 83.) can purchase Towgood's
paper of Mr. Sandford, who frequently advertises in "N. & Q." With
regard to his other Query, I think there can be no doubt of his being at
liberty to publish a photographic _copy_ of a portrait, Mr. Fox Talbot
having reserved only the right to paper copies of a _photographic_
portrait. Collodion portraits are _not_ patent, but the _paper_ proofs
from collodion negatives are.

GEO. SHADBOLT.

{111}
_Adulteration of Nitrate of Silver._--Will any of your chemical readers
tell me how I am to know if nitrate of silver is pure, and how to detect
the adulteration? _If so_ with nitrate of potash, how? One writer on
photography recommends the fused, as then the excess of nitric acid is
got rid of. Another says the fused nitrate is nearly always adulterated.
I fear you have more querists than respondents. I have looked carefully
for a reply to some former Queries respecting MR. CROOKES's restoration
of old collodion, but at present they have failed in appearance.

THE