cause for its beneficial action; for instance,
commercial iodide of potassium is generally alkaline, owing to impurities
present; the tincture of iodine in this case would render the collodion
neutral, and unless a very large excess of iodine were introduced, its good
effects would be very apparent. This, however, involving the employment of
impure chemicals, is a very improbable explanation of a phenomenon observed
by so excellent an operator as MR. HENNAH: there is most likely some local
cause which would be overlooked unless expressly searched for.

With regard to the point, whether the free iodine is the _sole_ cause of
the deterioration of old collodion, I should say decidedly not, at least in
a theoretical view; the liberation of free iodine necessitates some other
changes in the collodion, and the result must be influenced by these in one
way or another, but practically I have as yet found nothing to warrant the
supposition that they perceptibly interfere with the sensitiveness of the
film.

In the above I have endeavoured as much as possible to avoid
technicalities, in order to make it intelligible to amateurs; but if there
be any part which may be considered obscure, on its being pointed out to
me, I will endeavour to solve the difficulty.

WILLIAM CROOKES.

Hammersmith.

_Photographic Queries._--1. Would you, Sir, or DR. DIAMOND (DR. MANSELL is
too far off), be kind enough to inform your readers whether DR. MANSELL'S
process, recommended in No. 225., is equally applicable to _inland_ as to
sea-side operations; or must we, in the one case, follow DR. DIAMOND, and
in the other DR. MANSELL, and thus be compelled to prepare two sets of
papers?

2. DR. MANSELL recommends, as a test for the iodized paper, a _strong_
solution of bichloride of mercury; may we ask _how strong_?

3. MR. SISSON'S developing fluid has undergone so many changes, and has
been so much written about, that we are at a loss to discover or to
determine whether it has been at length settled, in the mind of the
inventor, that it will do equally well for negatives as for positives.

FOUR PHOTOGRAPHIC READERS.

[1. Both papers are equally available for both purposes. In actual
practice we have not ourselves experienced any difference in their
results.

2. It is quite immaterial. A drachm of bichloride dissolved in one
ounce of spirits of wine will cause a cloudiness and a precipitate, if
a very few drops are adde