ss.

The colour which iodized collodion assumes on keeping, I consider to be
entirely due to the gradual separation of iodine from the iodide of
potassium or ammonium originally introduced. There are several ways in
which this may take place; if the cotton on paper contain the slightest
trace of nitric acid, owing to its not being _thoroughly_ washed (and this
is not as easy as is generally supposed), the liberation of iodine in the
collodion is certain to take place a short time after its being made.

It is possible also that there may be a gradual decomposition of the
zyloidin itself, and consequent liberation of the iodide by this means,
with formation of nitrate of potassa or ammonia; but the most probable
cause I consider to be the following. The ether gradually absorbs oxygen
from the atmosphere, being converted into acetic acid; this, by its
superior affinities, reacts on the iodide present, converting it into
acetate, with liberation of hydriodic acid; while this latter, under the
influence of the atmospheric oxygen, is very rapidly converted into water
and iodine.

I am satisfied by experiment that this is one of the causes of the
separation of iodine, and I think it is the only one, for the following
reason; neither bromised nor chlorised collodion undergo the slightest
change of colour, however long they may be kept. Now, if the former
agencies were at work, there is no reason why bromine should not be
liberated from a bromide as well as iodine from an iodide; but on the
latter {207} supposition, could take place, the affinities of acetic acid
being insufficient to displace hydrobromic acid.

A great many experiments which I tried last autumn, for the express purpose
of clearing up this point, have convinced me that, _caeteris paribus_, the
addition of free iodine to the iodizing solution, tends to diminish the
sensitiveness of the subsequently formed iodide of silver. On paper, this
diminution of sensitiveness is attended with some advantages, so that at
present I hardly know whether to introduce the free iodine or not; but in
collodion, as far as my experience goes, I see no reason for retaining it;
on the contrary, everything seems to be in favour of its removal.

I can hardly imagine that the increased sensitiveness mentioned by MR.
HENNAH is really due to the free iodine which he introduces. Such a result
being so contrary to all my experience, I would venture to suggest that
there must be some other