ve: In the lilac blue substance we
have either pure silver in a soluble form or else a compound of
silver, with a perfectly neutral substance generated from citric acid
in the reaction which leads to the formation of the lilac blue
substance. If this last should prove the true explanation, then we
have to do with a combination of silver of a quite different nature
from any silver compounds hitherto known. A neutral substance
generated from citric acid must have one or more atoms of hydrogen
replaced by silver. This possibility recalls the recent observations
of Ballo, who, by acting with a ferrous salt on tartaric acid,
obtained a neutral colloid substance having the constitution of
arabin, C6 H10 O6.

To appreciate the difficulty of arriving at a correct conclusion, it
must be remembered that the silver precipitate is obtained saturated
with strong solutions of ferric and ferrous citrate, sodium citrate,
sulphate, etc. These cannot be removed by washing with pure water, in
which the substance itself is very soluble, but must be got rid of by
washing with saline solutions, under the influence of which the
substance itself slowly but continually changes. Next, the saline
solution used for washing must be removed by alcohol. During this
treatment, the substance, at first very soluble, gradually loses its
solubility, and, when ready for analysis, has become wholly insoluble.
It is impossible at present to say whether it may not have undergone
other change; this is a matter as to which I hope to speak more
positively later. It is to be remarked, however, that these allotropic
forms of silver acquire and lose solubility from very slight causes,
as an instance of which may be mentioned the ease with which the
insoluble form B recovers its solubility under the influence of sodium
sulphate and borate, and other salts, as described in the previous
part of this paper.

The two insoluble forms of allotropic silver which I have described as
B and C--B, bluish green; C, rich golden color--show the following
curious reaction. A film of B, spread on glass and heated in a water
stove to 100 deg. C. for a few minutes becomes superficially bright
yellow. A similar film of the gold colored substance, C, treated in
the same way, acquires a blue bloom. In both cases it is the surface
only that changes.

_Sensitiveness to Light._--All these forms of silver are acted upon by
light. A and B acquire a brownish tinge by some hours' exposure