It seems, therefore, clear that the lilac blue substance contains no combined citric acid.  Had the solubility of the silver been due to combination with either acid or alkali, the liquid from which it was separated by digestion at or below 100 deg.  C. must have been acid or alkaline; it could not have been neutral.

We have, therefore, this alternative:  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 to sunlight.  With C the case is quite different, the color changes from that of red gold to that of pure yellow gold.  The experiment is an interesting one.  The exposed portion retains its full metallic brilliancy, giving an additional proof that the color depends upon molecular arrangement, and this with the allotropic forms of silver is subject to change from almost any influence.