ts were made to get a decisive indication, and two series of
analyses, one a long one, to determine the ratio between the silver
and the citric acid present, without obtaining a wholly satisfactory
result, inasmuch as even these determinations of mere ratio involved a
certain degree of previous purification which might have caused a
separation.

This question has since been settled in an extremely simple way, and
the fact established that the soluble blue substance contains not a
trace of combined citric acid.

The precipitated lilac blue substance (obtained by reducing silver
citrate by ferrous citrate) was thrown on a filter and cleared of
mother water as far as possible with a filter pump. Pure water was
then poured on in successive portions until more than half the
substance was dissolved. The residue, evidently quite unchanged, was,
of course, tolerably free from mother water. It was found that by
evaporating it to dryness over a water bath, most of the silver
separated out as bright white normal silver; by adding water and
evaporating a second time, the separation was complete, and water
added dissolved no silver. _The solution thus obtained was neutral._
It must have been acid had any citric acid been combined originally
with the silver. This experiment, repeated with every precaution,
seems conclusive. The ferrous solution, used for reducing the silver
citrate, had been brought to exact neutrality with sodium hydroxide.
After the reduction had been effected, the mother water over the lilac
blue precipitate was neutral or faintly acid.

A corroborating indication is the following: The portions of the lilac
blue substance which were dissolved on the filter (see above) were
received into a dilute solution of magnesium sulphate, which throws
down insoluble allotropic silver of the form I have called B (see
previous paper). This form has already been shown to be nearly pure
silver. The magnesia solution, neutral before use, was also neutral
after it had effected the precipitation, indicating that no citric
acid had been set free in the precipitation of the silver.

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 alternati