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[Continued from Supplement, No. 706, page 11283.]

ON ALLOTROPIC FORMS OF SILVER.

By M. CAREY LEA.

In the first part of this paper were described certain forms of silver; among them a lilac blue substance, very soluble in water, with a deep red color.  After undergoing purification, it was shown to be nearly pure silver.  During the purification by washing it seemed to change somewhat, and, consequently, some uncertainty existed as to whether or not the purified substance was essentially the same as the first product; it seemed possible that the extreme solubility of the product in its first condition might be due to a combination in some way with citric acid, the acid separating during the washing.  Many attempts 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.