Time for complete development should always be allowed; with a hot bath fifteen seconds will be sufficient, but if a cooler development is used, or the prints are solarized in the shadows, more time should be allowed. When the deep shadows are solarized, or appear lighter than surrounding parts, a hot and prolonged development is required to obtain sufficient blackness, as they have a tendency to look like brown paper. I have found breathing on solarized shadows useful, as in the presence of slight moisture they begin to print out and become dark before development, getting black almost directly the print is floated on the oxalate. Three or four acid baths of about ten minutes each are used, and the prints are washed as before. The process throughout takes much less time than silver printing, and can be kept on all the winter, when it is nearly impossible to print in silver. Prints can be developed in weak daylight or gaslight, and prolonged washing is dispensed with.—N.P. Fox, reported in Br. Jour. of Photo.
A communication to the North London Photographic Society.
[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.