THE BRITISH
JOURNAL OF PHOTOGRAPHY.
A CHARACTER OF CERTAIN PRESERVATIVE FILMS.
Herr Schultz-Sellack has recently called attention to a remarkableagreement in certain properties between chemically-clean glass surfacesand photographic films coated with tannin or albumen. Theparticular property in which they appear to agree is this—that analbumenised or tannined surface, when breathed upon, takes themoisture evenly and loses it gradually by evaporation, just like aclean glass plate.
This wonderful discovery (!) is announced in Poggendorf’s Annalen,and a notice of it appears in the last Journal of the ChemicalSociety. If Herr Schultz-Sellack had endeavoured to make himselfacquainted with the literature of photography he would scarcelyhave taken the trouble to publish the statement of a fact well knownto all who have much experience in dry-plate work. A film protectedby albumen, tannin, or any other substance capable of absorbingmoisture, resembles a perfectly-clean glass plate in the mode ofcondensation of moisture and the manner in which an aqueous filmdisappears from the surface: but this property of a somewhat hygroscopicsurface is one which might be easily anticipated and has longbeen observed.
“Whether this hygroscopic property is beneficial or otherwise weare scarcely in a position to decide, as wide differences of opinionexist amongst practical men on this subject—on the one hand, Mr.M. Carey Lea considering nearly complete desiccation of a dry plateconducive to greater sensitiveness, and on the other, a large numberof operators declaring that a preservative capable of keeping theplate in a semi-moist condition is most advantageous. Our own experienceis in favour of Mr. Lea’s position; but it is by no meansimprobable that some of the so-called preservatives act best whenmoist, and others when the film is fully dried. We shall contentourselves with citing a single case in point.”
When a film of iodide of silver is washed free from extraneousmatter, and then covered with a solution of ferrocyanide of potassium,a very sensitive layer is obtained while the film is moist, but if driedfully the action of light upon the surface is very slow. Suppose,however, that we add to the ferrocyanide solution, previous to its applicationto the iodide film, a quantity of honey, a little glycerine, ora very minute amount of nitrate of magnesia, a comparatively sensitivefilm is obtained, which, though apparently dry, is still not completelyso. Here, then, is a remarkable case, parallel with theanalogous action of nitrate of silver moist and dry upon sensitivelayers.
A SHELLAC VARNISH.
In our last volume there appeared two very interesting notes—oneby Mr. G. Watmough Webster, F.C.S., and the other by Mr.A. R. Brown—on the preparation of a shellac varnish of a peculiarkind, and easily miscible with water. Though we have been longfamiliar with the mode of preparing such a varnish—thanks to thekindness of a friend—we have lately gone over the whole matter,and have been so interested that we now venture to recall thematter to the consideration of our readers—this week mentioningthe most convenient mode of p