The British Journal of Photography, No. 613, Vol. XIX, February 2, 1872

No. 613. Vol. XIX.—FEBRUARY 2, 1872.
Herr Schultz-Sellack has recently called attention to a remarkable agreement in certain properties between chemically-clean glass surfaces and photographic films coated with tannin or albumen. The particular property in which they appear to agree is this—that an albumenised or tannined surface, when breathed upon, takes the moisture evenly and loses it gradually by evaporation, just like a clean glass plate.
This wonderful discovery (!) is announced in Poggendorf’s Annalen , and a notice of it appears in the last Journal of the Chemical Society . If Herr Schultz-Sellack had endeavoured to make himself acquainted with the literature of photography he would scarcely have taken the trouble to publish the statement of a fact well known to all who have much experience in dry-plate work. A film protected by albumen, tannin, or any other substance capable of absorbing moisture, resembles a perfectly-clean glass plate in the mode of condensation of moisture and the manner in which an aqueous film disappears from the surface: but this property of a somewhat hygroscopic surface is one which might be easily anticipated and has long been observed.
“Whether this hygroscopic property is beneficial or otherwise we are scarcely in a position to decide, as wide differences of opinion exist amongst practical men on this subject—on the one hand, Mr. M. Carey Lea considering nearly complete desiccation of a dry plate conducive to greater sensitiveness, and on the other, a large number of operators declaring that a preservative capable of keeping the plate in a semi-moist condition is most advantageous. Our own experience is in favour of Mr. Lea’s position; but it is by no means improbable that some of the so-called preservatives act best when moist, and others when the film is fully dried. We shall content ourselves with citing a single case in point.”
When a film of iodide of silver is washed free from extraneous matter, and then covered with a solution of ferrocyanide of potassium, a very sensitive layer is obtained while the film is moist, but if dried fully the action of light upon the surface is very slow. Suppose, however, that we add to the ferrocyanide solution, previous to its application to the iodide film, a quantity of honey, a little glycerine, or a very minute amount of nitrate of magnesia, a comparatively sensitive film is obtained, which, though apparently dry, is still not completely so. Here, then, is a remarkable case, parallel with the analogous action of nitrate of silver moist and dry upon sensitive layers.