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.
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