“Thomas Wedgwood noted that a paper, or white leather, wet with a solution of silver nitrate, remains unchanged when kept in the dark; but, if exposed to sunlight, it quickly changes color and, after passing through various shades of gray and brown, it eventually becomes almost black. The color alterations occur with a speed proportionate to the intensity of light. Under direct sunlight, it takes two or three minutes to produce the full effect; in the shade it takes several hours and the light transmitted through variously colored glasses cause different degrees of intensity.”
Glaser, in his Traité de Chymie”- second book, does not point directly at the horny silver but speaks of “perpetual caustic”, that’s to say silver nitrate, or caustic moon. Here the operation he advises to adopt: Take two ounces of cupeled silver reduced in granules, or filings, dissolve it in a flask with double or triple good strong water, pour the solution into a cucurbit covered with its alembic, and light fire under the sand, remove about half the moisture from the strong water, and the water that comes out will be very weak, as the silver body retains to itself the strong water’s strength, then let cool the vessel for a few hours, and you will find the remaining material in the bottom of the cucurbit in salt form, which you will put in a good and big German crucible, and since he material when boiling tends initially to swell and can leak and get lost, place the crucible on a small fire, until the boiling have passed, and the matter has fallen to the bottom, and about that time you will increase the fire a little, and you will see the matter like oil on the bottom of the crucible, which you will pour into a well cleaned ingot form, which you have previously heated a little, and you will find it hard as stone matter, which you will keep in a box for use”.
To conclude, an excerpt from a discourse by Lady Elizabeth Eastlake published in London Quarterly Review, April 1857, pp. 442-68, in which we can find an interesting information elaborating Chevreul’s notes on colors. Eastlake’s speech was originally on the economic and sociological consequences of the photography medium and summarized the scientific prehistory of photography and the discoveries involved in its invention: “……… The investigation of the solar attributes, by the aid of photographic machinery, for which we are chiefly indebted to the researches of Mr. Hunt and M. Claudet, are, scientifically speaking, the most interesting results of the discovery. By these means it is proved that besides the functions of light and heat the solar ray has a third, and what may be called photographic function, the cause of all the disturbances, decompositions, and chemical changes which affect vegetable, animal, and organic life. It had long been known that this power, whatever it may be termed – energy – actinism – resided more strongly, or was perhaps less obstructed, in some of the colored rays of the spectrum than in others–that solutions of silver and other sensitive surfaces were sooner darkened in the violet and the blue than in the yellow and red portions of the prismatic spectrum. Mr. Hunt’s experiments further prove that mere light, or the luminous ray, is little needed where the photographic or “chemical ray” is active, and that sensitive paper placed beneath the comparative darkness of a glass containing a dense purple fluid, or under that deep blue glass commonly used as a finger-glass, is photographically affected almost as soon as if not shaded from the light at all. Whereas, if the same experiment be tried under a yellow glass or fluid, the sensitive paper, though robbed neither of light nor heat, will remain a considerable time without undergoing any change. We may add, though foreign to our subject, that the same experiment applied by Mr. Hunt to plants has been attended with analogous results”. Footnote to the article on Hunt and Claudet experiments: Bulbs of tulips and ranunculuses have germinated beneath yellow and red glasses, but the plant has been weakly and has perished without forming buds. Under a green glass (blue being a component part of the color) the plants have been less feeble, and have advanced as far as flower-buds; while beneath the blue medium perfectly healthy plants have grown up, developing their buds, and flowering in perfection.
- From Online Chemical Engineering Information. Dr. Drew Myers, Chemistry Coordinator Consultant in Surface, Colloid, and Polymer Science;