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Thiocarbamides Reversal

developer, eikonogen, results, thiocarbamide, bromide and added

THIOCARBAMIDES REVERSAL WITH.—It has recently been discovered that the addi tion of a thiocarbamide to the developer will produce a reversal of the image. The following remarks upon the subject are by Professor Bothamley, F. I. C.* "Phenyl-thiocarbamide added in a small quantity to an eikonogen developer containing eikon ogen 5 parts, sodium sulphite to parts, crystallized sodium carbonate 8 parts, water 400 parts, gives a reversed image with a normal and even with under-exposure. Over exposure prevents reversal, and an excess of the thiocarbamide is also injurious, the best proportions being 12 to 15 parts of a saturated aqueous solution (i : 1,000) to ioo parts of the developer. Potassium bromide in the proportion of o•5 to one part of a to per cent solution in 100 parts of the developer aids reversal, and in some cases a small quantity of ammonia is beneficial. Good reversal is difficult to obtain with landscapes having strong contrasts, but for copying the results are more satisfactory. Before development the plate should be flooded with dilute nitric acid (5 per cent.), which is quickly washed off, and before fixing a short immersion in a two to three per cent. solution of cupric bromide gives clearer whites, but too long an immer sion will weaken the lines. A bath of cupric bromide before develop ment destroys the image. Potassium dichromate does not act so well as a nitric acid when used as a preliminary bath or added to the developer ; potassium ferricyanide, when added to the developer acts as a power ful restrainer, and its clearing effect is not so good as that of nitric acid. Phenyl-thiocarbamide gives no similar reversal with ferrous oxalate, pyro, or quinol ; with the first two it acts as a restrainer, and with the last as a powerful accelerator, giving general fog. Thiosina mine (allyl-thiocarbamide) acts less regularly but more energetically than phenyl-thiocarbamide, giving clearer whites. With eikonogen it gives the best results in presence of sulphite ; with pyro containing sulphite it also gives reversal, but not with pyro in presence of much bromide ; with Thomas's quinol developer it gave no reversal, but good results were obtained with a developer containing quinol, one part ; sodium sulphite, two parts ; crystallized sodium carbonate, 10 parts ; water, 7o parts. With the eikonogen developer the addition of

about two parts of a io per cent, solution of potassium dichromate to 500 parts of developer gives clearer results.

Thiocarbamide (sulpho-urea) itself also gives reversal with the eikonogen developer, but the results are very irregular. Neither thiocarbamide nor allyl-thiocarbamide gives good results for copying, even with the preliminary bath of nitric acid. Very few experiments with the spectrum were made, but it seems that the reversal of any particular part of the spectrum is almost entirely a question of the exposure. In some cases with phenyl-thiocarbamide and allyl-thiocar bamide the image first develops normally, but as development con tinues reversal takes place ; in other cases the plate seems to fog all over, and the reversal is only evident after fixing.

All the thiocarbamides exert no reducing action on the haloid salts of silver in absence of an alkali, though thiocarbamide and allyl thiocarbamide form pasty compounds with them. In presence of an alkali, partial reduction takes place even without exposure to light, and the reduction proceeds further on addition of eikonogen. Simple carbamide (urea) has a slight reducing action on precipitated silver bromide in presence of an alkali, but has no actioe on gelatino-bromide plates, and gives no reversal when added in any proportion to an eikonogen developer.

No reversal takes place when any of these compounds is added to the hydroxylamine developer, though thiosinamine, like glucose seems to prevent the evolution of gas."