CHROMATE of potash is used in dyeing in conjunction with dextrine, &c., for discharging the colour from various fabrics; it seems, however, that a difficulty has been noted in using such a mixture, for it is found that certain of the goods become discoloured by the chromate, and that this undesired colour is not easily removed. Will our readers be “surprised to hear” that the cause of this circumstance appears to have been only just found out, if we may judge from a quotation in the _Journal of the Chemical Society_? It has been discovered that it is exposure to light during treatment which prevents the removal of the offending material. This circumstance indicates how little attention is in general paid to the phenomena attending the chemical action of light; and, notwithstanding the fact that the remarkable property possessed by the chromates of rendering insoluble gelatine, &c., has been utilised to an extraordinary extent in the various pigment printing processes, operative chemists seem to have only now become alive to the fact, and able to apply the obvious remedy in using yellow glass for the rooms in which the treatment of goods with the chromated dextrine is carried on.
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HAVING read with much interest the various remarks which have appeared in your columns of late on the blistering of albumenised surfaces, I will, with your approbation, state my conviction of the cause.
First, let us examine the blisters and get at a clear notion of their physical aspect.
All who have spoken on the subject appear to agree that the more highly a paper is albumenised the more do we suffer from the annoyance. Well, such a strong film gives us the opportunity to detach some of those large blisters which have been formed by the coalescence of a number of small blisters.
Proceeding after the manner of a microscopist in his dissection of animal structure, we depress the sheet or photograph to the bottom of a dish with water, and there introduce a sharp-pointed knife, whereupon air escapes. Here we simply note this fact, and continue the dissection. Having detached a vesicular film, and caught it on a small plate of glass with the innermost side upwards, it will be found in almost every experiment to have the surface fibre of the paper attached, thus showing that it does not part from the surface of the paper as a film pure and simple. We therefore dismiss from our minds the idea that the outer surface only had been rendered insoluble, while the inner portion had been dissolved; were this the case the blister would be _filled with the albumen solution_.