_Deepening Collodion Negatives._--I have lately been trying a method of deepening collodion negatives, so as to render instantaneous impressions capable of being printed from, which I have found to answer admirably; {283} and although it is but a slight modification of MR. LYTE'S process described in "N. & Q.," it is a very important one, and will be found to produce far better results. The picture having been developed in the usual way, with a solution of pyrogallic acid, is whitened by means of MR. ARCHER'S solution of bichloride of mercury. The plate is then washed with water and a solution of _iodide of cadmium_ poured on. This converts the white chloride of mercury, which constitutes the picture, into the yellow iodide, in the same manner as the solution of iodide of potassium recommended by MR. LYTE; but is much to be preferred, as it produces a more uniform deposit. The solution of iodide of potassium dissolves the iodide of mercury as soon as it is formed, and therefore cannot be left on the plate until the decomposition of the chloride is complete, without injury resulting to the picture, as the half-tones are thereby lost, and those parts over which the solution first flows become bleached before the other parts have attained their highest tone; whereas the solution of iodide of cadmium may be allowed to remain for any length of time on the plate, without any fear of its injuring the negative. J. LEACHMAN. _Caution to Photographers._--About six months since, I procured some gun cotton from a chemist which appeared very good, being quite soluble, and the collodion produced by it was excellent. That which I did not use I placed in what I believed to be a clean dry-stopped bottle, and put the bottle in a dark cupboard. I was much surprised the other day, upon going to the cupboard, to find the stopper blown out, and the cotton giving out dense red fumes of nitrous acid. It appears to me to be almost upon the point of combustion, and I have, accordingly, placed it under a bell-glass in a porcelain dish to watch the result. I feel satisfied, however, that there is some risk, and, as it may often be near ether, spirits of wine, or other inflammable chemicals, that caution is necessary not only in preserving it at home, but especially in its transmission abroad, which is now done to some extent. AN AMATEUR. * * * * * Replies to Minor Queries.