A Manual Of Photography

THE COLLODION PROCESS. 269 Prepare a saturated solution of bi-chloride of mercury in mu- riatic acid. Add one part of this solution to six of water. Pour a small quantity of it over the picture at one corner, and allow it to run evenly over the glass. It will be found immediately to deepen the tones of the picture considerably, and the positive image will almost disappear ; presently, a peculiar whitening will come over it, and in a short time a beautifully delicate white picture will be brought out. The negative character of the drawing will be entirely de- stroyed, the white positive alone remaining. This picture, after being well washed and dried, can be varnished and preserved as a positive ; but nevertheless, even after this bleaching, it can be changed into a deep-toned negative, many shades darker than it was originally, by immersing it, after a thorough washing, in a weak solution of hypo-sulphite of soda, or a weak solution of ammonia. The white picture will vanish, and a black negative will be the result. It is very singular that the picture can be alternately changed from a white positive to a black negative many times in succes- sion, and very often with improvement. Tims, by the above process, a most perfect white positive or a deep black negative is produced, quite distinct from each other. In the first part of this after-process it will be observed that the effect of this bi-chloride of mercury solution is to deepen the shades of the picture, and this peculiarity can be made available to-strengthen a taint image, by taking the precaution of using the solution weaker, in order that the first change may be com- pleted before the whitening effect comes on. The progress of the change can be stopped at this point by the simple application of water. The author first pointed out the remarkable action of corrosive sublimate, in his paper, published by the Royal Society, on the Daguerreotype process on paper. M. Adolphe Martin has published some remarks on the col- lodion in the Comptes Rendus of 5th July, 1852. The collodion he employs is made of— 30 grains of Cotton. 750 grains of Nitrate of Potash. 1500 grains of Sulphuric Acid. This is well washed and dissolved in 10 volumes of ether and 1 volume of alcohol: by this, 15 grains of gun-cotton are dissolved in 1860 grains of ether, and 930 grains of alcohol: add then to this collodion, 15 grains of nitrate of silver transformed into iodide,