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578 THE PHOTOGRAPHIC NEWS. [December 4, 1863. foot. The collodion that I use is Ponting’s iodized ; this I keep for several months, and mix with it, in a separate bottle, the bottoms from the bromo-iodized that I use for the instantaneous work, so that no collodion is wasted. After removing the plate from a 30-grain nitrate bath, made according to directions on the bottles of Ponting’s iodized collodion, I place it in a flat dish nearly filled with rain water, and having passed the hook of a little stick under it, keep lifting the plate up and down gently until all the greasy marks have disappeared. I then leave the plate, pour the collodion on another glass, and place it in the nitrate bath, and then I return to the plate left in the rain water. This I again move about with the hook to see if any lines exist, and if they do not, I transfer it to dish “No. 2,” also filled with rain water, where I again move it about for a minute, then, by the hook, draw the end of the glass over the edge of the dish, and (having blotting-paper between the first finger and thumb of my left hand and the glass) lift it by the corners on to the pneumatic holder. Having let the water drain for a few seconds, f pour on the tannin so as to run in a gentle stream into the same beaker from the end near myself to the corner from which the col lodion drained. I pour the next tannin on four times from beaker “No. 1,” which carries away any water, and then the same number of times from “ No. 2.” (As the tannin in “No. 1 ” gets wasted I add from “No. 2,” always putting the fresh mixture in “ No. 2,”) Having let the super abundant tannin run off, I leave the plate to dry in a box kept on purpose, and then return to the plate in the bath (about five minutes having elapsed since it was put in), and proceed in the same manner. I put fresh water in the first dish after every fourth, and in the second after every sixth plate. The preservative solution that I use is in the propor tion :— Tannic acid 15 grains Distilled water 1 ounce Methylated spirit ... } drachm The developers, pyrogallic acid, nitrate of silver, and citric acid, as recommended by Major Russell in his first edition, keeping one stock bottle of the pyrogallic acid and alcohol, and another of nitrate of silver, 20 grains to the ounce. At foot I put the quantities of these developers. The plates prepared as above I have kept more than six months, and they were apparently quite as good then as when freshly made. Not wishing to waste plates, I have not tried the shortest time of exposure that they would re quire, but I think that instead of five minutes, as before mentioned, a good picture might be got in three and a half or four, but as the over-exposure is easily remedied in the developing, and as I put on gelatine to save the trouble of doing the work again, for the same reason I prefer a little over-exposure to a weak or ruined plate. I develop as soon as possible, but have kept the negative a week, and it was then quite sharp and bright. Mr. King’s process, as described in his paper printed in the Photographic News of 27th November, differs for the most part from mine in manipu lation, with two exceptions, i. e., he does not use a gelatine coating, which I do; and he uses new bromo-cadmium iodized collodion, and I use old, simply iodized collodion, for the quantity of bromo-iodized from the bottoms of bottles is but trifling. With respect to the washing, we both agree as to the moving, but do it in different ways, and I think that mine is the least troublesome. We both use the same proportions of tannin, but Mr. King does not use the same portion twice, which I do. In developing, I lay the plate in a dish of common pump water, and let it stay there for four or five minutes; there is no danger of the gelatinized film coming off, or swelling, or cracking, it will bear a deal of rough usage. The levelling stand I never use. In developing, we use the same chemicals, but rather differently. If there are signs of over-exposure (which show themselves directly the developer is poured on) I pour back the developer into the beaker, and add a few drops of the nitrate of silver and citric acid mixture which corrects the evil. If there are signs of under-exposure, I pour back and add some more drops of the pyrogallic. Like Mr. King, I do not “ hope to communicate any novelty,” and a person conversant with Hardwich and Major Russell’s works will find much of the above therein. A photographer who has succeeded in one way of working had better persevere in that, although another may appear easier ; but to beginners who have not settled into one plan of working, practical suggestions are of the greatest use; following, therefore, Mr. King’s good example, I send you the above notes of my way of proceeding, hoping that they may be acceptable. Gelatine Solution. Nelson’s gelatine ... 40 grains Glacial acetic acid ... 40 minims Distilled water 8 ounces Iodide of cadmium ... ... ... 3 grains Bromide of cadmium ... ... ... 3 „ Small piece of iodine The three last to be dissolved first in a few drops of water, and then added— Developei's. No. 1. Pyrogallic acid 72 grains Alcohol (absolute) ... 1 ounce. No 2. Nitrate of silver ... 20 grains Citric acid ... ... 20 . This quantity reduced in cold weather. Distilled water 1 ounce. For use, diluted No. 1. No. 1. Developer 10 minims Distilled water ... 1 ounce. For developing a stereoscopic plate, to 3 drachms of diluted No. 1, add from 10 to 20 minims of developer No. 2. [It will be seen that the exposures of G. W. O. are longer than those of Mr. Jennings ; whether this be due to the latter using more bromide, or to the difference in the lenses, we cannot say.—En.] • ON IODIDE OF SILVER AS A SENSITIZING AGENT, AND ESPECIALLY OF THE ACTION OF TANNIN, AND SOME OF THE OXIDIZABLE SUBSTANGES, IN PHOTOGRAPHY. BY A. POITEVIN. The action of light upon iodide of silver is of three kinds, according to the condition of that body and of the substances it is associated with. 1st. It is nil upon chemically pure iodide, isolated from every substance which retains the iodine light tends to separate. 2nd. It modifies the iodide when metallic silver, nitrate of silver, and other soluble salts of this metal are present, and imparts to it the property of exciting the reduction of the acidulated solutions of silver by known developers, sulphate of protoxide of iron, gallic or pyrogallic acid, and also by mercurial vapours ; and it is this action that is turned to account to obtain negatives in the camera. 3rd. It brings the iodide to the state of inert iodide under the action of the developers, or when acted upon by light, it is covered with a solution of alkaline iodide. This action is the same with regard to metallic silver, which induces the belief that sensitized iodide of silver is brought partially to the metallic state when it has lost a certain portion of iodine to form a sub-iodide. These well known facts apply to the other insoluble compounds of silver, and particularly to the chloride, bromide, &c. Starting from this point, and wishing to explain the part played by the principal bodies which, up to the present