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THE PHOTOGRAPHIC NEWS. 103 . Intensify with bichloride of mercury and iodide of potas sium as described in former pages of the PHOTOGRAPMIO News. Varnish as usual. • IODIDE AND BROMIDE OF SILVER: RESEARCIIES into their Photogenic Properties according to the Circumstances under which they are obtained.* BY M. E. REYNAUD. From numerous experiments I have arrived at the conclu sion that when we make use of a collodion containing several iodides, the properties of the compound are not in relation to the several properties of each of the iodides when used separately. Thus, iodide of potassium, whose slowness when employed alone we well know, appears to me, on the contrary, to gain in sensitiveness when employed with the iodides of cadmium and ammonium. I shall now indicate some general rules to serve as a guide in the preparation of iodizing photogenic solution. These rules are derived from the observations stated above, and while leaving the operator free to choose which iodide he shall make predominate, they will serve as a basis of opera tions, and indicate to him the modifications he must make in his formulae, according to the circumstances in which he is placed. 1st. The quantity of iodine contained in the collodion (and here I understand not only the free iodine, but also that which exists in combination in the iodides employed), will be a mean of 1 gramme for every 100 cubic centi metres of normal collodion (1 per 100 grains, drachms, or ounces) t 2nd. In ordinary cases we shall find the advantage of employing together the iodides of potassium, ammonium, and cadmium. 3rd. When we desire to obtain the maximum of sensi tiveness, care must be taken that the collodion contains only a small portion of free iodine. 4th. If the photogenic film becomes spotted during sen sitising although its opaline hue may indicate that it does not contain so much iodide—we may attribute this defect to too great an excess of iodine ; but if the colour of the collo dion is not deeper than orange, we can only attribute this result to an excessive quantity of iodide of ammonium in proportion to the other iodides. 5th. Lastly, if the collodion film becomes fogged, wo must add some iodine or chloride of iodine until it assumes the orange hue. I recommend that an alcoholic solution of iodine be employed, so that the hue arrived at may be at once ascertained by merely shaking the mixture. I shall now make some remarks upon the action exercised upon the above-described phenomena by the state of the silver bath. A positively neutral silver solution always causes fogging, and in order to cause the fog to disappear, a small quantity °f acid must be added to the bath.t We may employ acetic or nitric acid ; but I prefer nitric to acetic on account of the formation, resulting from employing the acetic, of a Crtain. quantity of acetate of silver, which, by prolonged 18, is afterwards precipitated by the iodide of silver, with ten the solution becomes more and more saturated. The atate thereupon remains in suspension in the bath in the 8 a 'of Very minute crystals, and their injurious action in causing the collodion film to become riddled with holes is O"ByoQwell known to most photographers. 2 ne Use o f nitric acid this inconvenience is avoided; but i rnu 8 t be used with great moderation on account of its energy One drop usually suffices for four ounces of neutral silver soluticn. ” 7 Concludedfrom 7.87. , t 1.8089 antity of iodine, and not of iodide, for upon examining the euI miuI 1 the latter we perceive that a certain quantity of iodide of amnion , grammes, for example, contains 4 gr. 38 of iodine, while 5 Erommessornaide of cadmium contains only 3 gr. 45 of this metalloid, cilrneseystamized nitrate of silver of commerce is almost always suffi ciently acid to render the addition of any acetic or nitric superfluous. There is another acid the action of which upon the silver solution does not appear to have been observed; many experiments, however, have proved to me the advantage that may be derived in certain cases by giving it the pre ference, I refer to hyponitric acid. A nitrate solution into which we put nitric acid charged with nitrous compounds, appears to me to possess the property of giving very dense blacks, and lights free from fog; but what stiuck me as being very remarkable was, that the sensitiveness, far from being diminished, seemed in this case to become augmented. However, I do not positively assert this as a fact, not having repeated the experiment; but I recommend photographers to verify the fact, and to determine exactly what course to pursue in the use of this substance, especially in its applica tion to instantaneous photography. I shall not enter upon the means employed to neutralize a bath that is too acid, as they are sufficiently well known ; I shall content myself with condemning the employment of ammonia for this purpose, the much too intense neutralizing action of this substance almost always gives rise to acci dents resulting from the employment of an alkaline bath. Carbonate of soda or carbonate of lime are substances to which, I think, we ought in this case to give the preference. It now remains for me to speak of the bromides. We know that bromide of silver, compared with the iodide of that metal, is much more sensitive to the least refrangible colours, such as the red, yellow, &c. It is chiefly on account of this property that we are induced to employ the bromide in collodion. The bromides of ammonium and of cadmium are those which appear to me to possess this property in the highest degree. The bromide of potassium communicates to the collodion a sharpness and delicacy of detail similar to that produced by employing the iodide of that metal, and without dimi nishing its sensitiveness. The most suitable proportion of bromide in wet collodion appears to me to be 3 to 4 grammes to 12 of iodide in a litre of plain collodion. Too much bromide causes a super ficial fogging, which destroys all vigour in the proof. I now conclude my observations; there still remains much to be said on these subjects, especially upon the influence of iodide of silver upon the nitrate bath which becomes more and more saturated with it by use; and upon the changes induced in collodion by keeping, &c., &c. Upon these I hope to report on another occasion.—Bulletin de la Soc. Fr. de Photographic. PHOTOGRAPHIC SECRETS. BY COLEMAN SELLERS.* The beginner in photography and the unsuccessful operator alike are always on the look-out for, and most earnestly desire to learn, the “great secrets”—the wonderful instantaneous processes of this or that celebrated artist; and when they are told that these unknown processes are the same as what they have learned from the works of Waldack, Hardwich, &c., they cry out at once, “Oh, no! these men only publish what all the world knows already, and keep back the essential principles of their formula;and the proof of this they hold to be their own failures. This very thing is instanced in the complaints of some inquirers in this Journal. Some few years ago, being requested by a friend to give him some instruction in photography, I furnished him with a list of chemicals and fixtures needed, and as he was an ingenious man, I watched with interest the admirable arrangement of his dark room. At last he was ready for work, and an evening was spent at his house, directing the mixing of his bath, sensitizing his collodion, and preparing all things for an early trial of his skill. I had furnished him also with the balance of the collo dion in the bottle which I had used in showing him how to work. The next day but one he came to me with a sad tale to tell. He had followed my directions to the letter (he was a very precise man), and had “ exposed” any number of times in all kinds of light, and no image could he develop. “ What * From Humphrey^ Journal.