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164 THE PHOTOGRAPHIC NEWS. [March 16, 1883. considerably, after which the enamel goes into the furnace once more, and the firing is completed. Spoilt enamels are simply cleaned off with fluoric acid, and the plates used once more. Enamels of all sorts and sizes are here, some for tiny lockets not more than half-an-inch across, some with whole-plate portraits upon them. Here is an enamel of the late Prince Consort, and here one of the Grand Duke of Hesse and his wife, the late Princess Alice. Her Majesty the Queen is one of Mr. Henderson’s best customers, for she is a firm believer in imperishable photographs. Here is a picture on enamel of the Prince of Wales uncovering the monument to the late Prince Imperial at Woolwich— from a fine negative taken by Mr. Cobb, we believe—and here are some of Mr. Henderson’s wonderful Derby pictures. They are in all stages of progress, but no matter their nature or number, they must all perforce go under Mr. Henderson’s hands, for enamel photography, if simple to describe, requires undoubted skill and delicate manipula tion, if it is to be practised with success. NOTES ON PHOTOGRAPHY. Lecture XV.—Silver Printing (Theory of). BY E. HOWARD FARMER. For silver printing, it is generally found that, in order to get vigorous and brilliant prints, the following requisites are necessary:— 1st. A layer of silver chloride. 2nd. A considerable excess of silver nitrate. 3rd. A layer of some organic silver compound. 4th. That these should be retained on the very surface of the paper or other substratum. The functions performed by these four requisites are distinct and important. Silver chloride alone is reduced by light, darkening to a deep violet colour, with formation of silver subchloride and evolution of chlorine; the chlorine thus set free, how ever, by its tendency to re-combine with the silver, acts as a powerful retarder of the reduction. Silver nitrate, by reacting with this chlorine to form fresh silver chloride, prevents its retarding action; hence silver chloride, in presence of excess of silver nitrate, darkens far more rapidly than when alone When an image produced in this way, and consisting of silver subchloride, is placed in the fixing bath, to dis solve out the unreduced salt, it is decomposed into silver chloride and metallic silver ; Ag 2 Cl = AgCl + Ag. The consequence of this is, that the vigour or density of the image is enormously reduced ; in fact, so much so, as to render it almost invisible. We must, therefore, look for some additional action to account for the non-reduction of our prints in fixing, and we find this in the organic compound of silver. Albumen is the organic substance usually employed. When silver nitrate is added to this, it combines with it, forming a substance we may call, for convenience, silver albuminate, at the same time producing coagulation, or rendering the albumen insoluble in water. The silver albuminate, on exposure to light, darkens like the chloride ; but the colour, instead of being violet, is a brick red; or if the insolation be prolonged, the well- known bronze appearance occurs. The brick-red sub stance is a suboxide of silver, probably mixed with metallic silver, when bronzing occurs. Silver suboxide alone is like the sub-chloride, decom posed by hypo.; but when it is produced in presence of organic matter, such as paper, &c., it remains in combina tion with it, and resists the destructive action of hypo. Silver albuminate unreduced is, like the chloride, readily dissolved by hypo. The image on an ordinary silver print, when it is taken from the printing-frame, consists then of a mixture of violet sub-chloride and red sub-oxide of silver, and it is owing to the combination of these two that its pleasant colour is due. On immersion in the fixing bath, the sub chloride is decomposed, leaving a small quantity of metallic silver and the familiar red sub-oxide forming the image. If the excess of nitrate of silver be removed from sensitive paper, the reduction of the chloride is very much retarded, and, on taking a print with such paper, a red image is at once obtained, having a similar appearance to an ordinary print after fixing. It would appear from this theory that the silver chloride is of little, if any, use in the paper, and, according to Hardwich, it can be omitted with but little difference in the result, except that the time of printing is prolonged. To render the finest details visible by reflected light, and to obtain brilliancy, it is found to be of prime import ance that the sensitive compound should be retained as a smooth layer on the surface of the paper. And here again albumen becomes invaluable, for, owing to its glutinous character, it does not sink into the pores of the paper, but remains as a thin film on its surface, forming at the same time a vehicle to retain the silver chloride and nitrate in the same position. Toning.* If a thoroughly washed print be placed in a plain solution of gold chloride it will tone, but at the same time its vigour will be very much reduced ; the reason of this is that the chlorine from the reduced gold combines with the silver of the image to form white silver chloride, which dissolves out in the fixing bath. 3Ag.01+ AuCla = 6AgCl--Au In order to avoid this loss of vigour, a substance is added to the toning bath which will absorb the chlorine, an example of which we have in sodium acetate, which takes up the chlorine, forming sodium chlor-acetate. If the toning takes place with silver nitrate in the paper, a retarder, such as a soluble chloride or hydrochloric acid, is also required to be present, or the gold will be immedi ately thrown out of solution. Prints can be toned in the fixing bath to a colour very much resembling that given with gold, by adding to it an acid or oxidizing substance. In this case sulphur is deposited on the image, and the prints are not permanent. Fixing. On fixing a print, the unreduced silver chloride and albuminate are dissolved out, and the silver sub-chloride decomposed into silver and silver chloride, the latter also being dissolved. A bath which has been kept, or has already fixed a good many prints, is apt to become acid by decomposition of the silver thio-sulphate it contains in solution— Sulphuric Acid. 2Ag:S,03+H,0=Ag:S + H 2 SO 4 This sulphuric acid, by re-acting on the hypo., tends to deposit sulphur on the prints, and destroy their perman ency ; hence a small quantity of ammonia should be added to the bath, iu order to prevent acidity occurring. FREEING EMULSION FROM SOLUBLE COLLOIDS. BY A. HADDON. + Part II. Before giving the details and results of a few experiments I have been making on the removal of soluble from insoluble gelatine, I should like to read a few short extracts on dialysis and the preparation of colloids in a state of purity from Watt’s Dictionary of Cheniistryj page 715, vol. iii., second edition, 1872 :—• “ Membranes and septa of the colloid class possess the property which is very useful in assisting diffusive separations. The jelly of starch, that of animal mucus, of pectin, of Poyen’s vegetable • “ Instruction in Photography.” . t Read before the London and Provincial Photographic Association,