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THE PHOTOGRAPHIC NEWS. Vol. VI. No. 180.—FeLruanj 14, 1862. THE PROJECTED ADDITIONAL PHOTOGRAPHIC EXHIBITION. The negotiations for holding a photographic exhibition in connection with the South London Photographic Society, to which we referred in our last, arc in progress with every probability of a satisfactory and successful termination, under conditions most favourable to photographers. It is hoped that facilities will be secured for the exhibition of new appaiatus and inventions in connection with the art, which will be satisfactory to many houses who have failed to secure allotments at South Kensington. If the arrangements terminate as suncessfnlly as we have reason to hope, the causes for regret arising out of the inadequate representation of photography at the International Exhibition during the coming summer will be deprived of very much of its force. We hope shortly to be able to announce full particulars. Scientifit Gossip. Preparation of Pure Nitrate of* Silver—Properties of the Pure Salt—Not Alkaline, as usually reported, but Neutral—Solution under Pressure. Wb have considered it of interest occasionally to lay before our readers in these columns short abstracts of what has lately been done by the Belgian chemist, M. Stas, in his researches on the atomic weight of silver, and its compounds. I erhaps amongst all these facts, the most interesting and important (to us) are those which relate to nitrate of silver; wo will therefore follow up our previous articles on the sub ject by condensing from the voluminous memoir before us a few particulars respecting the formation, purification, com position, and properties of this salt. The most advisable way to prepare nitrate of silver was ascertained by M. Stas to be to dissolve the pure metal in an excess of nitric acid in a Bohemian glass flask, to the neck of which was attached an arrangement of tube, and bulbs containing liquid so as to prevent silver being carried away by the evolved nitrous gas. The silver being dissolved, which requires from twenty- four to forty-eight hours, according to the quantity of th. metal employed, (M. Stas worked upon from 100 to 200 gramnes of silver at a time,) the flask is inclined, and a feceiver attached to the neck. The liquid is then caref’ My evaporated to d ryness ; the temperature being raise ■ r, but not quite, to the boiling point; this takes .rom forty- eight to seventy-two hours. After the evaporation of the liquid, the temperature of the dry nitrate is raised to its fusing point, and kept ein re until if is of a constant weight. To hasten its drying and esPucic y to drive away the last traces of nitric acid,which the salt retains, there is passed through the vessel a current f air, which has been first deprived of organic matter by being passed through a red hot glass tube containing tarnished copper (which yields its oxygen to the carbon and hydrogen of the organic matter), and then through tubes containing dried chloride of calcium to remove the moisture. These precautions were necessary, since it was found that the air ot the laboratory, or even the external air simply filtered through cotton and then dried, slowly but continuously reduce? nitrate of silver heated to ing point, and still mere quickly when fused, with elimination of nitric acid. When it is und that the weight of the dried salt is perfectly constant, tho temperacure is again increased, so as to fuse it, and it is then kept in fusion in a. current of air deprived of organic matte, and water until its weight becomes absolutely constant . M. Stas states that in many experiments he has kept nearly 500 grammes of nitrate of silver in a fused state, fdnight io the morning till ten at night without in the least diminsh- ing its weight. Whenever purified air was made use of in any of these experiments, the nitrate of silver obtained was'perfect!; colourless in the fused state, and after solidification wa of a pearly white colour, with a radiating fracture. It was found to be very slightly hygroscopic after fusion; a pro perty which the crystallized salt does not appear to possess, and was always neutral to test paper. This is a very impor tant observation, as it is in opposition to statements which have frequently been made by photographic authorities when writing on this subject. Thus, Hardwich in his “ Photographic Chemistry ” says, that the aqueous solution does not redden blue litmus paper; on the contrary, tho pure recrystallized and dried nitrate of silver restores the blue colour of paper previously reddened. This statement has been repeatedly made and copied into other works. It is probable that the slight decomposition which M. Stas mentions as always taking place when nitrate of silver is prepared in contact with air not previously purified, and which consists in the decomposition and removal of nitric acid, and the separation of metallic silver or its oxide as a black powder, may be the cause of this alkaline reaction. It is not yet definitely settled whether the black deposit is silver or oxide of silver, but in either case it might give rise to an alkaline reaction. Supposing it to be the metal, it is known that when finely divided metallic silver is boiled with nitrate of silver in solution, it is dissolved with evolu tion of nitric oxide forming a light yellow liquid, contain ing basic nitrite of silver, which has an alkaline reaction. If on the other hand it be the oxide of silver, it is equally certain that this would dissolve in the nitrate of silver solu tion, and give it an alkaline reaction. In either case then, nitrate of silver made in the way universally adopted, would, even if all the usual precautions were taken, be liable to communicate a blue colour to reddened litmus paper. This, however, after the experience of M. Stas, does not prove that pure nitrate of silver has an alkaline reaction, but merely showy that the specimen, upon the examination of v«>se property this character has been ascribed to it, was not aolutely pure. o omn to the properties of the pure nitrate. The erstall. ; -nern dried in a current of air at its fusing point, still weigavt when it is actually fused. The loss is however very ifing. lure crystallized nitrate of silver kept for six monus und er a behi-jar in an atmosphere arti- ficially dried with encenitrated sulphtiq acid, lost in an experiment tried on at durably large scale, one four- thousandth of its weight 1 on fusion. This salt which would be considered by all chemists as anhydrous, loses therefore three times as much water as the salt obtained by direct synthesis and dried at its fusing point. It appears probable, however, that all the loss experienced by the crys tallized celt upon fusion should not be attributed to the disengagement of water; it may possibly be that the air, condensed by the surfaces ol the small crystal 1 ; nc plates, has something to do with it. The nitrate of silver was net in every case prepared in tho manner described above. It was found that there is a