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May 9,1862.] THE PHOTOGRAPHIC NEWS. chance of their adhering either to the bath or to the plate (fig. 19). Some glass dippers have lately been made for sale in one piece, like the porcelain dippers, the lip being turned up instead of stuck on. These are better in principle than those which are of two pieces cemented together, but the speci mens I have seen of them were not of a satisfactory form, the top of the lip being turned inwards, which would in evitably touch and tear the collodion film. Metal dippers are made of a loop of silver wire, or an iron wire coated with gutta percha, bent into the form shewn in fig. 18; they are useful for large plates which might be too heavy to trust upon a dipper of glass or porcelain, but their cost is too great for them ever to come into general use for ordinary work. Dippers are sometimes constructed of glass rod bent into the same shape as the above, but they are not to be depended on, and should only be used with very small plates except •on an emergency. Gutta-percha dippers are made like porcelain dippers, but these also should only be used for small plates. When a dipper is made of such a flexible material as gutta-percha it is apt to bend under the weight of a heavy plate, and force it off the lip. A dipper intended for carrying about in a portable bath should always be made specially for it, with the head cut into a diamond shape, (fig. 20) so that it may lie diagonally inside the bath when the water-tight cover is screwed on ; and Yig. 20, if this be well devised the soft gutta-percha cover will press lightly upon it and keep it from shaking about and getting broken. The top of the dipper will of course get wetted with the nitrate of silver splashing on it when carried in this manner, and must be wiped with a piece of filtering paper or rag, before being used. Generally speaking, the vertical baths and dippers are Very convenient for plates of moderate size, and present no difficulties in practice ; but for large plates their use is not Vuattended with risk from the fragile nature of the dipper, and the excessive, as well as rather awkward handling a plate has to undergo. Also, the great quantity of bath liquid such baths require, when very large plates have to be Bensitized, is a serious consideration for amateurs especially ; and in my opinion it will be found more suitable for such purposes to have recourse to a horizontal dipping bath, the construction of which I propose next to examine. {To be continued.) PHOTOGRAPHIC CHEMICALS : Tima MaxUFACTUEE, ADULTEEATIONS, and Analysis. Sulphides (concluded).— Few of the other sulphides possess any interest to the photographer. Sulphide of gold is obtained from the toning baths, either mixed with, or free from silver, according to the kind of solution employed, and, therefore, a brief account of its properties may not be out of place. Formed by precipitation, it is a heavy black powder which very readily decomposes with separation of metallic gold. At a gentle heat the sulphur goes off, and if left in contact with the atmosphere in a moist state the gold is likewise separated with formation of sulphuric acid; the oxygen being absorbed from the atmosphere. Ignition, ' either per se, or with the usual mixture of carbonate of soda and nitre used for decomposing the sulphide of silver, at once reduces it to the metallic state. Platinum is sometimes used for toning pictures, instead of gold ; it is a valuable metal, and would also be recovered in the form of sulphide. Sulphide of platinum is a black, heavy powder, decomposed by heat and fusion with nitre, but not so readily as sulphide of gold. When exposed to the air in the moist state it becomes oxidised, and then con tains a quantity of free sulphuric acid, along with platinum- black. When this compound is heated in contact with the air, it takes fire and burns with a hissing noise and violet flame, continuing to glow with evolution of sulphuric acid. Nitrites.—A brief account may here be made of some of these salts. Nitrite of silver, along with nitrate of silver in the sensitising bath, communicates several curious proper ties to the collodion plate, which deserve further investiga tion. In order to facilitate this promising branch of research, if any of our readers feel inclined to pursue it further, we will briefly describe one or two of the most important nitrites. Nitrite of Potash.— 1 This salt is formed in the following way :—Heat nitrite of potash in a crucible, and keep it at a red heat for a long time. The extra equivalents of oxygen in the nitric acid gradually go off, and there remains nitrite of potash, mixed, however, with a considerable excess of undecomposed nitrate. Dissolve the mass in boiling water, and allow to crystallise by cooling. The undecomposed nitrate of potash crystallises out first. On concentrating the mother-liquor, neutralising it with acetic acid, and then mixing it with twice its volume of strong alcohol, crystals of the nitrite are deposited, which must be collected on a filter, washed with alcohol and rapidly dried. It forms a white crystalline deliquescent salt, without alkaline reaction. Nitrite of Soda is prepared from nitrate of soda in exactly the same manner as the potassium salt. The only reason for its being referred to here is, that it is not deliquescent (ac cording to Gmelin), and is, therefore, a much better salt to work with than the potash nitrite, which is generally used. Nitrite of Silver.—This is the most important of the nitrites for photographic purposes. It may be prepared in several ways. If an aqueous solution of nitrate of silver is boiled for several hours with finely divided silver, the metal is dissolved with formation of a light yellow liquid, which, when evaporated down to a density of more than 2’4, solidifies after some time, on agitation. If this mass is extracted with water, a basic salt remains un dissolved, in the form of a yellow powder. Cold water or alcohol, likewise throws down this powder from the yellow solution. Some time ago, we remember mention having been made of some valuable properties which were communicated to a nitrate of silver bath, by boiling it with finely-divided silver for a few hours. We are not aware that the subject has been pursued further, but it is evident from the above that this process would give rise to a certain quantity of basic nitrite of silver in the bath to which the properties would be due. Nitrite of silver, in the basic form, blackens when exposed to the light: it dissolves readily in ammonia, but only sparingly and partially so in water, flakes remain ing behind. A monobasic nitrite of silver may also be prepared, by mixing together aqueous solutions of crude nitrite of potash or soda with nitrate of silver. The precipitate is filtered, washed, and then dissolved in boiling water, filtered from the precipitated oxide, and allowed to cool. The salt thus pre pared forms long and slender needle-shaped prisms, having a wavy lustre; they are colourless when immersed in the liquid, but acquire a slight sea-green tint when taken out. It requires three hundred times its weight of cold water to dissolve it, but is more soluble in hot water; it is insoluble in alcohol. According to Proust, the yellow liquid obtained by boil ing finely-divided silver with an aqueous solution of nitrate
