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January 9, 1863.] THE PHOTOGRAPHIC NEWS. 15 heat, the water is decomposed, its hydrogen uniting with the chlorine to form hydrochloric acid, which goes off, and its oxygen uniting with the aluminium to form a resi due of alumina. It is not possible to avoid this decom position, except by setting to work in a somewhat round about way. An intimate mixture of alumina and carbon is prepared by thoroughly incorporating together alumina, powdered charcoal, sugar, and oil, to the consistence of a thick paste. This is introduced into a crucible, and heated, out of contact with the air, to redness. The sugar and oil are in this manner decomposed, and the carbon of their composition is left in a state of intimate mixture with the alumina and powdered charcoal. The mass is introduced warm into strong porcelain or glass retorts, through which a slow current of perfectly dry chlorine gas is allowed to pass. When all the air has been swept from the apparatus, heat is applied to the carbonized mass, when the alumina is decomposed by virtue of the double chemical persuasion to which it is subjected : the carbon takes the oxygen from the alumina, forming carbonic oxide, whilst the chlorine unites with the aluminium to form chloride of aluminium. This is carried along by the carbonic oxide gas, and con denses principally in the form of a fine powder, and partly as a solid mass at the extremity of the neck of the retort, which must be very large, or it will be stopped up by it. The alumina must be precipitated from a solution which contains no sulphuric acid, as otherwise the alumina will carry some of this down with it, and will give rise to the formation of chloride of sulphur, which will mix with the chloride of aluminium. Chloride of aluminium forms a lemon-coloured waxy mass, transparent, and of a crystalline texture resembling talc. It fuses when in large masses, a small quantity immediately evaporating on the application of heat; it boils at about 300° Fah., and fumes slightly in the air, smelling of hydrochloric acid. It rapidly deliques ces in the air, forming transparent drops, and dissolves in water with a hissing noise and evolution of heat. In this state it has fixed to itself the elements of water which can not be driven off again by heat, the compound splitting up, as we have above described, into hydrochloric acid and alumina. When the anhydrous chloride of aluminium is heated with potassium or sodium (the latter method is always employed at the present time), metallic aluminium is separated from it with formation of chloride of potassium or of sodium. If the aqueous solution of chloride of aluminium, or the liquid obtained by saturating hydro chloric acid with hydrate of alumina, is allowed to evaporate in a warm and very dry atmosphere, crystals are deposited, which are very deliquescent, and very soluble in alcohol. They contain twelve atoms of water of crystallisation, and 43-96 per cent, of chlorine. This salt has not been much used in photography ; but since chlorides which arc very soluble in alcohol are not very common, it is likely that chloride of aluminium could be employed for several useful purposes, especially in the collodion process. Nitrate of Alumina.—This is a very deliquescent salt, formed by saturating nitric acid with hydrate of alumina. Upon evaporating the solution carefully down to a syrupy consistency, and allowing it to stand in a cool dry place, the nitrate crystallises out in thin colourless lamina?. It has been successfully applied, like nitrate of magnesia, for the preservation of sensitive collodion plates. Sulphate of Alumina and Ammonia, or Ammonia-alum, as it is generally called.—This is the most important salt of alumina, and is prepared by hundred of tons at a time. Alum shale is boiled out In dilute sulphuric acid, and the solution is mixed with sulphate of ammonia derived from the gas residues. The solution is boiled down rapidly, and the salt is deposited in the form of a fine white crystalline powder, known by the name of alum meal. This is rinsed from the impure mother-liquor by a small quantity of water, then dissolved in hot water, and allowed to crystallise in large wooden tubs, capable of holding some tons’ weight at a time. After the magnificent display of alum which our readers have recently seen- at South Kensington, no detailed account of the appeaiance of this body is needed. Al though called alum, this name strictly speaking, belongs to potash alum, a compound exactly similar to the one we have now been describing, but containing potash instead of ammonia. Until within the last few years, potash alum was always made. It was found, however, that ammonia, which would confer equally good crystallising powers upon the alumina salt, could be obtained in almost unlimited quan tities from the gas-purifying liquors; and it has, there fore, so completely replaced potash alum, that it is a diffi cult matter to obtain even a small specimen of the latter compound. PHOTOGRAPHY AND THE HEALING ART. [We are desirous of giving especial prominence to the appeal of Dr. Wright to photographers. No more interest ing or important application of photography can be made than that which makes it an adjunct to the art of healing. A collection of good photographs of all kinds of medical and surgical cases, placed in the library of the Medical and Chirurgical Society of London, cannot fail to be of the utmost value in aiding the study of such cases. Wo feel sure that any of our readers possessing negatives, prints from which will add to the completeness of the collection, will gladly respond to the appeal of their brother photo grapher, our friend Dr. Wright.] Sir,—Physicians and surgeons desirous of retaining lasting representations of important cases (on the accuracy of which might depend decisions as to life and death in similar cases occurring to others) were formerly compelled to resort to the pencil of the artist. But the reliable talent available for such a purpose was not readily procured, and when obtained involved a large expense. The photo graphic art supplies a simple and cheap method of perpetuating the appearance presented by any particular form of diseased structure at any stage of its progress, or of its cure, and medical men are daily more and more availing themselves of the opportunities thus presented. It took hours to make an exact sketch where now it requires only as many seconds, and the presentment of that seen is not only accurate in every detail, but gives facilities for such comparative measurements as the hand following the eye would never have obtained. Hence it follows that photo graphy has been of vast service in this department of its working, and there is scarcely a professional photographer who has not been called on, at some time or other, to picture the appearances produced by deformity or disease, or to perpetuate some triumph of surgical skill. Such records are of lasting value ; they do not tell merely of that which they represent; they also afford assistance, such as no verbal description could supply to those who may be in doubt when similar cases occur. Hence it well may happen that the exact verisimilitude which a photograph affords will determine doubts on which the issues of life and death may depend. Photography is, par excellence, an art of exactitude, it neither exaggerates, or mitigates what the eye of the camera sees, and therefore is practically reliable even where so much depends on its evidence. The importance of the photographic representations of professional subjects has led the Medical and Chirurgical Society of London to determine on adding to their extensive library a collection of photographs of subjects of professional interest. As a Fellow of the Medical Chirurgical Society, and a Member of the Council of the Photographic Society, I have willingly undertaken to gather and arrange the contribu tions. In the hands of professional and amateur photo graphers in various parts of the kingdom are numerous negatives, for the most part taken at the request of medical men. I believe, that considering the object in view in making this collection, I am justified in asking your readers
