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1862. tetobe e holder liquid is r the cut at 80°to be sees that this led after hat each ' several different solution t be dis- asses, it gress of ent this .ISIS. iossesses ds, and ier. Ito is weak i, unless lourless, of sorb r spirit lightest a water, cess of alts are. f their lement, rchloric lumina, a preci- mt, the Potash nations stals of d com- ngdom, .re con- 1 is con- icb, by er, cot- etables he pro- rom it almost source of the ted in mufae- ; same ash is ilution lidifies ced in y over rganic ■hat is solved being ren, is entra- in the ed off. JUNR 6, 1862.] THE PHOTOGRAPHIC NEWS. 271 * Read on the evening of Wednesday, May 21, at the North London Pho tographic Association. ON THE PRACTICAL WORKING OF THE PANORAMIC LENS. BY GEOEGE DAWSON* Lecturer on Photography at King’s College, London. This paper is meant to be thoroughly practical. I do not, therefore, intend to discuss the scientific principles on which the panoramic lens has been constructed; nor, indeed, is this course necessary, inasmuch as Mr. Sutton, the inventor (to whose papers I refer you), has on several occasions fully entered into that portion of the subject, and explained the circumstances which compel the use of a curved glass for the reception of the negative. For my own part, I confess that the extraordinary appearance of the whole apparatus, so different to the traditions and ordinary requirements for the practice of photography, pre judiced me strongly against the estimation of it for practical utility. The difficulties attending the coating with collodion and evenly developing curved surfaces seemed to me almost in surmountable, and it was with strong misgivings as to the result that I'determined to spend a few days of my holiday in t rying the capabilities of the lens, and how far the difficulties above indicated could be overcome. I have succeeded beyoml my expectations, and can now truly say that there is no plea sure greater than that of agreeable disappointment. The specimens before you, good, bad, and indifferent, are my first attempts. Some of them, the architectural for instance—in Soda.—This is a very common, but not a very important impurity ; it may be detected by heating a little of the dry salt on platinum wire in the flame of a spirit-lamp ; if soda be present the colour of the flame will be a pure yellow: the separation of soda from potash would be attended with too many difficulties for an amateur to attempt to perform. Carbonate of lime is also sometimes present. The solution in this case deposits crystals of carbonate of lime when allowed to stand for some time; if it be neutralized with acetic acid, and then a drop of oxalic acid added, a white precipitate of oxalate of lime will be produced. Silica is a very common impurity in commercial carbonate of potash. When it is present in large quantities the solution yields a gelatinous precipitate of silica when an excess of hydro chloric acid is added, especially on boiling. Smaller quan tities may be detected by evaporating the acid solution to dryness and gently heating, the silica will be left behind upon redissolving in water. Oxide of copper, which is sometimes present, is detected by adding an excess of hydrochloric acid and then an excess of ammonia. A blue colour will show the presence of copper, which may be verified by adding to the solution a drop of ferro-cyanide of potassium, a brown precipitate will in that case be produced. Pure carbonate of potash forms a white solid mass, fusible at a bright red heat. Being only united to one equivalent of carbonic acid, it has a strong alkaline reaction both to the taste and litmus paper. It rapidly deliquesces in the air, forming an oily liquid. If this liquid is allowed to stand in a dry atmosphere for some time, it forms crystals. Carbonate of potash is capable of absorbing another atom of carbonic acid, forming a bicarbonate. This salt is much less soluble in water than the mono-carbonate, and there fore may be precipitated from a strong solution of the latter by passing carbonic acid through it, or better still by only conducting it to the surface of this solution. The absorption takes place completely, although slowly, sometimes not being finished by the end of a week, The crystals are washed in a little cold water, and then drained on blotting paper. They form large oblique prisms, transparent, of a saline and slightly alkaline taste, but no longer caustic. Its reaction is still slightly alkaline to test paper. When dry, the crystals remain unaltered in the air; but when heated strongly, they lose half the carbonic acid, and then return to the condition of mono-carbonate. Upon boiling the clear solution down to the density of about 1'43, most of the carbonate of potash will crystal lize out on cooling and stirring. The crystals of carbonate are lastly washed with a small quantity of cold water, and then rendered anhydrous by ignition in a cast-iron, silver, or platinum vessel. When prepared from crude tartar this is to be purified by one or two crystallizations, and then heated to low redness in a covered iron crucible, the mass is then extracted with hot water. After filtration the liquid must stand for some days in order to allow the carbonate of lime to settle, and the clear solution is then evaporated to dryness in a vessel of either cast-iron, silver, or platinum. (As a rule we may observe that whenever alkaline substances are to be evapo rated down, or to be ignited, the operation should always be performed in vessels made of one of these three metals, silver being the best.) The dry residue is covered with three times its weight of cold water, and allowed to stand hr three days, and the solution is then filtered or decanted bom the insoluble matter and evaporated to dryness. It is hecessary to purify the crude tartar before employing it for fhis operation, as otherwise the carbonate of potash would b contaminated with cyanide of potassium formed from the nitrogenous matter which it always contains. What is known as black flux, a substance of considerable "ein reducing photographic residues to the metallic state, lf a very impure mixture of carbonate of potash and char- (9al; it may be readily prepared by mixing together two Parts of crude tartar and one part of nitre. The mixture is then to be thrown into a red-hot iron crucible. Upon cooling it will form a black mass which may be powdered and kept for subsequent use. It thus forms a mixture which, from the presence of carbon (from the decomposition of the acid of the tartar) and cyanide of potassium, has eminent re- dtcing properties. The white flux, which is a constant Aecompaniment of the former mixture in chemical labora- tories is formed in a similar way, but with an increase in the quantity of nitre, equal weights being taken. In this May all the carbon is burnt. It is used as a flux when no Beducing action is desired. The impurities found in commercial carbonate of potash Gre very numerous, and as a knowledge of the means of etecting them is of some consequence, we will proceed to Moe them in detail. . ^phate of potash is detected by adding an excess of ydrochlor ic acid, and then chloride of barium; the formation 0fa white insoluble precipitate is a conclusive proof of the "sence of this substance. For many purposes it is im- Enant that it be got rid of; this is effected without much tality by dissolving in water and evaporating once or bl to a syrupy consistency. The sulphate of potash, a 598 much less soluble than the carbonate, crystallizes out nay be strained off. ih,‘hloride of potassium.— 1 This impurity is detected by add- 8 su excess of nitric acid, and then a drop of nitrate of ./'f; it may be removed in a similar way to the sulphate "Botash. oPhosphate of potash may be detected by adding an excess hydrochloric acid, and boiling for some time until all the abonis acid is expelled, and then adding an excess of fomonia and a little solution of chloride of calcium. A oulentprecipitate indicates the presence of phosphoric acid. °f potash is detected by dissolving the salt in m1.88 of sulphuric acid, and then adding a crystal of dP i ° of iron ’ the well-known reddish halo will be pro- presdnround the crystal if the slightest trace of nitric acid be hitnteof potash may be detected in the same way. ehloyan ■ 9— 1 To detect this add excess of hydro- bnronc acidaand then a little sulphate of iron which has (Fosrtsvpartialy. per-oxidized by exposure to the air. M) feVeloping solution will answer the purpose very Mt a tnessmallest trace of cyanide of potassium be pre- ' P "CPI ate of prussian blue will be formed. i