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244 THE PHOTOGRAPHIC NEWS. [May 23,1862. or black precipitate shows the presence of a metallic im purity. For photographic purposes caustic potash is gene rally required pure, and when used for the preparation of oxide of silver, by precipitating nitrate of silver with it, the presence of many of the above impurities would be very injurious, as the oxide of silver is usually wanted absolutely pure for neutralizing minute traces of nitric acid, or other delicate reactions. We will, therefore, give a process by which impure caustic potash may be purified, and also another by which a perfectly pure article may be prepared ab initio, in case the photographer has no crude caustic potash to purify. The caustic solution, after evaporation to a syrupy consis tency, is mixed with about one-third of its volume of alcohol, and well shaken together in a close vessel, it is then left to settle. Three strata are formed, the bottom one consisting of a precipitate which may contain lime, oxide of iron, and sulphate of potash ; over this is an aqueous solution of chlo ride of potassium, carbonate of potash, and sulphate of potash, together with a portion of caustic alkali; on this floats the upper stratum, consisting of an alcoholic solution of caustic potash. This is to be poured off, and the spirit separated by distillation or evaporation in a silver vessel, until the hydrate begins to sublime. A resinous matter will be formed by the decomposition of the alcohol; this will be found swimming on the surface, it must be removed, and the hydrate of potash poured out on plates. The only impurities which this alcoholic potash is likely to contain arc chloride of potassium, together with traces of carbonate and acetate of potash. The next method of preparation to which we alluded is more expensive, but yields a very pure substance. Sulphate of potash, which is readily obtained in a state of perfect purity, is powdered finely, and then boiled in a strong solu tion of caustic baryta, until a small portion of the liquid filtered off from the precipitate gives a turbidity upon super saturation with hydrochloric acid, and addition of chloride of barium. This proves that the sulphate of potash is pre sent in excess. This is now decomposed by careful addition of baryta water in slight excess, and the decanted solution is evaporated, the small excess of baryta being precipitated during evaporation by the carbonic acid in the air. Solid caustic potash must be preserved in very well stop pered glass bottles. Both in the case of the solid and solution of potash it is advisable that the stopper of the bottle should be rubbed over with solid paraffin, in order to prevent the adhesion of the stopper to the bottle, which almost always takes place after a certain time when this precaution is not taken. It should also be preserved in hard German glass bottles, in preference to the lead glass in general use in England, as the latter communicates silica, lead, and other impurities to the potash. The solid hydrate is white, hard, brittle, and very dele- quescent. Below redness it melts to a clear oily liquid. It requires but half its weight of cold water to dissolve it, the solution being attended with considerable rise of tempera ture. In more dilute solution it forms the liquor potass of the druggists. In this state it is a clear, colourless, inodorous solution, possessing intense alkaline properties, shown by the perfect manner in which it neutralises the most powerful acids; the strong blue colour which it communicates to reddened litmus paper, the brown colour which it imparts to turmeric paper ; the decomposing power which it exerts on most, if not all metallic salts, withdrawing their acid and setting free their oxide ; and finally by the strong power which it possesses of disorganizing the skin, communicating to the fingers, for instance, a greasy feeling, owing to its having dissolved the epidermis, and converted it into a kind of soap. CELESTIAL PHOTOGRAPHY. MR. Warren De la Rub has presented to the Academic des Sciences, a collection of astronomical photographs and engravings, upon which M. Faye reports that he has care- iat may be derived fro® ie ive as a velocity of the currents of the photosphere. fully examined them, and will endeavour to make their importance in a scientific point of view, fully appreciate! He says—The total eclipse of the year 1860, is too familiat to you to render it necessary for me to detail the great enter prises it occasioned. On the other side of the channel, Mr. a * It must not be forgotten that Mr. De la Rue’s photographs rlas p phenomena of totalily, and that the photographic impression or presents much greater difficulties than that of a partial eclipse- fidelity, a continuous history of the solar disc. Let us for a moment suppose the Academic to b‘ possession of such designs, continued perseveringly .a many years ; what problems connected with the consitug, of the sun should we not be permitted to solve? Wlor the present time can tell, by an attentive but excess" 3 special study of the spots in the sun, that the appearad these spots is a periodical phenomenon. It is even susPCy that the appearance of these spots is connected with w variations of terrestial magnetism. In taking the rers of one of our members on the motions peculiar to these sp j starting point, a learned German has, by the A long-continued observations determined the direction De la Rue was charged with the photographic department and here we have the results of this portion of the English expedition into Spain, placed before our eyes. The original proofs have been enlarged by well known methods, in ordet that the details of the mysterious phenomena may be bettet appreciated. Among these photographs, some faithfully represent the first impressions with all their defects, oc0k sioned by accidental jarring: others have been retouched remove these defects of which the origin is known: all deserving of the lively interest you have shown in them. Ii is in fact a real triumph of modem science to be able in th® manner to transmit to the remotest posterity, the brilliant b® transient phenomena of a total eclipse. It would be usel to repeat in this place the results that may be derived fn® these remarkable designs for the solution of the problel which astronomers have pursued since the eclipse of 184: the Academic has received ample details upon this subjod from Padre Secchi, who, in the south of Spain, has also su ceeded in obtaining photographs of this same eclipse. 1 shall limit myself to remarking that the fact of thi pression of the protuberances does not prove that thef appearances are real objects floating in the supposed atu sphere of the sun. Simple flashes of light would show jus as well upon the plates, on the sole condition of having the same intensity. To speak decidedly on this particular. " should find a much more significant argument in correspondence established by Padre Secchi between t proofs obtained by Mr. De la Rue and his own, obtained 111 intervals of a few minutes at stations separated by t entire width of the Spanish territory. Whatever it may be, the success of my honourable colleag" of the Royal Astronomical Society of London, is a g 1 ® step in a new and fertile field of observation. It will M desirable that the Academic, in thanking Mr. De la Rue his present, should enquire of him the details of the metb. he has pursued, and particularly respecting the improt. ments which his recent experience in Spain may have 805 gested to him for future operations. But while rendering full justice to these beautiful wor it must not be supposed that we remain strangers, or 9 indifferent to the progress which is based upon a Frep discovery. I will remind you, therefore, that in the y® 1858, there were presented to the Academic at its sitting A Monday the 15th of March, some beautiful large proof** the principal .phases of this phenomenon, proofs which " susceptible of exact measurements and obtained din without the intermediate operation of enlarging* UP the first glance at these proofs we may distinguish q smallest spots, and even the very curious and complict, undulations of the marginal facula ; and on this same0 | sion you were reminded how useful to science it would b^ ' collect, on the same scale, day by day, and with the 8