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The photographic news
- Bandzählung
- 35.1891
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- 1891
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1720, August 21, 1891
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The photographic news
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596 THE PHOTOGRAPHIC NEWS. [August 21, 1891. of fainter stars, which become richer and more intricate in the irregularly rifted zone of the Milky Way. We, who form part of the emblazonry, can only see the design distorted and confused—here crowded, there scattered, at another place superposed. The groupings due to our position are mixed up with those which are real. Can we suppose that each lumino us point has no closer relation to the others near it than the accidental neighbour ship of grains of sand upon the shore, or of particles of the wind-blown dust of the desert ? Surely every star, from Sirius and Vega down to each grain of the light-dust of the Milky Way, has its present place in the heavenly pattern from the slow evolving of its past. We see a system of systems, for the broad features of clusters and streams and spiral windings which mark the general design are reproduced in every part. The whole is in motion, each point shifting its position by miles every second, though, from the august magnitude of their distances from us and from each other, it is only by the accumulated move ments of years or of generations that some small changes of relative position reveal themselves. The deciphering of this wonderfully intricate constitu tion of the heavens will be undoubtedly one of the chief astronomical works of the coming century. The primary task of the sun’s motion in space, together with the motions of the brighter stars, has been already put well within our reach by the spectroscopic method of the measurement of star-motions in the line of sight. Many other problems of the newer side of astronomy would claim attention if time permitted. The researches of the Earl of Kosse on lunar radiation, and the work on the same subject and on the sun by Langley. Observa tions of lunar heat with an instrument of his own invention by Mr. Boys, and observations of the variation of the moon’s heat with its phase by Mr. Frank Very. The dis covery of the ultra-violet part of the hydrogen spectrum, not in the laboratory, but from the stars. The confirma tion of this spectrum by terrestrial hydrogen in part by H. W. Vogel, and in its all but complete form by Cornu, who found similar series in the ultra-violet spectra of aluminium and thallium. The discovery of a simple formula for the hydrogen series by Balmer. The important question as to the numerical spectral relationship of different substances, especially in connection with their chemical properties; and the further question as to the origin of the harmonic and other relations between the lines and the groupings of lines of spectra. On these points contributions during the past year have been made by Rudolf v. Kdvesligothy, Ames, Hartley, Deslandres, Rydberg, Grunwald, Kayser and Runge, Johnstone Stoney, and others. The remarkable employment of interference phenomena by Professor Michelson for the determination of the size, and distribution of light within them, of the images of objects which, when viewed in a telescope, subtend an angle less than that subtended by the light wave at a distance equal to the diameter of the objective— a method applicable not alone to celestial objects, but also to spectral lines and other questions of molecular physics. PHOTOGRAPHIC Club.—Subject for August 28th, “ Films ” ; September 2nd, “ Testing Lenses.” Outing Saturday next, August 22nd, Cobham (Surrey). Train from Waterloo at 2.27. Photographic Society of Great Britain. — Technical meeting at 8 p.m. on August 25th ; subject for discussion, “Printing Processes.” THE DECOMPOSITION OF SILVER CHLORIDE BY LIGHT. BY A. RICHARDSON, PH.D. Although the fact that silver chloride darkens on expo sure to light has long been known, the composition of the product formed has not yet been satisfactorily determined; the observation of Schelle, however, that the darkening process is accompanied by an evolution of chlorine, leads to the conclusion that the substance formed is either a subchloride or an oxychloride of silver, whilst its sparing solubility in dilute nitric acid seems to preclude the pos sibility of the reduction of the chloride to metallic silver. Fischer, in 1814, assumed that the darkened product was a subchloride, and this idea has been generally ac cepted until recent years. When, however, doubt was cast on the existence of Wohler’s silver suboxide by the experiments of Muthman {Ber., 1887, 20, 983), and of Bailey and Fowler (Trans. 1887, 51, 416), and the exist ence of the suboxide of Pfordten (Ber., 1887, 20, 1458, 3375; 1888, 21, 2288) was disproved by Freidheim (Ber., 1887, 20, 2554), the evidence in favour of the subchloride theory was somewhat shaken. Quite recently, however, Giintz (Compt. Bend., 112, 861) claims to have obtained silver subchloride by the action of gaseous hydrogen chloride on silver subfluoride, as also the suboxidc and other similar compounds; the subchloride so obtained has not, however, been identified with the darkened pro duct formed by the action of light on silver chloride. The other view, namely, that the compound formed is an oxychloride of silver, has lately been received with considerable favour, and seems to be supported by some experiments of Dr. Hodgkinson, in which the presence of the oxychloride appears to have been detected in the darkened compound (Meldola, “ Chemistry of Photo graphy,” p. 856). As, however, the evidence in favour either of the subchloride or oxychloride theory is far from con clusive, it was thought desirable to examine the darkened silver chloride in order to determine whether it contained oxygen, and further, to study the decomposition of the chloride when exposed to light under water. The silver chloride used in all the experiments was obtained by adding hydrochloric acid to a solution of pure silver nitrate, the precipitate being washed by decantation till free from acid. Action of Silver Chloride on Water when Exposed to Light.—In the early experiments, the chloride was ex posed to light in sealed tubes containing water, the space above being occupied by air. When opened after long exposure (about six months), it was found that a con siderable quantity of gas had been formed, and that it contained sufficient oxygen to rekindle a glowing match (8 grams of the chloride gave 7 c.c. of gas). The evolu tion of oxygen under these conditions was confirmed by later experiments, and was studied more closely in conjunction with the other changes which occur. An experiment is recorded by Robert Hunt (“ Re searches on Light,” p. 81), in which absorption of gas was observed to take place when the chloride was darkened, leading him to the conclusion that the product contained oxygen; the conditions, however, were somewhat different, and this may account for the want of agreement between our results.* During the exposure of the chloride under water, it * It does not appear that in Hunt’s experiment a large volume of water was present.
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