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The photographic news
- Bandzählung
- 24.1880
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- 1880
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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. 1163, December 17, 1880
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The photographic news
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Band 24.1880
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- Register Index 631
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Band 24.1880
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DECEMBER 17, 1880.1 THE PHOTOGRAPHIC NEWS. one; it differs slightly from the figure to be seen in most text-books on organic chemistry. Now the great difference between benzene derivatives and other forms of carbon compounds is, that the spectra of the former are remarkably short. 2. Being liquid or solid bodies, when they are dissolved in, and enormously diluted with, some transparent fluid—such as water or alcohol— they show the presence of absorption bands, that is to say, intervals or bars, which intercept the chemical rays. These bands are caused by the peculiar grouping or link ing of the carbon atoms. It follows from this that if the chemical constitution of a substance be unknown, we may readily ascertain by photographing its absorption spectrum whether it contain the benzene nuclus as the basis of its structure, because, if so, characteristic bands will make their appearance. More than this, from the character of its transmitted spectrum being similar to that of some body of a known constitution, we may ascertain whether it is an allied substance or not. This is a matter of the greatest possible importance, for if we know the constitu tion of a body, we have half conquered the difficulty of forming the substance artificially. The most transparent of substances containing the benzine nucleus—namely, benzene itself—withstands a dilution with 2,400 parts by weight of alcohol, without the bands of absorption dis appearing. These, however, are not all the peculiarities of this class of compounds. It is well known that they form numerous isomerides—or compounds—identical in chemical composition and constitution, but with a difference in molecular structure, which difference is marked by their physical properties being different. Amongst such com pounds we have three nitro-phenols. Now a phenol is a benzene molecule, in which hydroxyl replaces an atom of hydrogen ; and a nitro-phenol is one in which two atoms of hydrogen are replaced ; the first by hydroxyl, the second by an atom of nitrogen, linked to two atoms of oxygen, and called nitroxyl. The difference between the first, second, and third nitro-phenols is supposed to be due to the relative positions of the two groups of atoms OH or thydroxyl, and NO, or nitroxyl. In representing these three substances by the formula made use of before, we obtain the following figures : — Without entering into forther particulars, it may be stated that isomeric substances yield very different spectra. In conducting these researches, a method of showing the nature and the extent of absorption of substances was employed which had not been used by previous observers. It consisted in knowing the exact thickness of the layer of liquid examined, and the exact proportions of the trans parent liquid used as a dilutent ; diagrams were then drawn from the photographs taken, which show the position of the absorbed rays in solutions of various strengths. Thus, in the diagrams of benzene, we can see the effect produced by a layer of liquid fifteen millimetres in thickness, or less than three-quarters of an inch, in any stage of dilution from one part of benzene in fifty of alcohol to one part in 2,400. In other cases the range of dilution extended to as much as one part of substance in five or even ten millions of the solvent. Professor Huntington and myself examined a large number of essential oils, and found that they rarely exhi bit absorbtion bands, which shows that for the most part they are not benzene derivatives. These oils, which trans mitted spectra showing bands of absortion, were known from the previous researches of other chemists to contain benzene derivatives. We were led to this part of the research on account of the great extent of the chemical and physical research which has been devoted to this class of bodies, partly because of the doubt which existed as to their precise constitution, and partly because many of the oils are articles of great commercial value, which makes them subject to adultera tion. There is one substance, cymene, a benzene derivative, which is readily produced by chemical reaction from essen tial oils, that is to say, the oils themselves which are classed under the name of terpenes are easily converted into cymene. Now, cymene exhibits a well-defined and easily- recognised absorption band, and it was found that many essential oils contain cymene as a constituent. More than this, by noting the intensity of this band the proportion of cymene present was ascertained. In conclusion, I may add that spectrum photography can be employed as a means of identifying organic sub stances, and as a most delicate test of their purity ; hence the method of research here shortly described is capable of very extensive application in chemical analysis. Some of the advantages derived from the employment of photography in the study of spectra are the following :—. The measurements are free from any personal error ; the photographs are permanent and unmistakable records easily referred to; and the observations are made more rapidly and with much less fatigue to the eye than by any other means. When the apparatus is once properly ad justed, but little skill is required in making observations. By lengthening the exposure of the photographic plate, rays too feeble to be seen either on a fluorescent screen or by means of a fluorescent eyepiece are easily reproduced. 1 believe that photography of the ultra-violet rays is the only means by which spectra can be properly studied, since the most characteristic lines of metallic spectra lie beyond the region of the luminous rays. There is one conclusion of special interest to photogra phers which I have arrived at. I believe it to be perfectly possible to obtain a medium which will transmit white light, but intercept all chemically active rays. Liquids are known which almost fulfil these conditions. Perhaps at a future date another communication on the subject may be forthcoming. Original Papers to which Peference is made in the Foregoing Article. Miller.—“On the Photographic Transparency of Various Bodies,” &c.—Philosophical Transactions of the Regal Society, 1863, Part I. Stokes. —“ On the Long Spectrum of Electric Light.”—Ibid. Soret.—“ Recherches sur 1’Absorption des Rayons Ultra-violets par Diverses Subtances.”—Archives des Sciences Physiques et Naturelles, Geneva, January, 1878. Hartley and Huntington.—“ On fhe Action of Organic Sub stances on the Ultra-Violet Rays.’ ’ Parts I. and II.—Philosophi cal Transactions, 1879, Part I. Hartley and Huntington.—“On the Action of Organic Sub stances on the Ultra-violet Rays.” Part II. Essential Oils.— Proceedings of the Royal Society, 1880. Hartley.—“An Examination of Terpenes for Cymene by means of the Ultra-violet Spectrum.”—Journal of the Chemical Society, 1880;
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