Dr. Gregory on a volatile Liquid procured from Caoutchouc by Destructive Distillation; with Remarks on some other d Mr. J. T. Graves's Explanation of a remarkable Paradox în the Calculus of Functions, noticed by Mr. Babbage Mr. J. Bishop's Experimental Researches into the Physiology of the Human Voice (concluded) Mr. W. C. Williamson on the Limestones found in the Vicinity Mr. J. Saxton on his Magneto-electrical Machine; with Re- marks on Mr. E. M. Clarke's Paper in the preceding Number 360 Mr. W. Hopkins's Reply to Dr. Boase's "Remarks on Mr. Hopkins's Researches in Physical Geology'," in the Num Ehrenberg's Fossil Infusoria.... Mr. W. R. Birt's Meteorological Observations made during the Mr. H. F. Ta'bot's Facts relating to Optical Science. No. IV. 401 Mr. L. Thompson on a new Method of preparing Iodous Acid 442 Mr. J. T. Graves's Explanation of a remarkable Paradox in the Dr. J. Inglis on the Conducting Power of lodine for Electricity 450 Rev. J. W. MacGauley's Account and Explanation of some Mr N. T. Wetherell's Observations on some of the Fossils of the London Clay, and in particular those Organic Remains SUPPLEMENTARY NUMBER. I. A Portrait of the late FRANCIS XAVIER BARON VON ZACH, Director of II. A Plate illustrative of Dr. FALCONER and Capt. CAUTLEY'S Paper on III. A Plate illustrative of Mr. J. BISHOP'S Experimental Researches into the Physiology of the Human Voice. IV. A Plate illustrative of Mr. O. WARD'S Physiological Remarks on the THE LONDON AND EDINBURGH PHILOSOPHICAL MAGAZINE AND JOURNAL OF SCIENCE. [THIRD SERIES.] JULY 1836. I. Facts relating to Optical Science. No. III. By H. F. Talbot, Esq., F.R.S.* Optical Properties of the Iodide of Mercury. CHEMICAL writers have observed that this substance exhibits remarkable changes of colour. It is orange-red when cold, but becomes yellow when moderately heated. As it grows cold again, the red tint reappears. Sometimes, however, the yellow exhibits more permanency, as has been remarked by Dr. Inglis in his Essay on Iodinet. He says "that the yellow crystals of the biniodide retain that colour for a considerable time, unless suddenly cooled or agitated, when the characteristic crimson tint of the biniodide again appears." Wishing to examine into the cause of these facts, I placed a small portion of the red iodide between two plates of glass, and warmed it over a spirit-lamp. It immediately sublimed into a yellow powder composed of minute crystals. As it cooled, blood-red spots appeared upon the surface of the yellow mass and gradually spread themselves over the whole, with the exception of some portions around the circumference which usually remained yellow. When examined with a microscope this powder presented the curious appearance of Communicated by the Author. Nos. I. and II. of these " Facts" will be found in Lond. and Edinb. Phil. Mag., vol. iv. pp. 112, 289. † See the Number for January last, (vol. viii.) p. 18. Third Series. Vol. 9. No. 51. July 1836. B orange-red crystals lying interspersed among yellow ones, which resembled them in size and shape, and were different in no respect but in colour. Upon repeating this experiment I found that by continuing the heat a little longer I could obtain much larger crystals, such that the field of view of the microscope would only contain a few of them at a time; and with these it became possible to see the phænomenon much more distinctly and advantageously. These large crystals have the shape of thin flat lozenges or oblique-angled parallelograms of a pale yellow colour. They are very transparent, act strongly upon light, and form a very pleasing and convenient object for the polarizing microscope. But the most important and singular phænomenon which they exhibit, is the sudden change of colour which they are capable of undergoing, and to which I do not think the science of optics has hitherto furnished any parallel. The change takes place in some of the crystals during the process of cooling, in others shortly afterwards, while in others the yellow tint remains permanent for many hours, or even days. In general, a crystal which is about to change colour, is known by the appearance of a red streak along one of its sides or edges. If then the observer selects one of these and fixes his attention upon it, he will shortly afterwards see it change colour from pale yellow to a fine and deep orange-red. The change generally occupies only a few seconds, and the red tint advances uniformly across the crystal, i. e. the boundary of the red and yellow is a straight line parallel to two sides of the rhomboid, and its motion is across the crystal from one of these sides to the opposite one. The change of colour is accompanied by a visible internal motion in the crystal, like a sinking or giving way of successive ranks of particles, one consequence of which is, that the crystal after the change is generally less transparent than it was before. This phænomenon is, I think, the most evident proof which we yet possess of the dependency of colour upon internal molecular arrangement. As this substance sublimes very readily, I tried what might be the effect of placing a hot piece of glass over the crystals while they were under examination with the microscope. Immediately each crystal became surrounded with a cloud of little particles which appeared to me to be rhomboids like the larger ones from which they were derived. The cracking of the larger crystals from heat must be an effect of unequal expansion, and it appears not unlikely from this experiment that the sublimation of the substance is pro duced by this cause acting more energetically at a high temperature. On Prismatic Spectra. It is much to be desired that an extensive course of experiments should be made on the spectra of chemical flames, accompanied with accurate measurements of the relative position of the bright and dark lines, or maxima and minima of light which are generally seen in them. The definite rays emitted by certain substances, as, for example, the yellow rays of the salts of soda, possess a fixed and invariable character, which is analogous in some measure to the fixed proportion in which all bodies combine, according to the atomic theory. It may be expected, therefore, that optical researches, carefully conducted, may throw some additional light upon chemistry. Some experiments which I formerly made upon this subject will be found in Brewster's Journal for 1826*. In addition to the substances there enumerated as giving a peculiar optical character to flame, I have found that the salts of copper are exceedingly remarkable. They give spectra so covered throughout with dark lines as to resemble in that respect the solar spectrum. The flames of boracic acid and nitrate of barytes also possess somewhat of a similar character. The most convenient way of obtaining brilliant spectra of these substances is to deflagrate them with chlorate of potash, but this is attended with the inconvenience that the spectrum produced by the chlorate is seen in conjunction with the other, and an allowance must be made accordingly. Another good method is to sprinkle the substance in powder on the wick of a spirit-lamp, and direct a current of oxygen upon it. With regard to the accurate measurement of the lines, it requires the use of very superior apparatus. I have sometimes made approximate measurements by fixing a divided scale transversely to the linear aperture through which the light of the burning body was observed. This aperture was then expanded by the prism into a spectrum parallel to the scale, by means of which it could then be measured. An objection may, perhaps, occur to the reader, that the scale would thus be as much refracted as the light itself, and therefore could not serve to measure it. But this difficulty was avoided by a simple contrivance, viz. by illuminating the scale with homogeneous light. Spectra of various Galvanic Flames. Silver leaf deflagrated by galvanism gave a spectrum with * Vol. v. p. 77. See also the present Journal, vol. iv. p. 114. |