The Optical Magic Lantern Journal (November 1893)

analysers would be missing at the close of the performance. Mr. Anderton has done much to reduce outlay, by bringing the analyser down but to two mounted bundles of good glass, yet he, probably, would be glad to see some addi! tional reduction in this item of expense. A somewhat troublesome way out of the difficulty might be to charge individuals for the analyser as they enter the place of entertainment, and to return them the money as they come out, and give it back. First-class polariscopic apparatus has had a tendency to become dearer of late years, because of the Iceland spar famine. The world practically had to depend upon but two or three mines in Iceland for the supply of the crystals. A landslip covered up one of them, and the others are now stated to be exhausted, and partly filled with water. Better methods of cutting avd building up Iceland spar prisms have been devised by Mr. Ahrens and others, and this has aided the economical use of what little Iceland spar is now to be had. So great is the spar famine that some men of science have looked with greedy eyes at the gigantic pair of Iceland spar prisms presented years ago by Mr. William Spottiswoode to the Royal Institution ; they long to cut them up for optical instruments innumerable. Photographic Work recently suggested that attempts should be made to produce large crystals of what have been called artificial tourmalines, but which consist of a salt which the late Mr. William Herapath, who discovered its optical properties, called sulphate of iodoquinice. Mr. Herapath was a noted analytical chemist, and he knew it, which perhaps may account for his also putting ‘‘ Herapathite,’’ as a name of the salt, in his first detailed publication of the method of making it in large crytals, as published in The Philosophical Magazine of | November, 1853. The name of ‘‘ Herapathite ” originated with Professor Haidinger. Mr. Herapath stated therein that two plates scarcely thicker than gold-leaf may be rendered ‘“‘totally”’ impervious to light when crossed at right angles, and of a specimen pair sent to the editor of the Magazine, he said that one was six-tenths of an inch long and three-tenths of an inch broad; the other was of the same length as the first, but one-tenth of an inch broader. He remarked that they were as good for polarisers as tourmalives, for which four guineas each plate would be charged. Mr. Herapath’s method of manufacture, as described by himself, was to dissolve pure disulphate of quinine in pyroligneous acid of specific gravity 1-042, diluted with an equal | 167 quantity of proof spirit made by adding rectified spirit of wine, sp. gr. 0°837, to an equal bulk of distilled water. He next dissolved 40 grains of iodine in a fluid ounce of rectified spirit of wine, and then, after the preceding details, gave the following formula :— Disulphate of quinine... 50 grains. Pyroligneous acid 2 fluid ounces. Proof spirit 2 fluid ounces. Spirituous solution of iodine . 00 drops. He dissolved the quinine salt in the pyroligneous acid mixed with spirit, warmed the solution to 130° F., and immediately added the solution of iodine; drop by drop, with occasional agitation of the mixture. He said that this formula gives to the matter liquid after crystallisation at 52° F., a specific gravity of 0-986, which appears highly favourable to the deposition of the majority of the crystalline production, and yet allows only the very broad and thinner plates to float, thus getting them perfectly free from all interfering and adhering plates. The temperature must be maintained a little after the addition of the iodine, and the solution then set aside to crystallise under certain delicate conditions. The room should have an equable temperature of about 45° or 50° F., or the currents produced by changes of temperature will destroy the parallelism of the crystals; a rise of temperature to 60° F., dissolves the thinner plates. The liquid must be free from vibration during the whole act of crystallisation, the room must be free even from ordinary vibrations. He preferred the method of Mr, Thwaites, who suspended the flask by the neck With strong twine to a string stretching across the apartment from one wall to the other. It is necessary that the surface of the liquid should ' not be exposed to too rapid evaporation, as its temperature would fall too quickly. The advantage of having a broad surface in proportion to the depth of the liquid is great, as the thinner and more easily-reached plates form on the surface. If after six hours the required broad plates do not make their appearance, it is but necessary to apply a spirit flame to the bottom of the flask to dissolve the deposited crystals, then add a little spirit and a few more drops of iodine, and again wait for crystallisation. When a crop of broad floating plates is formed, they should be allowed to remain in the liquid to from twelve or twenty-four hours to attain a proper degree of thickness, but they must not be allowed to remain in the liquid too long. iThe string suspending the flask is then taken by the fingers and cut at its upper end with