Elements of chemistry : including the most recent discoveries and applications of the science to medicine and pharmacy, and to the arts / by Robert Kane.

  • Kane, Robert, 1809-1890.
Date:
1842
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Licence: Public Domain Mark

Credit: Elements of chemistry : including the most recent discoveries and applications of the science to medicine and pharmacy, and to the arts / by Robert Kane. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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    farther, that in the case of quartz the cross is not produced, the central space being occupied in succession by all the prismatic colours. Even in quartz there have been found two modifications of this property; with one, the analyzing plate must be turned from right to left, to obtain the spectral colours from red to violet; but with the other, the rotation must be in the opposite direction, to show them in the same order. The molecules of the quartz cause these colours to appear along the axis by turning the plane of polarization of each colour round in a different degree, and thus opening out into a fan shape those combined lights, which had previously affected the eye only as white or black. This faculty does not depend upon the manner of arrangement of the particles of the quartz ; it involves the chemical na- ture of the molecules; and, although some observations appeared to connect it with the crystalline structure, it is now fully established to be independent of it. In fact, this property of circular polarization, as it is termed, belongs to certain bodies, independent of their arrangement, and even in many cases accompanies them when they enter into combination. It is even found in liquids, particularly the volatile oils; and when oil of turpentine is converted into vapour, its molecules preserve unaffected their rotative power. Its existence is, however, subjected to remarka- ble anomalies ; thus, when oil of turpentine combines with muriatic acid and forms artificial camphor, it retains its power of rotation ; but when the artificial camphor is decomposed and the oil of turpentine got back again, its power of changing the plane of polarization of the ray of light has totally disappeared. [These phenomena of circular polarization may be readily traced. If from a crystal of quartz a disk is cut transversely, a system of rings will be seen enclosing a circular coloured space. If the disk be turned round, no change takes place ; but if the analyzing plate turns, the colour passes through a series of tints, which, after 100° of rotation, may end in a sombre violet. If, now, we cut from the same crys- tal another disk twice the thickness of the former, and make use of it, we shall find the tint different from what it last was; but, by turning the analyzing plate 100°, we may bring it back again to the same sombre violet: with a plate three times as thick, we should have to turn 100° still farther to produce the same tint, and for each additional thickness an additional 100°. We therefore infer that, when polar- ized light passes along the axis of a crystal of quartz, its planes of polarization ro- tate circularly, or, rather, spirally, in the crystal; and this takes place in some spe- cimens from right to left, and in others from left to right. Under these circum- stances, light is said to undergo circular polarization.] In cases, therefore, where bodies exhibit this action upon light, their power of rotation becomes an important numerical fact in their descriptions, and it may be measured by the angle through which a certain thickness of the body is capable of moving the plane of polarization of a ray of homogeneous light, such as the pure red given by glass coloured by sub-oxide of copper. It may also be expressed, when white light is used, by the angle at which the pure violet is produced, and the direction of rotation is expressed by an arrow turned either to the right or left, ac- cording as it is necessary to make the analyzing crystal revolve to the one or the other side. Thus, the rotative power of oil of turpentine, contained in a tube six inches long, is for red light 45° < <sg, and of oil of lemons, in the same length, 84° 3»» >. The rotative power of quartz is about 68 5 times greater than that of oil of turpentine. This property is beautifully applied to trace the changes which occur during the saccharine fermentation: a solution of starch possesses a high 3»—> power; but it gradually changes into the sugar of grapes, the rotative power of which is < ■«£■ Hence the action of the starch, when fermentation has com- menced, rapidly diminishes, untd there is so much sugar formed that the -ga—>- and -«£ exactly balance, and the solution is totally without action upon a polarized ray ; after that, the quantity of sugar still increasing, the rotation becomes ■ and increases until all the starch has been decomposed. With such a solution, knowing the total quantity of starch originally dissolved, the measure of its rotative power enables the quantity of sugar present to be at once calculated. The juices of plants which contain sugar, as the beet-root, the maple, the sugar-cane, may be ex- actly valued by a simple determination of their rotative power compared with their specific gravities. This property of the circular polarization of a ray of light, which at the first aspect might appear so far removed from proper chemical inquiry or useful application, becomes thus an instrument from which the distiller or sugar- boiler may every day derive advantage ; and when we come to discuss the means by which we endeavour to learn the internal constitution of bodies produced by chemical affinity, we shall find that the light which ordinary polarization throws upon F