or in their mode of arrangement. "The crystal," says Fresnel, "cannot be constituted from right to left as it is from left to right, either in virtue of the arrangement of its particles, or of their individual constitution." If it be a molecular property, it must be acquired in the act of crystallization, by the mutual action of the molecules on each other, for other forms of silica, as well as melted quartz, are devoid of it. An helicoidal arrangement (right or left-handed, as the case may be) of the molecules furnishes a physical explanation of the fact above referred to. It has been objected to this hypothesis, that it is not applicable to the case of circularly polarizing liquids. But as the circular polarization of quartz is dependent on direction, while that of liquids is independent of it, it is tolerably clear that the cause must be different in the two cases. In the first, it may depend on the arrangement of the molecules; in the second, on some peculiarity in the molecules themselves. If two plates of quartz, cut obliquely to the principal axis of the crystal (fig. 49, bb, bb), be superposed crosswise and examined in the polariscope, they present a series of parallel coloured bands or stripes, with a central black or white stripe. When the tourmaline plates are crossed, the central stripe is black, when they coincide, it is white. The lateral coloured stripes seen in the one case, are complementary to those seen in the other. Amethyst is a mixture of right and left-handed quartz, and will be hereafter noticed among tesselated crystals. 5. Circular Polarization by Fluids. Some liquids possess the remarkable property of circularly polarizing light. The following are the most important: Volatile oils (those of mustard and bitter almonds excepted). Naphtha. Aqueous solutions of several kinds of sugar, dextrine, tartaric acid, and tartrates (tartrate of alumina excepted). Diabetic urine. Albuminous urine. Alcoholic solutions of camphor and artificial camphor. Most vegetable juices. Biot found that vaporization did not destroy the circular polarization of oil of turpentine. The following liquids have been found devoid of this property: Water. Pyroxilic spirit. Pyroacetic spirit. Glycerin. The apparatus necessary for observing this property of Huds consists essentially of three parts; viz., a polarizer, a tube to contain the fluid, and an analyzer. The polarizer is an unsilvered glass mirror, a bundle of pasar alled glass plates, or a Nichol's prism. Both Biot and Proteased Powell use the first, while Ventzke employs the last. A pland glass, blackened at the posterior surface, answers very well. Sometimes a second mirror (of silvered glass) is used to the light on the polarizing plate. The tube, to hold the liquid, should be from six to twenty-four inches long. In general, it is to be filled with the flu examination, and to be closed at each end by a that gloss p wk. Professor Powell, however, employs a fills West-tiek, kjal at the top, and having the usual nem spretical wwwn. To Set cases it is desirable to have two or three perforated asparagus of sheet silver or platinum, placed at intervals in the tube, No exclude the light reflected from the sides of the tube, but to admit those rays which traverse the axis of the tube. The analyzer should be either au achromatic, doubly refracting prism, or a Nichol's prism. Biot uses a doubly refracting prism of cale spar, made of a rhombohedron of this substance, rendered achromatic by replacing a portion of the crystal by a glass prism. Achromatic quartz prisms are objectionable, since they are never so accurately prepared as to yield two images only, but always four; of which two, however, are very faint. Professor Powell employs a rhombohedron of cale spar, in its natural state, as his analyzer, and a lens to magnify the separation of the images. Ventzke uses a Nichol's prism as the ana lyzer. The amount of rotation which a ray of light suffers during its passage through the liquid, is measured by an index attached to the analyzer, and moving on a graduated circular metallic plate. Before the tube containing the liquid is introduced, we must fix the zero, or Oo. If a doubly refracting prism be the analyzer, the index is made to point to zero, or 09, when the ordinary image alone is seen. If, however, a Nichol's prism be used as ana H lyzer, the index is arranged to point to O° when the light is excluded; or, in other words, when the light, transmitted by the polarizer, is extinguished by the analyzer. Homogeneous light is generally employed when we wish to measure the arc of rotation affected by a liquid on a luminous ray. Red light is usually selected, because this is the only homogeneous light which can be isolated by coloured glass. We, therefore, place a plate of red glass between the eye and the analyzer. If, when the index points to zero 0°, the tube containing a circularly polarizing liquid be introduced, the second or extraordinary image immediately becomes evident, if the doubly refracting prism be used as analyzer. By turning the latter round to the right or to the left, as the case may be, this second image disappears (when homogeneous light is used), and the arc traversed by the index from the zero 0°, measures the angle of deviation of the ray. If, however, a Nichol's prism be employed, it no longer excludes the light when the index stands at Ò°, but requires to be rotated a certain number of degrees to do so, and the arc of rotation is here a measure of the rotative power of the liquid. The explanation of the action of these liquids on the incident rectilinearly polarized light is similar to that already given for the axis of quartz; with the exception, that in the case of quartz, the circular double refraction may depend on the arrangement of the molecules, whereas in liquids it must arise from some property of the molecules themselves. I proceed now to examine some of the liquids which possess the property of circular polarization : First, with regard to the essential or volatile oils. Most of these bodies are circular polarizers: indeed, I know but two exceptions to this statement, viz. oil of mustard and oil of bitter almonds. Some turn the planes of polarization to the right, others to the left, but the intensity of their rotative power varies considerably, as the following table shows: This table is a very instructive one. It shows that isomerism has no connection with circular polarization, for of three isomeric oils (turpentine, lemon, and bergamot) mentioned in this table, one is laevogyrate, the others dextrogyrate. We see also, that oils derived from plants of the same natural family (as the oils of anise, fennel, and caraway from the umbellifere, and those of mint, lavender, and rosemary from the labiata) differ in respect of their circular polarization. In some cases, perhaps, this fact might be available to the Pharmaceutical Chemist in detecting mixtures of one oil with another, as the adulteration of oil of peppermint with oil of rosemary, recently mentioned by Mr. Herring. (See Pharmaceutical Journal, vol. i., p. 263). Some kinds of sugar, when dissolved in water, yield solutions which have in a greater or less degree the property of rotating the planes of polarization, some to the right, others to the left. Hence polarized light may be sometimes used as a test of the presence of sugar, and the degree of rotation becomes an indication of the quantity and even quality of the sugar present. Biot examined by this test a specimen of sugar-cane juice, and found that it indicated the presence of 20 or 21 per cent. of sugar. Peligot subsequently analyzed it, and found 20.9 per cent. of sugar. Biot, therefore, suggests that those who make, as well as those who refine sugar, might resort to this test as a means of determining the amount of sugar in different juices or solutions. To the colonist it would prove useful by pointing out the saccharine strength of the juice at the mill, and to the sugar refiner it would be valuable by enabling him to determine the absolute strength of raw sugar. The sugars are prepared for examination by dissolving them in water, and decolorizing the solutions when necessary, by filtering them through purified granulated animal charcoal. Several sweet or saccharine substances do not indicate any circular polarization, and of those that do, some indicate righthanded, others left-handed circular polarization. I 2 The grape sugar referred to in this table exists ready formed in honey, and in diabetic urine. It is deposited when the acidulous juices of fruits have been saturated and sufficiently concentrated. Moreover, it is produced by the action of diastase on starch, as well as when syrup, obtained by the action of weak acids on starch or sugar, is abandoned to itself. The following table, taken from a memoir, by Biot, shows the extent of rotatory power possessed by different sugars: ROTATION OBSERVED WITH RED GLASS THROUGH A SOLUTION OF Cane sugar, dissolved in water, causes right-handed polariA strong syrup made with refined sugar shows the zation. |