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 0°, 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 lævogyrate, 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 umbelliferæ, 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. H 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. colours most brilliantly. When this kind of sugar is subjected to heat, especially in contact with acids, it loses its crystallizability, and then acquires left-handed polarization. In the manufacture of barley-sugar, hard bake, &c., the makers of these kinds of hard confectionary use a little cream of tartar to destroy the crystallizability of sugar. Soubeiran found that a syrup of cane-sugar heated by a salt-water bath, the air being excluded, underwent a series of remarkable changes in respect of its rotative power. In sugar-refining the object is never to let the syrup get beyond the first zero; that is, not to convert crystallizable unto uncrystallizable sugar. Raw sugar contains, however, both crystallizable and uncrystallizable sugar: the latter alone should constitute treacle. But, from Soubeiran's optical examination, it appears that treacle contains a portion of crystallizable sugar. The optical characters of sugar have been made use of to detect fraud in Pharmacy. In 1842, more than a ton of a substance purporting to be manna was offered for sale in Paris at less than fivepence per pound, the excuse given for the unusually low price was, that cash was immediately required. Suspicion was raised, and the substance was submitted to careful examination, the result of which was the establishment of the fact, that it was not manna, but potato-sugar. Its aspect, taste, fermentibility (mannite not being fermentible), and the presence of sulphate of lime proved this. Biot submitted it to a very careful optical examination, and found its characters to be those of a starch-sugar. Manna contains two kinds of saccharine matter, one called mannite, and the other a fermentable sugar. Now mannite, when pure, has no rotative power on polarized light, but commercial manna has a slight effect, owing to the presence of a small quantity of fermentable sugar. This fictitious substance, however, had the same effect on plane polarized light, as sugar prepared by the action of acids on starch, when the action is arrested at the first phase of its transformation. Vinous fermentation has been studied by the aid of polarized light. Take a solution of cane-sugar which has right-handed circular polarization. As soon as it begins to ferment it loses this property, but acquires left-handed polarization. Polarized light has been prepared and used as a test of the presence of sugar in urine. To render diabetic urine available for this purpose, it must be decolorized by agitation with fresh prepared granulated animal charcoal, and subsequent filtration. The process is troublesome, tedious, and can only prove successful in the hands of persons familiar with the phenomena of polarized light. With all due deference to Biot, I do not think it will ever prove of much value in practical medicine. We have other, simpler, less laborious, and cheaper methods of detecting sugar in urine than the one now referred to. Moreover, it should be remembered, that albuminous urine possesses the property of circular polarization. The substance called dextrine is starch-gum, and is soluble in water. It is usually prepared from potato-starch, either by torrefaction or by the action of a small quantity of nitric acid. A solution of it possesses the property of right-handed circular polarization, hence the name dextrine applied to it by Payen and Persoz. Properties of circularly polarized Light.-Common, rectilinearly polarized, and circularly polarized, lights are undistinguishable by the eye. All three may be coloured or white. The properties which distinguish the latter from the two former are as follows: 1. A ray of circularly polarized light is capable of reflection by a reflecting plane, as of glass, in every azimuth of the plane of reflection. For the circular vibrations of the ethereal molecules may be resolved into two equal rectilinear vibrations, one parallel, the other perpendicular, to any arbitrary plane. By this property, therefore, circularly polarized light differs from rectilinearly polarized light, but agrees with unpolarized light. 2. A ray of circularly polarized light is capable of transmission through a plate of tourmaline (cut parallel to the axis of the crystal), in every azimuth of the axis of the crystal. For in this case also, the circular vibrations of the incident ray may be resolved into two equal rectilinearly vibrations, one parallel, the other perpendicular, to any azimuth. One of these vibrations is transmitted by the tourmaline, the other is suppressed. |