Imagens das páginas
PDF

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, hardbake, &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.

Arc of Rotation/or mean Yellow Ray

for a length of 100 Millimetre*. Jf

Syrup, primitive +71 ^"^

"After twenty hours O

"After twenty-five hours —11 ^^

"After sixty-four hours O

"After seventy-two hours -f- 5

Here then it appears, that cane-sugar^^ gradually lost its

rotative power O°, and then became ^>^. In this latter state it was probably incrystallizable sugar. But this in its turn lost its rotative power 0°, and became^^ . The precise nature of

the latter kind of sugar is not known.

In sugar-refining the object is never to let the syrup get beyond tbe 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 righl-kanded 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.

[graphic]

In this property also circularly polarized light agrees with common or unpolarized light, but differs from rectilinearly polarized light.

3. Analyzed by a doubly refracting prism of Iceland spar, a ray of circularly polarized light gives constantly two equal images, in whatever plane the principal section of the prism be placed. For, as I have already stated, a ray of circularly polarized light is the resultant of two rays placed at right angles and differing in their phase by a quarter undulation; and, therefore, it must give equal images by the doubly refracting prism, in the same way that common or unpolarized light does, for the difference of phases has nothing to do with this character.

In this respect circularly polarized light agrees with common or unpolarized light; but is distinguished from rectilinearly (plane) polarized light, which in certain positions (before specified) yields one image only.

4. By two total internal reflections in the interior of glass, at an angle of about 54£°, circularly polarized light is converted into rectilinearly polarized light. Thus if light circularly polarized be incident on Fresnel's rhomb, it emerges rectilinearly polarized, and the position of the plane of polarization at emergence makes an angle of +45° or—45°, with the plane of reflection according as the incident light was right-handed or left-handed. This experiment may be readily understood from the explanation already given of the action of Fresnel's rhomb in converting rectilinearly polarized light into circularly polarized light (fig. 46, p. 88). In fact, the two experiments are the converse of each other; the light called incident in the one case, being termed emergent in the other, and vice versd.

In this character, circularly polarized light differs equally from both unpolarized and rectilinearly polarized lights. For by two reflections of this kind, common light suffers no obvious change; while rectilinearly polarized light, under the same circumstances, is converted into circularly polarized light, provided that the plane of reflection be at an azimuth of 45° to that of primitive polarization.

5. If a ray of circularly polarized light be transmitted through a thin film of a doubly refracting crystal, and the emergent light be analyzed by a doubly refracting prism, two rays of complementary colours are produced.

In this character, circularly polarized light is decidedly different to common or unpolarized light, which when submitted to the same examination presents no colour. Rectilinearly polarized light, however, agrees with the circular light in producing complementary tints; but they are not the same in the two cases; those produced by circular light differing from those of rectilinear light by an exact quarter of a tint, either in excess or defect, as the case may be.

[graphic]

To illustrate these facts, place a film of selenite, of uniform thickness, in the polariscope, and observe the tint which it yields by rectilinearly polarized light. Then interpose, between the polarizing plate and the selenite film, a circularly polarizing apparatus (as Airy's mica plate, or Fresnel's rhomb), and the tint seen by the analyzer immediately changes.

If a plate of calcareous spar, cut to show the circular rings and cross by rectilinearly polarized light, be placed in the polariscope, and circularly polarized light be used, we observe a system of rings and a cross (fig. 53), but which are very different to those seen by rectilinearly polarized light.

Fl° 53- The rings are divided into quadrants by the

cross, every other quadrant being similar, while the adjacent ones are dissimilar. The rings : appear to be abruptly and absolutely dislocated, \those in the two alternate quadrants being pushed outwards or from the centre, by J of an order, and those of the intermediate quadrants being as it were pulled inwards by £ of an fP7r'7So?uZ'dfyCcair.TMter- Instead of a black cross, we have a cuiariypoiarucd light.luminous one, the intensity of its light being uniform, and about equal to the mean intensity. If the plane of incidence pass through 135° and 315°, the phenomena of adjacent quadrants are exactly interchanged. But the most important difference produced by circularly polarized light, is, that no alteration is made by turning the analyzing plate round the incident ray.

If a plate of a biaxial crystal, as of nitre, be examined by circularly polarized light, we observe the double system of rings, but the black cross disappears. Every alternate semicircle of rings presents the appearance of dislocation.

The origin of the tints produced by circularly polarized light, have been so clearly and concisely explained by Sir John Herschel, that I cannot do better than use his words:

"When," says this eminent philosopher, " a ray propagated by circular vibrations is incident on a crystallized lamina, it may be regarded as composed of two; one polarized in the plane of the principal section, the other at right angles to it, of equal Intensity, and differing in phase by a quarter undulation. Each of these will be transmitted unaltered ; and, therefore, at their emergence, and subsequent analysis, will comport themselves in respect of their interferences, just as would do the two portions of a ray primitively polarized in azimuth 45°, and divided into two by the double refraction of the lamina; provided that a quarter undulation be added to the phase of one of these latter rays. Now, such rays will produce, by the interference of their doubly refracted positions, the ordinary and extraordinary tints due to the interval of retardation within the crystallized lamina. Hence, in the present case, the tints produced will be those due to that interval, plus or minus the quarter of an undulation added to, or subtracted from, the phase of one of the portions; and, consequently, will differ one-fourth of a tint in order from that which would arise from the use of a beam of ordinary polarized light, incident in azimuth 45° in the lamina."

[graphic][graphic]

t 6. If a ray of circularly-polarized light be transmitted through a column of syrup or oil of turpentine, lemon, &c., and then analyzed, either by a Nichol's prism, or a doubly-refracting prism, no colour is produced. For the circular wave is propagated along the liquid without suffering subdivision, and, therefore, at its emergence, no colour can be produced by the analyzer.

In this character circularly-polarized light agrees with common or unpolarized light; but differs from plane polarized light.

7. " Circularly-polarized light," says Fresnel, "differs from plane polarized light in not sensibly developing colours in plates of quartz perpendicular to the axis." According to the wave hypothesis this ought to be the case; for " a ray propagated by circular vibrations, when incident- on rock crystal in the direction of the axis, will (by hypothesis) be propagated along it by that elasticity which is due to the direction of its rotation, the wave then will enter the crystal without further subdivision, and there will be no difference of paths or interfering rays at its emergence; and, of course, no colours produced on analyzing by double refraction."

I confess, however, I have not been able precisely to verify this statement, though, I doubt not, my failure has arisen from some defect in the apparatus used to produce circular polarization. 1 have always found a very feeble tint of colour in the axis. As Mr. Airy has very accurately described the phenomena which I myself have repeatedly seen, I prefer quoting his words:

Fio. 54. "If circularly-polarized light pass through

the quartz, on applying the analyzing plate, in,-<-' .''*•-'''lT}K stead of rings, there are seen two spirals mu

tually enwrapping each other [as in fig 54.]. If the [Fresnel's] rhomb be placed in position 135", the figure is turned through a quadrant If the quartz be left-handed, the spirals are turned in the opposite direction. The central tint appears to be white. With the rhomb which I have commonly used (which is of plateglass, but with the angles given by Fresnel for crown-glass), there is at the centre an extremely dilute tint of pink : I think it likely that this Spirals of Quarts, arises from the error in the angles, as the in

produced by circularly- tensity of the colour bears no proportion to that polarised light. in other parts of the spiral."

If a plate of right-handed quartz be superposed on a plate of left-handed quartz of equal thickness, and examined by circularly-polarized light, the left-handed slice being nearer to the

[graphic]

r

« AnteriorContinuar »