MINERALOGY 375 cube, the planes of double refraction will be, as in fig. 245, a plane passing through the two diagonals of each face of the cube. The tints vary as the square of the distance from the nearest plane of double refraction. Pleo- Pleochroism. Closely connected with double refraction chroism. is that property of transparent minerals named pleochroism (of many colours), in consequence of which they exhibit dis tinct colours when viewed by transmitted light in different directions. Crystals of the cubic system do not show this property, whilst in those of the other systems it appears in more or less perfection, in tetragonal and hexagonal minerals as dichroism (two colours), in the rhombic and clinic systems as trichroism (three colours). In most cases these changes of colour are not very decided, and appear rather as different tints or shades than as distinct colours. The most remarkable of dichromatic minerals are the magnesian mica from Vesuvius, the tour maline, and ripidolite ; of trichromatic, iolite, andalusite from Brazil, diaspore from Schemnitz, and axinite. In a specimen of yellow Iceland spar the extraordinary image is of an orange -yellow colour, while the ordinary image is yellowish white. Along the axis of double refraction the colour of the two pencils is exactly the same, and the difference of colour increases with the inclination of the refracted ray to the axis. This is the invariable law of the phenomena in uniaxal crystals. Sir John Herschel found several tourmalines to have a blood-red colour along the axis, and at right angles to it to be yellow-green. There can be little doubt that this property will be found in every crystal of sufficient refraction. Even if the crystal is colourless, a slight inequality in the intensity of the two images may be observed; and when it is distinctly coloured the difference of intensity is very easily seen, even when the two colours are not of a different kind. The phenomena of dichroism are best seen in crystals with two axes of double refraction, and are well exemplified in iolite, a mineral which crystallizes in six- or twelve-sided prisms. These prisms are of a deep blue colour when seen along the axis, and of a yellowish brown colour when viewed in a direction perpendicular to it. If abed (fig. 246) is a section of the prism of iolite in a plane parallel to the axis of the prism, the transmitted light will be blue through the faces ab and dc, and yellowish brown through ad, be, and in every direction perpcndicu- lar to the axis of the prism. If we grind down the angles a, c, b, d, so as to replace them with " faces mn, m n and op, o p , inclined 31 41 to ad, or to the axis of the prism, then, if the plane abed passes through the optic axes, we shall observe, by transmitting polarized light through the crystal in the directions ac, bd, and subse quently analysing it, a system of rings round each of these axes. The system will exhibit the individual rings very plainly if the crystal is thin ; but if it is thick, we shall observe, when the plane abed is perpendicular to the plane of primitive polarization, some branches of blue and white light diverging in the form of a cross from the centre of the system of rings, or the poles of no polarization, as shown at p and p (fig. 247), where the shaded branches represent the blue ones. The summits of the blue masses are tipped with purple, and are separated by whitish light in some specimens and yellow ish light in others. The white light becomes more blue from p and p to o, where it is quite blue, and more yellow from p and p to c and d, where it is completely yellow. When the plane abed is in the plane of primitive polarization, the poles p, p are marked by spots of white light, but everywhere else the light is a deep blue. In the plane cadb (fig. 247) the mineral, when we look through it by common light, exhibits no other colour but yellow, mixed with a small quantity of blue, polarized in an opposite plane. The ordinary image at cand dis yellowish brown, and the extraordinary image faint blue, the former receiving some blue rays and the latter some yellow ones from cand dto a and b, where the difference of colour is still well-marked. The yellow image becomes fainter from a and b to p and p t till it changes into blue, and the faint blue Fig. 246. image is strengthened by other blue rays, till the intensity of the two blue images is nearly equal. As the incident ray advances from c and d top and_p , the faint blue image becomes more intense, and the yellow one, receiving an accession of blue rays, becomes of a bluish white colour. The ordinary image is whitish from p and p to o, and the extraordinary is deep blue ; but the whiteness gradually diminishes towards o, when they are both almost equally blue. The principal axis of double refraction in iolite is negative. The most refracted image is purplish blue, and the least refracted one yellowish brown. Brewster found that the dichroism of several crystals is changed by heat, and that in some cases this property may be communi cated to them. Babinet found that all negative crystals, such as calcareous spar, corundum (including ruby and sapphire), tour maline, and emerald, absorb in a greater degree the ordinary ray, with the exception of beryl, apatite, and some apophyllites ; while positive crystals, such as zircon, smoky quartz, sulphate of lime, and common apophyllite, absorb in a greater degree the extraordi nary ray. Babinet found also that certain crystals, such as red tourmaline and ruby, transmit rays of their peculiar colour without being polarized, in which cases the black cross of their system of rings is coloured, and this unpolarized light exists both in the ordinary and extraordinary ray. Haidinger devised an instrument for showing and for testing the pleochroism of minerals. In fig. 248, p is an oblong cleavage- rhombohedron of Iceland spar which has two glass prisms w,iv of FIG. 248. Section of Dichroiscope. 18 cemented to its ends with Canada balsam. This combination is placed in a metallic case, which has a convex lens I at one end and a square hole o about the fifteenth of an inch in width at the other. The lens is of a focal distance which shows an object held about half an inch from the square hole. On looking through the lens and prisms two images of the square hole are seen just touching each other. The light of the one image is polarized in the plane which intersects the short diagonal of the prism ; that of the other is polarized in the plane of the longer diagonal. When a pleochroic crystal or fragment is held at focal distance and examined by transmitted light, then, on tho turning of the instrument bringing the polarization of its planes into coincidence with those of the crystal, the two images of the square opening will show the colours of the oppositely polarized pencils of which the light transmitted by the crystal is composed ; this constitutes its pleochroism. The dichroism is then seen by looking through the crystal in one direction only, and the contrast of the two colours is made more obvious. Phosphorescence. This is the property possessed by par- Phosphor ticular minerals of emitting light in certain circumstances, escence. without combustion or ignition. Thus some minerals appear luminous when taken into the dark, after being for a time exposed to the sun s rays or even to the ordi nary daylight. Many diamonds, and also calcined barytes, exhibit this property in a remarkable degree ; less so aragonite, calc-spar, and chalk. Many minerals, including the greater part of those thus rendered phosphorescent by the influence of the sun, also become so through heat. Thus some topazes, diamonds, and varie ties of fluor-spar become luminous by the heat of the hand ; other varieties of fluor-spar, and phosphorite, require a temperature near that of boiling water ; whilst calc-spar and many silicates are only phosphorescent at from 400 to 700 Fahr. Electricity produces phosphorescence in some minerals, as in green fluor-spar and calcined barytes. In others it is excited when they are struck, rubbed, split, or broken ; as in many varieties of zinc-blende and dolomite when scratched with a quill, pieces of quartz when rubbed on each other, and plates of mica or needles of pectolite when suddenly separated. The light emitted by phosphorescent minerals is of various tints. The variety of fluor called chlorophane emits, as its name expresses, a green light. The same particle may emit varying tints, as in the fluor from Aberdeenshire, which, as the heat falls, or the energy of the phosphorescence wanes, emits tints which pass from violet, through blue, green, and yellow, to dull purplish red. The yellow blende from the same place is vividly phosphorescent when heated. Fluor generally phosphoresces with a tint of its own colour. Too high a heat destroys the phosphorescence, which may, how
ever, be restored by either exposure to sun s light or to electricity.