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:
1848
Catalogue details

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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    bend with the convexity towards the centre of the figure. In substances, as topai and carbonate of soda, where the axes make a largo angle with each other, the complete system of rings cannot be at once seen, and only one half, or the portion round one axis, as in the case of topaz in figure c, is visible in one direction. The angle of the axes in topaz is 18° 30', but in Other cases it may be much greater; thus with green sulphate of iron they are at right angles with each other. The physical production of these beautiful phenomena involves optical principles loo recondite to be here introduced. It is, for the purposes of the chemist, sufficient to say that they arise from the mutual action of the two rays, which are produced by the double refraction of the crystal; and hence, if there be not double refraction, there can be no colours produced. With crystals of the regular system there ia consequently no such result, and hence such crystals are recognised by the com plete absence of coloured rings. The optical properties of the different systems of crystallization may be thus summed up. 1. Regular system. Single refraction. No rings by polarized light 2. Rhombohedral system . . ) Double refraction < Simple system of rings by 3. Square prismatic system . . J with one axis. < polarized light. 4. Right prismatic system . . i Doub,e refraction ( Double system of rings by 5. Oblique prismatic system ^ with two axes. ) polarized light. b. Doubly oblique prismatic system ) ( ' & When crystals form in a crowded or confused manner, it frequently happens that not merely are their surfaces modified in a complicated way, but that several crys- tals become soldered together so completely as to simulate a single form which does not belong to the substance of which the crystal is composed. These crystals are called macks, or twin or hemitrope crystals. Some bodies have a remarkable tendency to crystallize in this way ; thus, sulphate of potash had been long consid- ered as crystallizing in six-sided prisms, terminated by six-sided pyramids; and such crystals of it occur with almost exactly the proportions of the rhombohedral system ; but by optical examination, this figure was found to be composed of three or six of the true crystals, which are right rhombic prisms of the fourth system. These being laid together, form, by their angles exactly joining, a six-sided prism; but when tested by polarized light, they show, in place of the system of rings which a true crystal should pro- duce, the tesselated structure of the figure. In many cases, the agglutination of the crystals is less complete, and irregular figures, with the sides channelled by the imperfect joints, are found. A substance which il- lustrates remarkably this tendency to the macled form is the mineral analcime, which is termed also cubizite, from its forms belonging most perfectly, so far as ex- ternal characters go, to the regular system. It has, however, no distinct cleavage planes, and refracts doubly. When examined by a ray of polarized light, the cube of analcime gives a most beautiful appearance. The diagonals of each surface become occupied by lines, which are alter- nately black and white, according as the analyzer is made to revolve, and in the intervening triangu- lar spaces the richest colours of the rainbow suc- ceed one another, according to the optical laws. This crystal is therefore made up of a great number of other crystals belonging to some one of the more complex systems ; but its structure is so ex- traordinary, that the determination of the form of its real crystal has been as yet impossible In this instance, and in that of boracite alrea .y no- ticed, the optical properties have been the means of showing the true nature of bodies which, from their external form should oth erwise have been ranked among those which crystallize in the forms o'f the regular system. fe It has been noticed as a general character of the crystals of the rhombohedral and square prismatic systems, that by the analysis of a beam of polarized lieht transmitted along the principal axis, there is seen a system of coloured rinsrs trav ersed by a cross, alternately black and white, as the analyzing plate revolves out