FORMATION OF CRYSTALS 



1095 



of arrangement of the structural units gives rise to the phenomena 

 of double refraction, circular polarisation, pleochroism, &c., observed 

 with crystalline bodies. The important results to be anticipated 

 from the microscopic examination of crystalline preparations such as 

 rock sections, etc., was pointed out by H. C. Sorby in 1858 ; the micro- 

 scopic methods as at present applied to pure crystallography have 

 been fully described by P. Groth l and by Th. Li ebisch, 2 whilst their 

 Applicability to the identification of the crystalline constituents of 

 rocks has been exhaustively treated by H. Rosenbusch. 3 



The study of crystalline materials in such minute crystals as are 

 appropriate subjects for observation by the microscope is not only 

 t\ very interesting application of its powers, but is capable of 

 affording some valuable hints to the designer. This is particu- 

 larly the case with crystals of snot/:, which belong to one of the 

 * hexagonal systems,' the basis of every figure being a hexagon of 

 six rays ; for these rays ' become incrusted with an endless variety 

 of secondary formations of the same 

 kind, some consisting of thin lamina 

 iiione, others of solid but translucent 

 prisms heaped one upon another, and 

 others gorgeously combining lamina' 

 and prisms in the richest profu- 

 sion,' 4 the angles by which these 

 figures are bounded being invari- 

 ably 60 U or 120. Beautiful ar- 

 borescent forms are not imfrequently 

 produced by the peculiar mode of 

 aggregation of individual crystals ; 

 of this we have often an example on 

 a large scale on a frosted window ; 

 but microscopic crystallisations some- 

 times present the same curious phe- 

 nomenon (fig. 814). Avanturine, 

 lapis lazuli, crystallised silver, &,c. 

 make very good specimens ; whilst 



thin sections of granite, gabbro, and other crystalline rocks, also of 

 agate, aragonite. piedmontite, the zeolir.es, and other minerals, are 

 very beautiful objects for the polariscopo. 



The actual process of the formation of crystals may be watched 

 under the microscope with the greatest facility, all that is necessary 

 being to lay on a slip of glass, previously warmed, a saturated solu- 

 tion of the substance, and to incline the stage in a slight degree, so that 

 the drop shall be thicker at its lower than at its upper edge. The 

 .crystallisation will speedily begin at the upper edge, where the pro- 

 portion of liquid to solid is most quickly reduced by evaporation, and 

 Avill gradually extend downwards. If it should go on too slowly, 



FJI;. 814. Crystallised silver. 



1 Physikalische K r ij st all a g rapine, Leipzig, 1895. 



'-' Grumlriss der physikalischen Erystallographie, Leipzig 



1896. 



Microscopical Physiography of the Rock-maJcing Minerals, London, 1895. 

 4 Glaisher on ' Snow-crystals in 1855,' Quart. Journ. Micros/-. Sci. vol. iii. 1855, 

 1>. 179. See also C. A. Hering, Zcita.f. Krijst. Bd. xiv. 1888, p. 250. 



