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The Development of Crystal Faces

Nature volume 88, page 360 (11 January 1912) | Download Citation



IN a memoir by M. P. Gaubert entitled “Recherches recentes sur le Facies des Cristaux,” published by the Société de Chimie physique, some remarkable new facts are described regarding the influence of foreign substances on the development of crystal faces. The most interesting relate to the influence of colouring matters, such as methylene blue, eosine, fuchsine, and picric acid, which are shown to be capable of passing into the crystal substance in two different ways. According to one, the colouring matter is deposited on the crystal in the course of growth; this occurs when the solution is saturated with the colouring matter, which thus deposit crystals. An excellent example is afforded by lead nitrate, which when pure usually deposits colourless octahedral crystals. When the mother liquor is saturated with methylene blue, the crystals first deposited exhibit cube faces modifying those of the octahedra, and these cube faces alone are coloured blue; they also exhibit striations like pyrites, corresponding to the faces of the pentagonal dodecahedron. The colour thus not only attaches itself to the faces of a particular form, but indicates the true class of symmetry of the system, in this case a class lower than the holohedral. Similar phenomena are described with gypsum, copper sulphate, thallium sulphate, and morphine, all of which develop specific unusual forms and faces, coloured by the dye, when the latter is methylene blue. Strong polychroism is also introduced in the case of copper sulphate.

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