^18 Boyal Society. 



are less acquainted with the single cyanides of iron, copper, &c.: 

 but the disposition of the protocyanide of iron, and of the cyanide of 

 copper to combine with two atoms of cyanide of potassium, may de- 

 pend on the cyanides of iron and of copper possessing, like the cor- 

 responding chlorides, two atoms of constitutional water, which are 

 displaced by two atoms of the alkaline cyanide in the formation of 

 the double cyanides. 



December 15, 1836. — A paper was read, entitled, " Further Ob- 

 servations on the Optical Phenomena of Crystals." By Henry Fox 

 Talbot, Esq., F.R.S. 



The author had described, in a former paper*, the remarkable cir- 

 cular mode of crystallization frequently occurring from a solution 

 of borax in phosphoric acid, and producing, when examined by the 

 polarizing microscope, the appearance of a black cross, with four 

 sectors of light, and occasionally coloured rings, upon each crystal. 

 In the present memoir, he describes some deviations from the usual 

 forms of crystalline circles ; the most striking varieties consisting in 

 the cross being itself highly coloured, instead of black, upon a 

 white ground. The author shows that these crystals consist of bo- 

 racic acid alone, resulting from the decomposition of the borax by 

 the phosphoric acid. He gives an explanation of the optical ap- 

 pearances they present on the hypothesis of their being constituted 

 by an aggregate of acicular crystals, radiating from a central point, 

 and the whole circle being of variable thickness at different distances 

 from its centre, and acting with great energy on polarized light. 

 Other modes of crystalline formation, dependent chiefly on the pre- 

 sence or absence of combined water, are next described. These 

 sometimes produce crystals composed of two opposite sectors of a 

 circle, united at the centre j at other times, they exhibit irregular 

 elongated shapes, having a stem, either subdivided at both extremi- 

 ties into minute diverging fibres, or abruptly truncated ; and occa- 

 sionally they present regular geometric forms : but, whatever be 

 their shape, they undergo, in general, spontaneous changes in the 

 course of one or two days after they have been formed. 



The author then notices a property belonging to some crystals, 

 similar to that possessed by the tourmaline, of analysing polarized 

 light ; for which reason he denominates them analytic crystals. As 

 an example, he mentions those obtained by dissolving sulphate of 

 chromium and potash in tartaric acid by the aid of heat. A drop 

 of this solution, placed on a plate of glass, soon yields, by evapora- 

 tion, filmy crystals, which frequently have this propert}^ The 

 plumose crystals of boracic acid, when crystallized from a solution 

 of borax in phosphoric acid, also possess this analytic power, and 

 present very beautiful appearances when viewed with the polarizing 

 microscope. Another instance occurs in the oxalate of potash and 

 chromium, a salt whose optical properties have been investigated by 

 Sir David Brewster. If gum arabic be added to a solution of this 

 salt, and a drop of it be put between two plates of glass, a very 

 beautiful arborescent, but microscopic crystallization takes place, 

 * See our last volume, p. 288. 



