455 



ther evidence, the opinion he formerly advanced ; that as bisulphate 

 of potash is a double sulphate of water and potash, and therefore 

 neutral in its composition, so, with the sole exception of the anor- 

 mal class already noticed, all salts, usually considered as bisalts are, 

 in like manner, really neutral in composition. He shows that this 

 theory is strictly applicable to the red chromate of potash, which 

 appeared to present a difficulty. 



The chlorides are next considered. The law followed by the 

 chlorides of the magnesian class of metals appears to be that they 

 have two atoms of water strongly attached to them, and which may, 

 therefore, be regarded as constitutional. Thus, chloride of copper 

 crystallizes with two atoms of water, and with no lower proportion; 

 but several chlorides of this class have two or four atoms more; the 

 proportion of water advancing by multiples of two atoms. The 

 chlorides have probably their analogues in the cyanides, although we 

 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. 



WILLIAM LAWRENCE, Esq., V.P., in the Chair. 



Thomas Graham, Esq., M.A., was elected a Fellow of the So- 

 ciety. 



A paper was read, entitled, " Further Observations 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 

 polarising 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 polarised light. 

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



2n2 



