45G 



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 property. 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, 

 composing a multitude of minute prisms, growing, as if by a species 

 of vegetation, and variously arranged in sprigs and branchlets, often 

 resembling in miniature, the tufts of marine confervse. A similar 

 plumose appearance, accompanied with the same analytic proper- 

 ties, is obtained from the evaporation of a drop of a mixed solution 

 of nitre and gum arabic. This analytic effect is shown to be the 

 consequence of the high degree of doubly refractive power pos- 

 sessed by these crystalline filaments, and which exists even in those 

 whose diameter is evanescent on microscopic examination. The 

 author entertains hopes that it will be possible to obtain large and 

 permanent artificial crystals, which may possess the advantages of 

 the tourmaline, without the inconvenience resulting from its dark 

 colour. 



December 22, 1836. 



FRANCIS BAILY, Esq., V.P. and Treasurer, in the Chair. 



William Page Wood, Esq., was elected a Fellow of the Society. 



" First Memoir on the Theory of Analytical Operations." By 

 the Rev. Robert Murphy, M.A., F.RS., Fellow of Caius College, 

 Cambridge. 



The author considers the elements of which every distinct analy- 

 tical process is composed, as of three kin(!s ; the first, being the sub- 



