Reports and Proceedings — Mineralogical Society. 143 



II. — MlNEEAliOGIOAL SoCIETY OF LONDON. 



January 23rd, 1906 ; Professor H. A. Miers, F.E.S., President, in 

 the chair. — Studies in Crystallisation, Sodium Nitrate, by H. A. 

 Miers and J. Chevalier. Microscopic observations were made upon 

 solutions of known strength contained in open tubes or sealed tubes 

 maintained at a known temperature, or in the form of drops upon 

 a slide, with the object of comparing the growth of crystals in 

 metastable and labile solutions respectively. The limits of the 

 labile state (in which the solution can crystallise spontaneously) 

 have been fixed by previous experiments by H. A. Miers and Miss 



F. Isaac. If a crystal of the salt be introduced into a supersaturated 

 solution which is not labile, the centres of growth of new crystals 

 are on its surface, and they grow in parallel positions upon it; 

 if it be introduced into a labile solution the new centres of growth 

 are in its neighbourhood, and the crystals fall upon it in various 

 positions. If it be moved about in either, a cloud of crystals 

 is produced ; but in the metastable solution this appears to be due 

 to minute crystals which are swept from its surface. A crystal 

 having appeared spontaneously, can continue to grow in a labile 

 solution without producing others in its neighbourhood; but if 

 introduced, it at once produces a cloud. This may be because the 

 growing crystal is surrounded by a zone of metastable solution. — 

 Geikielite and the Ferro-magnesian Titanates, by T. Crook and 

 B. M. Jones. Geikielite occurs in association with magnesian 

 menaccanite and common ilmenite (menaccanite) in the gem gravels 

 of the Balangoda and Eakwana districts of Ceylon. A considerable 

 number of analyses indicate that Geikielite varies in composition, the 

 iron oxides ranging from 8 to 14 per cent. No specimen has hitherto 

 been found which contains less than 84 per cent, of iron oxide. For 

 this reason the formula (Mg Fe) Ti O3 is preferable to Mg Ti O3, as 

 expressing the true composition of Geikielite. Magnesian menaccanite 

 containing about 28 per cent, of iron oxide is very closely allied to 

 Geikielite in all its properties, more so than to common ilmenite. 

 The alteration products of Geikielite are similar to those of ilmenite, 

 consisting of rutile and so-called leucoxene ; the latter is a mixture 

 of amorphous titanic acid, sphene, and limonite. It seems advisable 

 to classify the ferro-magnesian titanates as Ilmenites and Geikielites, 

 treating magnesian menaccanite (which has the formula (Fe Mg) 

 Ti O3 where Fe : Mg =: 1 : 1) as the middle member of the series. — 



G. F. Herbert Smith exhibited and explained the use of a diagram 

 for the graphical determination of the refractive index from the 

 prism angle and the angle of minimum deviation. He also explained 

 a simple test for ascertaining the pair of faces corresponding to any 

 refracted imaa;e. 



MACHINE-MADE IMPLEMENTS. 

 Sir, — Since this article appeared, I have been able, in company 

 with Mr. C. Bird, F.G.S., of Rochester, to visit a chalk wash-mill at 

 the Borstall Cement Works near that city. 



