Reports and Proceedings — MineraJogical Society. 237 



III. — MiNERALOGICAL SoCIETT. 



March 23, 1909.— Principal H. A. Miers, F.R.S., President, in the 



Chair. 



On a Stage Goniometer for nse with the Dick pattern of microscope ; 

 by Professor H. L. Bowman. The form of goniometer, intended 

 to be screwed to the stage of a microscope with rotating nicols, 

 which was designed by Principal Miers, has been slightly modified 

 by the author with a view to securing increased rigidity and ease of 

 control. The instrument is adapted for supporting and manipulating 

 a small crystal during the examination of etching-figures or other 

 features requiring high magnification, as well as for the determination 

 of its optic axial angle in air or oil, and the extinction angles and 

 other optical characters of the various faces. It is provided with 

 screw motions for adjusting and centreing the crystal, and for 

 regulating the height of the axis above the stage. — On the Electro- 

 static Separation of Minerals ; by Mr. T. Cook. Conductivity is 

 a much more important factor than specific gravity in determining 

 the behaviour of mineral fragments under the influence of an 

 electrostatic charge. The greater susceptibility of good conductors 

 as compared with bad conductors can be made still more pronounced 

 by providing for the escape of the repelled opposite charge, which 

 takes place rapidly in good conductors and slowly in bad conductors. 

 It was shown that, in consequence of this fact, grains of such good 

 conductors as ilmenite, pyrites, galena, or wolfram can be easilj^ 

 separated by means of a rubbed piece of sealing-wax from admixed grains 

 of bad conductors such as calcite, quartz, fluor, or monazite. Minerals 

 having a metallic lustre are good conductors, whereas those which 

 are colourless and highly transparent are bad conductors. It is 

 suggested that there is probably a much closer connection between 

 the conductivity of a mineral and its general optical properties than 

 has been hitherto suspected. — On the identity of Guarinite and 

 Hiortdahlite ; by Dr. F. Zambonini, with chemical analysis by 

 Dr. G. T. Prior. The rare mineral guarinite, which occurs sparingly 

 in small yellow crystals in the sanidinite bombs of Monte Somma, 

 has been hitherto regarded as ortho - rhombic, and as essentially 

 a complex silicate of lime, alumina, and soda. A new investigation 

 made by the author on crystals showing terminal faces shows that 

 the mineral is really triclinic, and identical, both crystallographically 

 and optically, with hiortdahlite. Crystals of guarinite show poly- 

 synthetic twin lamellae with oblique extinctions like those exhibited 

 by crystals of hiortdahlite. The chemical analysis showed that the 

 mineral is essentially a fluo-silicate and zirconate of lime and soda, 

 practically identical in composition with hiortdahlite, although the 

 percentage of fluorine was lower than that given in Cleve's analysis 

 of the latter mineral. The numbers obtained in the analysis 

 correspond closely to a formula 3 Ca Si On . Ca (OH, F) Na . Zr Og. — 

 Note to a paper "On the Comparison of Refractive Indices of Minerals 

 in Thin Sections"; by Dr. J. W. Evans. Parallel nicols are placed 

 so as to bisect the angle between the directions of vibration in the 

 adjoining crystal sections which make the smaller angle with each 



