Reports and Proceedings — Mineralogical Society. 45- 



retardation is nil. This may be observed by using a gypsum plate 

 or the double quartz wedge devised by the author. In the latter 

 case the positions in question are marked by the coincidence of the 

 bands in the two halves of the wedge. This gives a very exact 

 reading if strictly parallel light be employed. — Mineralogical Notes 

 (Diopside and Albite), by Professor W. J. Lewis. A large tabular 

 crystal of white diopside, a brown diopside of unusual habit, and 

 a Carlsbad twin of albite were described. — Note on the Crystallisa- 

 tion of Drops, especially of Potash-alum, by Mr. J. Chevalier. The 

 President described observations made by Mr. Chevalier on the 

 crystallisation of drops of solution of potash-alum. These generally 

 yield in succession (a) bii'efringent spherulites, (6) octaliedra, and 

 (c) a fine rectangular network, (o) is probably a less hydrated 

 alum, and it becomes isotropic on exposure to moist air by 

 conversion into (6). (c) is ordinai-y alum which is in a state of 

 strain, owing to its rapid crystallisation, and becomes white and 

 opaque after a time owing to the development of cracks. Drops 

 observed upon a slide under the microscope behave differently 

 according as they are in the metastable or labile condition. A meta- 

 stable drop inoculated with (a) or (6) or (c) deposits octahedra. 

 A labile drop inoculated with (a) deposits spherulites, but inoculated 

 with (h) or (c) deposits the rectangular network. When a metastable 

 drop containing either octahedra or spherulites, or both, passes into 

 the labile condition (by cooling or by evaporation) they may continue 

 to grow unchanged. If, however, a fragment or germ of octahedral 

 alum be introduced into a labile drop, the network (c) is immediately 

 produced. An alum crystal growing in a labile solution is 

 surrounded by a zone of metastable liquid which prevents it from 

 starting the network (c) chai'acteristic of a labile drop. Experiments 

 were made upon the action of various mineral substances in inducing 

 crystallisation in metastable and labile drops. Among these the 

 holosymmetric cubic crystals, and especially galena, exercise 

 a remarkable effect in producing the network (c) in labile drops. — 

 Note on the Formation of Gypsum Crystals in a disused Well at 

 Chemical Works, by C. J. Woodward. Groups of gypsum crystals 

 were exhibited which were found thirty years ago studding the walls 

 of an old well at Messrs. Chance's Chemical Works at Oldbury. — 

 Notes on Minerals recently found in the Binnenthal, by Mr. E. H. 

 Solly. The minerals described were : (1) Ilmenite, in brilliant 

 crystals, displaying marked hemihedrism and showing five new 

 forms. It is associated with quartz, adularia, magnetite, and mica, 

 on mica-schist. (2) Seligmannite ; an exceptionally large and well- 

 developed crystal in dolomite. Unlike any previously described, it 

 is untwinned ; altogether 45 forms were observed, of which 21 

 are new. (3) Marrite ; two more crystals of this rare mineral 

 were found, one tabular and the other sharply pointed in habit. 



(4) Proustite ; a minute crystal deposited on a crystal of rathite. 



(5) Ti-echmannite; a crystal of this rare mineral displayingasymmetric 

 hemihedrism, deposited on a crystal of binnite. (6) Hyalophane, 

 in crystals of an unusual green colour. 



