536 SCIENTIFIC RECORD FOR 188-2. 



solution is even more useful for the se])aration of two or more minerals 

 when mixed together in the form of small fragments, the solution being- 

 graded by successive dilution so that in succession one constituent after 

 another of the mixture sinks and is removed while the others float. This 

 method of procedure is much used in connection with the study of the 

 minerals in rocks, and for the development of it science is especially 

 indebted to Thoulet and Goldschmidt. Another solution of similar 

 properties has been proposed by D. Klein ; this is best made, as he states, 

 from the boro-tungstate of cadmium, and may be obtained with a specific 

 gravity up to 3.0. Breon has obtained remarkable results from a liquid 

 obtained by fusing lead chloride and zinc chloride together at about 

 400° C. A liquid with a specific gravity above 7.5 has been obtained, 

 and although the practical use of such a liquid is not very convenient, 

 Breon has been able with it to separate as many as twelve constituent 

 minerals from a sample of sand; he has also used it for minerals as 

 heavy as wolfram. 



In optical mineralogy the subject which has attracted most attention 

 during the period under consideration, as indeed for the few years pre- 

 ceding it, is the cause of the so-called "optical anomalies" of crystals. 

 Since the time of Brewster it has been well known that many species 

 exhibit in polarized light optical phenomena not in accordance with their 

 apparent geometrical form ; for example, that alum, fluorite, the diamond, 

 and other species show double refraction, while their crystalline form is 

 apparently isometric. Later investigations have largely increased the 

 list of crystallized minerals exhibiting these "optical anomalies," as they 

 have been called, and various explanations have been offered for them. 

 At present there may be said to be two opposed theories, both having 

 strong supporters, and the weight of evidence is hardly sufficient to 

 make it possible to decide finally between them. 



In the view of Mallard, first advanced In 1876, the apparent symmetry 

 exhibited by the crystals of the species under discussion, which show opti- 

 cal anomalies, is only pseudo-symmetry ; the optical properties observed 

 belong inherently to the molecular structure of the individual parts of the 

 crystal ; and the external crystalline form observed is really due to the 

 grouping; or twinuing, of several simple crystals, each of a lower grade of 

 symmetry than that which the complex whole so formed simulates. For 

 example, according to this view, an apparent isometriccube may in fact be 

 made up of six four-sided pyramids, each optically uniaxial, and united so 

 that their bases form the sides of the cube and their vertices are grouped 

 at the center. Similarly, an apparent regular octahedron may be made 

 up of eight uniaxial triangular pyramids; or, still more generally, a hex- 

 octahedron of garnet may be composed of forty-eight triangular pyra- 

 mids, each optically biaxial, and four together forming a pseudo-rhombic 

 pyramid. The theory of Mallard has been warmly supported by Ber- 

 trand ; and the labors of the French mineralogists in general, includ- 

 ing Des Cloizeaux, have increased largely the list of so-called pseudo- 



