MINERALOGY. GG5 



lous double refraction arises with the admixture of an isomorphous salt. 

 For example, pure crystals of potash alum were isotropic, while crystals 

 formed of isomorphous mixtures of potash-alum and ammonia-alum 

 showed a more or less distinct double refraction. From this it is argued 

 that the mixtures of unlike molecules must exert an influence upon each 

 other, causing in some way not clearly made out a sort of internal ten- 

 sion. Further investigations in the same field and on a wider range of 

 isomorphous compounds are much to be desired. 



The species boracite is one about which much interest has centered 

 in connection with this subject, and the thorough investigations of Klein, 

 alluded to in the Eeport for 18S2, had seemed to prove pretty clearly 

 that it was to be referred back to the isometric system, and its optical 

 characters designated as true " anomalies" due to molecular tension. 

 The subject has been further investigated by Mallard (Bull. Soc. Min. 

 France, vi, 122, 129), chiefly with reference to the effect of heat upon 

 its optical properties. His results, as interpreted by Klein, go to prove 

 that boracite is dimorphous, since only at a temperature of about 300° 

 do the sections become isotropic. At the elevated temperature, then 

 the molecules satisfy the demands of isotropism corresponding to au 

 isometric form, while at ordinary temperatures the condition of the 

 molecules is abnormal and corresponds to their arrangement, or their 

 own symmetry corresponds to an optically biaxial crystal. 



Some additional observations have been made by Ben-Saude (Bull. 

 Soc. Min. France, vi, 200), whose investigations of analeite and perofs- 

 kite are well known. These relate to rock salt and sylvite; he finds 

 the probable cause of their doubly refracting character, as observed by 

 him, in part by irregularities produced by rapidity of crystallization and 

 in part by a cause like that suggested by Brauns, as stated above. 



Klein has published (Jahrb. Aim., 1883, 1, 87) results of an exhaustive 

 study of garnet, with reference to its anomalous optical character. 

 These are of especial interest, because garnet is often taken by the 

 French school (Mallard, Bertrand) as a typical example of "pseudo- 

 symmetry," they explaining the optical character as really belonging to 

 the molecular structure and the outward form a complex twin. Kleiu's 

 observations are most conclusive in showing that the latter explana- 

 tion is not the correct one; on the contrary, that garnet is to be still 

 considered as a true isometric species, and that the anomalous optical 

 characters are due to secondary causes. This memoir of Klein is one 

 of the most important that has been devoted to this subject. 



The memoir of Groth (Zeitsch. Kryst., vn, 375, 457) upon the members 

 of the cryolite group has been concluded (the first part was alluded to 

 in the report for 1882). The same subject has been written upon by 

 Des Cloizeaux (Bull. Soc. Min. France, V, 310, vi, 254) and by Krcnner 

 (Berichte aus Ungarn, i). Further, Cross and Hillebrand (Amer. Journ. 

 Sc, xxvi, 271) have published an exhaustive account of the minerals 

 of this group from the Pike's Peak region, Colorado. The crystallo- 



