548 SCIENTIFIC RECORD FOR 1884. 



butiou to the optical limits of the individuals ; from these and other simi- 

 lar facts, together with the most important point of all, that mentioned 

 above, that the crystals become isotropic at a high temperature, he 

 reaches the conclusion stated, that the orthorhombic form and anisotro- 

 pic optical characters are both of secondary origin. This view seems all 

 the more probable in the case of leucite, since we know that it must 

 have been formed at very high temperatures, an assumption which is 

 not so natural for boracite, although we know here that artificial crys- 

 tals formed at a high temperature were originally isometric and isotropic, 

 though losing the optical simplicity at ordinary temperatures. Increase 

 of temperature is not the only change of conditions, however, which 

 may bring about a change of optical characters. It has recently been 

 shown by Mallard and Le Chatelier {Bull. 8oc. Min., vii, 478), that hexa- 

 gonal silver iodide, which becomes isotropic at a temperature of 146° C, 

 undergoes the same change at ordinary temperatures (20oC.) if subjected 

 to a pressure of 2,500 kilograms per square centimeter. A mineral, 

 then, which has crystallized at a high temperature, or under a great 

 pressure, or both, may have taken a form which does not correspond to 

 its molecular structure under ordinary conditions of pressure and tem- 

 j)erature. This change may manifest itself in optical character alone 

 (as boracite) or in this and in geometrical form as well, as in leucite. 



Some other contributions to the same subject deserve to be mentioned. 

 Merian has proved {Jahrh. M«.,1884, i, 193) that crystals of tridymite, 

 wliich in optical character correspond, as shown by Lasaulx, to complex 

 triclinic twins, become normally uniaxial in accordance with their hex- 

 agonal form at an elevated temperature. Analcite underwent consid- 

 erable change in optical character when heated, although it did not be- 

 come comi)letely isotropic. Merian did not succeed with leucite, but 

 Penfield {Jahrb. Min., 1884, ii, 224) confirms Klein's results in this 

 respect, though he shows that elevation of temperature does not re- 

 move the optical anomalies of garnet. 



Klein has also shown {Jahrh. Min., 1884, ii, 49) that aragonite, a 

 mineral which is optically biaxial and orthorhombic in form, though 

 often imitating hexagonal forms by twinning, becomes uniaxial upon 

 heating, the section changing into an aggregate of uniaxial particles 

 with negative double refraction. In other words, the molecular change 

 brought about in aragonite by elevation of temjierature corresponds to 

 a change of aragonite to calcite. It may be remembered that G. Eose 

 showed a long time since that the powder into which heated aragonite 

 sei>arated was probably calcite, as suggested by its lower specific grav- 

 ity. A similar optical change takes place, according to Miigge {Jahrh. 

 Min., 1884, i, 63), in leadhillite (monoclinic) upon carefully raising its 

 temperature. These results recall the discussion of a similar subject by 

 Prof. J. P. Cooke, in his paper on the vermiculites (1873), and later in 

 that on haloid compounds of antimony (1877). Cooke shows that at a 

 moderate temperature (about 114° C.) the yellow orthorhombic antimony 



