MINERALOGY. 453 



portant contributions bave been made, but for tbe niost part they are 

 of such a nature as not to allow of being mentioned briefly — titles are 

 given on a following page. Among the more popular articles may be 

 mentioned several on the subject of the specific gravity. Brauns discusses 

 the use of methyl iodide for petrographic and optical investigations. 

 Its specific gravity is 3.33; it has a very high index of refraction (1.7424 

 for Na at 14° 0.) and remains unchanged in the air. It can be used in 

 much the same way as the Tboulet solution for the accurate determina- 

 tion of the specific gravity, or the separation of different minerals me- 

 chanically mixed. It has the disadvautage that it can not be diluted 

 with water, but with benzol, and moreover its specific gravity changes 

 rather rapidly with cliange of temperature. A method of obtaining the 

 specific gravity of small fragments of a mineral applicable, as the 

 Thoulet solution is not, to minerals of specific gravity over 4, or, on the 

 other hand, to porous bodies, is described by Joly. It consists briefly in 

 determining the specific gravity, as by the Thoulet solution, of a little 

 ball of paraffin in which the mineral fragments have been imbedded 

 by careful heating; the weight of the paraffin and its specific gravity 

 are known by previous observations. Goldschmidt has discussed the 

 degree of accuracy attainable in the different methods of obtaining spe- 

 cific gravity and thrown some light upon the subject. In one of* his 

 papers he shows that the temperature-correction may in all ordinary 

 mineralogical work be neglected, since it is considerably less in amount 

 than the usual errors of observation. He also urges a point, the impor- 

 tance of whicli is too little understood, that the apparent wide variation 

 in the specific gravity of a given mineral is in most cases due simply to 

 the use of poor material, or to faulty determinations. 



Miigge has made some additions to a subject previously developed 

 by him : TLe existence of secondary twinning and the change in position 

 of crystalline faces due to it; also the production of a twinning structure 

 by pressure, as in bismuth, antimony, and tliopside. This is a method 

 of especial interest in the case of diopside, since natural crystals often 

 show twinning lamella) parallel to the basal plane, and in consequence 

 there is often the distinct parting in this direction which was long called 

 the basal cleavage. Judd has followed out a related line of investiga- 

 tion in his discussion of what he has called " schillerization," that is, 

 the production of the peculiar, nearly' metallic reflection called ''schiller" 

 by tbe secondary formation of inclusions in parallel position. He argues 

 for the existence of planes of easy solubility ("solution-planes") along 

 which chemical action takes place more readily, as in the formation of 

 negative crystals. These planes of chemical weakness, he says, have 

 with the cleavage planes and gliding-planes (Gleitfliichen) a definite 

 relation to the symmetry of the crystal. These solution-planes are 

 hence connected with the planes of secondary twinning. 



An interesting series of papers have been given by Kundt and Blasius, 

 Mack and Schedtler, on the pyro-electricity of different minerals. The 



