532 Journal of Applied Microscopy. 



Goldschmidt, V. Ueber Definition eines Zwill- The author seeks to find a definition 

 ings. Zeit f. Krystallographie. 30: 254, .898. ^^^ ^ ^^^.^^ ^^^^^^j ^j^.^j^ ^j^^j^ ^^ ^^^.^_ 



factory, a. genetically, expressing the manner of the combination of the embryonic 

 particles, d. physically, expressing the arrangement of physically equivalent direc- 

 tions in the two individuals, and r. which shall finally express the arrangement of the 

 equivalent faces of the two individuals. The definitions of the type : " An inter- 

 growth such that individual I can be brought into position of individual II by 

 rotation of 180 degrees around some axis," are rejected as genetically of no sig- 

 nificance since such a rotation can not be assumed to have taken place, and since 

 also some observed twins can not be explained by one rotation, but require the 

 assumption of two. The definition chosen is : "A twin is a symmetrical 

 intergrowth of two equivalent individual crystals." Or more briefly : " A twin 

 is a symmetrical crystal pair." Under this definition, symmetrical combinations 

 of right and left forms, such as occur in quartz, are not twins. 



A. J. M. 



Viola, C. Ueber Bestimmung und Isomorphismus der Feldspathe. Zeit. f. Kryst. 30: 232, 1898. 



Hankel, W. G. Ueber die thermo-und piezo- This is the twenty-first paper by Prof, 



Elektrischen Eigenschaften der Krystalle __,, ,. ,. .j- c 4-u^ 



des ameisensaurin Baryts, Bleioxyds, Stron- Hankel, recordmg his Studies of the 



tians und Kalkes, des salpetersauren Baryts, electric charges developed by heat and 

 und Bleioxyds des Schwefelsauren Kalis, des ,.„ ^ r r ^1 



Glycocolls, Taurins und Quercits. Abhand. Pressure on different faces of crystals, 



der math-phys. Classe der K. Sachs. The methods are described in earlier 



-■ ^ ' papers. a. j. m. 



Goldschmidt, V. Ueber Stereographische Pro- A description of a new and simple 



iection. Zeit. f. Kryst. 30: 260, 1898. ^1 j r u^ • • i. u- 



■• •' ^ method for obtaining a stereographic 



projection from the position angles cp and p obtained in measuring with a two- 

 circle goniometer. The writer, however, reaffirms the inferiority of the stereo- 

 graphic to the gnomonic projection for crystal work. a. j. m. 



Stoeber, F. Notice sur un appareil permettant Apparatus has the precision of the 



de tailler un crista! suivant une direction , . , ,,,..,,- j -t- i.^ u i. 



determinee et sur une methods de taille des machines of Wulfing and Tutton, but 



plaques aux faces parallelles. Bull. Ac. de is much simpler, being in fact a hand 



e gique, s. 3. . 43, i 97. form of grinding apparatus. It depends 



upon the fact that the position of a face is determined when its inclination to a 



given plane is known, and also the line in that plane through which the given 



face passes. The method of attaching the crystal so that any desired face or 



plane may be ground upon it is described. l. McI. l. 



INDIVIDUAL SPECIES. 



Cedarite. R. Klebs. Jahrb. der K. preuss. A new Canadian fossil resin, having 

 Geol. Land. P. i, 1896. composition C 78.15, H 9.89, S 0.31, 



O 11.20, ash 0.45. It melts at 340 degrees. 



Lanarkite, Reproduction artificielle. A. de Prepared by the action of basic acetate 



Schulten. Bull. Soc. Min. 21: 142, 1808. , , , t-.i_ //^ tt /-^\ -m. r\ 



^ ' ^ of lead. Pb (Cj H3 0)2, Pb O upon 



sulphate of sodium. Comparison given between the artificial and natural 



crystals. l. mci. 



