Journal of Applied Microscopy. 417 



the etch-figures indicate the symmetry of the crystal, but that the shape of the 

 etch-figures and their orientation with respect to cleavage lines would be essen- 

 tially constant for the same crystal face in different crystals of the same com- 

 pound. 



The figures were examined by reflected, vertically incident light, with rela- 

 tively low powers, and the general shape was determined by measuring the 

 angles between straight sides, and their relation to some fixed base line as a 

 crystal edge or a cleavage. 



All the non-aluminous amphiboles gave essentially the same figure upon the 

 prismatic face or cleavage, the differences being so minute as to defy measure- 

 ment. The hornblende crystals yielded etchings upon the prism face, the main 

 characters of which were constant, but in which four subtypes could be distin- 

 guished. 



In etching the clinopinacoid of actinolite two quite diiiferent kinds of figures 

 were developed, small, light, four-sided figures elongated parallel to the side most 

 oblique to the cleavage, and larger, dark, four-sided figures elongated parallel to 

 the side most nearly parallel to the cleavage. When the strength of the acid 

 was varied the light pits were unchanged, but the dark pits were rotated about 

 30 degrees for a change from pure acid to 10 per cent. acid. All these characters 

 showed similarity to Baumhauer's apatite. 



In comparing amphibole and pyroxene, the author points out that actinolitic 

 amphiboles give one class of etch-figures, aluminous amphiboles another ; that 

 there is a similar contrast between diopside and augite, and that the etching 

 phenomena of diopside with hydrofluoric acid are hardly distinguishable from 

 those of actinolite. a. j. m. 



Sohncke, L. Einfluss der Entwasserungstemp- This paper well illustrates the careful 



eratur aui die Vervvitterungsflecke des attention that is, in these days, given to 

 Gypses. Zeit. f. Kryst. 30: i-8, 1898. ' ^ '_* 



phenomena of apparently minor im- 

 portance. Cleavage plates of gypsum, when heated, develop spots, some of 

 which Pape described* as of elliptical shape and composed wholly or in part of 

 the dehydrated material. Blasius and Weiss were unable to obtain the elliptical 

 spots, but Weiss described almost rectangular spots resembling an envelope, 

 with four arms nearly diagonal. It is these envelope-like spots which Sohncke 

 has examined. 



The gypsum cleavages were heated in an air bath, either being suspended 

 therein by a wire or laid upon asbestus paper. The spots were examined under 

 the microscope, and were found to consist of four branching lines starting not 

 exactly from a common center, nor quite at right angles. The ends of these 

 determine a quadrilateral of which they are approximately the diagonals. When 

 perfectly developed the quadrilateral is crossed by fine stripes parallel to two of 

 the sides and nearly parallel to the conchoidal fracture, and in the triangles 

 opposite the obtuse angles of the diagonals there appears another set of stripes 

 parallel to the fibrous fracture. 



Denoting the dimension of the figure parallel to the first set of stripes by /, 



. Ann. 125, 113, 1865; 133, 364, i{ 



