116 G. H. Williams — llemiherfrism in 



1. We may select two planes which are parallel (ill) and 

 (ill); and, if the other pair of parallel planes, (in) and (in) 

 disappear, there results a tetrapyramid composed of two planes, 

 which is the most general form possible in the triclinic system 

 (fig. 2). Such a form 'would he truly hemihedral because the 

 planes are similarly grouped about each extremity of the axis 

 of symmetry. A combination of such forms similarly devel- 

 oped would not, however, differ from a triclinic combination 

 and this system may therefore be regarded as a kind of mono- 

 clinic hemihedrism. Orthoclase is sometimes quoted as an 

 instance of this triclinic hemihedrism because its imperfect 

 prismatic cleavage is often developed parallel to only one face 

 of the prism. This was first noticed by Haiiy and has been sub- 

 stantiated by many subsequent observers. 



2. We may select the two planes which meet at an end of 



the axis of symmetry, b, (ill) and (ill). If the corresponding 

 pair, (in) and (nl) should disappear (fig. 3) there would result, 

 as Groth remarks,* not a hemihedral, but a hemimorphic form. 

 By this method of selection those holohedral forms belonging 

 to the orthodiagonal zone must remain unaltered, while all 

 others, including the clinopinacoid, may occur with only half 

 of their planes developed. The plane of symmetry here dis- 

 appears as in the preceding case. 



This case is illustrated by the crystals of several organic 

 compounds, among which are tartaric acid (C 4 H 6 6 ),t cane 

 sugar (sucrose, C 12 22 H n );{: and quercite (pentacid alcohol, 

 C„H 12 6 ).§ These crystals have furthermore all been shown to 

 be pyroelectric — a property which is especially characteristic 

 of hemimorphic substances. 



3. "We may, as a last possibility, select two of the planes of 

 the monoclinic hemipyramid which intersect in the plane of 

 symmetry, (ill) and (nl). If the corresponding pair, (in) 

 and (In), disappear (fig. 4) there results an apparent hemi- 

 morphism with reference to either or both of the axes in the 

 plane of symmetry — vertical or clinodiagonal. The only holo- 

 hedral form which cannot appear with only half its planes by 

 this method of selection is the clinopinacoid. All others may 

 be present with only half their planes, although of course both 

 halves of a form may appear on the same crystal. If this be 

 the case in the vertical zone there results apparent hemi- 



* Phvsikalische Krystallographie, 2d Ed., p. 481. 1885. 



f De'la Provostaye, Ann. Ch. et Phys., Ill, vol. iii, p. 139. 1841. 



% Wulff, Jour, fur prakt. Chemie, vol. xxviii,"p. 1 29, 1843. L. Wulff, in his recent 

 article on the crystallization of cane-sugar (Zeitschr. fur Kryst., vol. xiv, pp. 552- 

 562, 1888), finds that the face g, Pa> (Oil) generally occurs with only a single 

 plane, which forces him to regard the substance as an instance of either mono- 

 clinic tetartohedrism or triclinic hemihedrism. 



§ Lewis, Proc. Cryst. Soc, p. 51, 1882 : cf. also Groth, Physikalische Krystal- 

 lographie, 2d Ed., pp. 512 and 514. 



