T. L. Walker — Examination of Triclinic Minerals. 179 



must coincide with the lines of symmetry of the basal planes. 

 A glance at figures 4 and 5 will show that this is the case. 

 Although the trigonal axis of symmetry of tourmaline com- 

 pels the figures to be built on the plan of the equilateral tri- 

 angle, yet while conforming to this requirement the difference 

 in the faces 0001 and 0001 is very clearly indicated by the dif- 

 ference of the etching figures. 



Acid Dextro- Tartrate of Strontium. — This is one of the 

 best known and most easily prepared of all the substances crys- 

 tallizing in the asymmetric or hemihedral division of the tri- 

 clinic system. Dr. Steeg & Co. of Homburg v. d. Hohe 

 supplied me with a few very fine crystals for examination. 

 These crystals possess one very perfect cleavage direction, 

 which, according to the orientation proposed by A. Scacchi* 

 and adopted by Liebisch,f is taken to be the basal pinacoid 

 (001) and (001). Very thin cleavage plates are easily prepared 

 which serve very well for etching. As the substance is readily 

 soluble in water, good figures were prepared by placing the 

 cleavage fragment between folds of a piece of very slightly 

 moistened calico and pressing between the finger and thumb 

 for twenty seconds. A microscopic examination showed that 

 the figures on 001 were four-sided with their sides parallel to 

 the edges of the cleavage plate. The four sides of the corro- 

 sion pits are nearly equal (fig. 6). On the parallel face 001 the 

 figures are also four-sided but very slender, almost reduced to 

 mere lines at times (fig. 7). After etching and examining a 

 large number of cleavage plates, I satisfied myself that these 

 figures are unmistakably different and eminently characteristic 

 for the two faces upon which they occur. It would be impos- 

 sible to confuse them. So much is this so, that if a crystal 

 were split into half a dozen cleavage plates and these etched, 

 then by means of the difference of the figures the upper and 

 lower surfaces could be distinguished and the crystal recon- 

 structed with all the fragments properly oriented. 



So far as I know, the only previous experiments made with 

 etching figures with a view to determining the crystallographic 

 equivalence of pairs of parallel faces, are those made by Miers 

 on spangolite. This hemimorphic hexagonal mineral had been 

 described by Penfield from a single specimen from Arizona, 

 when Miers found it on some specimens of Cornish copper 

 minerals. In his description in the Mineralogical Magazine he 

 says : " In order to obtain independent evidence of the hemi- 

 morphic character of this mineral, experiments were next 

 made upon etched figures, it being thought that possibly some 



* A. Scacchi, Atti. Accad. delle Sc. Fis. e Mat., Napoli, 1, 1863. 

 \ Grundriss der Krystallographie, p. 176, Leipzig, 1896. 



