Blake — 8ome Isomorphotis Triple Thiocyanates, 13 



sphenoidal character of the crystals was more fully established 

 for all of the salts of the series by the binary character of the 

 etchings on basal cleavage surfaces. The basal cleavage is 

 a pronounced feature of all the salts, hence basal sections for 

 etching and for optical examination could readily be obtained. 

 The etched figures, in no case very distinct, were best obtained 

 by using dilute ammonium hydroxide as the solvent. The 

 characters obtained are sketched in figure 4, and exhibit plainly 

 the binary symmetry of the vertical axis. Moreover, on oppo- 

 site faces of the same basal section the figures were orientated 

 at right angles to one another, as indicated by full and dotted 

 lines in the figure. In this way the sphenoidal character of 

 the silver-barium salt was established, although the sphenoidal 



faces, ^ (111) were not present on any of the crystals observed. 

 Indeed, the sphenoidal character of this salt, the first one of 

 the series which was prepared and studied, was unsuspected 

 until after the other salts had been examined. 



The crystals studied were from one millimeter to one centi- 

 meter in thickness, with the proportionate lengths represented 

 in the figures, and exhibited some tendency to arrange them- 

 selves in parallel groups. One large individual of the silver- 

 strontium salt in particular seemed to be made np of a collection 

 of smaller ones in parallel position, the separate pyramidal faces, 

 ^(201), being readily discernible. The crystals are clear and 

 colorless when not tinged with impurities, and, with the excep- 

 tion of the silver-strontium salt, give good reflections with the 

 goniometer. 



Basal sections of all of the salts, when examined with the 

 polariscope, exhibit the normal interference- figure of uniaxial 

 crystals, and, when tested with the mica plate, the salts were 

 proved to be optically negadve. The well developed pyra- 

 midal faces, ^, 201, and ^", 201, of the several salts served as 

 prisms whereby the indices of refraction were determined for 

 sodium light. 



