Properties of some Minerals and their varieties. 257 
The view of Franz,* that the direction of the stream (+) in 
bismuth depends upon the cleavage lamella, offers a satisfac- 
tory explanation of some of the phenomena.t On the other 
hand, the right or left hemihedrism entirely fails to account for 
the change of + in holohedral or amorphous fused materials; 
and in such cases other causes must exist and must be sought 
for. 
was with great pleasure, consequently, that we availed 
ourselves of the kindness of Prof. Stefan in putting at our dis- 
which has been made use of and fully described by Prof. 
Stefan and G. Rose. We mention here merely that the homo- 
wire when warmed and brought in contact with the cold wire 
left the needle in complete rest,f so that no considerable error 
in consequence of subordinate currents 1s possibl 
cy 
A series of preliminary experiments was devoted to estab- 
: is . . * 
(c.) In tetrahedrite and chalcopyrite, which crystallize hemi- 
hedrally, and. wi e most complete opposition of right and 
left, no change of + could be discovered.| 
* Franz, Pogg. Ann., vol. Ixxxiii, 375; vol. Ixxxiv, 388; vol. xevii, 34. ; 
+ Schrauf (Lehrbuch d. Phys. Min., 11 Ba, p. 386) has adopted this explans- 
tion. In accordance with the results of the present investigation, 
the electrical phenomena must be altered. : ae 
+ We may inention in addition that a brass wire in contact with itself (cut 9m 
the same piece) caused a considerable deflection of the needle, in the directio 
| Rose (1. c.) found the same constancy in the sign in chalcopyrite. 
Am. Jour. Sc1.—THIRD iy Vou. VIII, No. 46,—Oct., 1874. 
