498 Profs. F. C. Brown and L. P. Sieg on the Seat of 



forms like (c) and (d) the branches grew entirely together 

 as the crystals developed, so as to form an apparently plane 

 surface. They sometimes formed a solid lamellar crystal 

 form, and were then excellent reflectors of large dimensions. 



t cm. 



The form (e) is made up of four rows of branches placed 

 90° apart. A large number of other crystal forms have 

 been described (loc. cit.), all of which are light-sensitive. 

 However, the forms described in this paper were best adapted 

 tojthe argument. 









Fig. 2. 















frton/T 









4- 







P c 







c 



\ 



T= 



-.9 r 



4^ 



"fl 



^T 



^ 





fji 





II v II 







R 





C-j 















1 



o 



L 



\ 



o 



~" S" 





Back 



This crystal was laid on a glass plate, Gr, and silver elec- 

 trodes, P, P, were pressed against the upper surface of the 



