70 Penfield and Ford — Slibiotantaliie. 



test. Resting on this secondary material are a few very 

 minute pyramids of cassiterite. 



Pyroelectrieity. — The pyroelectrie phenomenon which is 

 exhibited by all of the crystals is a matter of interest. It 

 reveals a complex polysynthetic twinning structure, as the 

 result of which the crystals with few exceptions appear as if 

 they possessed normal orthorhombic symmetry. Each crystal 

 has its own peculiar polysynthetic development, and it seems 

 sufficient to select a few examples as types, in which the dis- 

 tribution of the electrically excited surfaces is indicated dia- 

 grammatically, as in the accompanying plate, page 63. The 

 crystals are represented, as it were, unfolded ; there is given 

 first a top view, then in turn views of the front, right-hand 

 side, back and left-hand side. Lower ends have not been 

 represented because, with few exceptions, they are fracture 

 surfaces. As stated on page 63, the crystals were tested for 

 pyroelectrieity with a mixture of carmine, sulphur and lycopo- 

 dium, with which negatively and positively electrified surfaces 

 are coated respectively, white by lycopodium, and red by sul- 

 phur coated with carmine. In the diagrams the negative sur- 

 faces are indicated by white (lycopodium), the positive by 

 cross lines (red, sulphur and carmine), while the neutral, brown 

 surface of the crystals is simply lined. The diagrams illustrate 

 how irregularly the lamellae are intergrown and, to a certain 

 extent, how complex the twinning really is, but they do not do 

 the subject full justice, for often the lamellae are very thin 

 and red and white alternate in bands, too narrow to be repre- 

 sented by line drawings. 



Two pebbles of the stibiotantalite from G-reenbushes, Aus- 

 tralia, belonging to the Brush Collection, were tested for pyro- 

 electricity, but in their entirety gave no effect ; when broken 

 up, however, small homogeneous pieces gave the reaction dis- 

 tinctly, and it may be assumed that failure to get a response 

 from the larger pebbles is because of the numerous cracks which 

 permeate the material. 



Crystal Measurements. — The measured and calculated 

 angles are as follows : — 

















Measured. 



Calculated. 



9 



/\ 



g\ 



130 



s\ 



130 



— 



45° \§'* 





9 



s\ 



W, 



130 



/\ 



4.12.9 



— 



39 20* 





m 



/\ 



m'", 



110 



A 



110 



— 



approximately 



IT 18' 



w 



y\ 



w' 



4.12.9 



s\ 



4.12.9 



z= 



34 43 



34 38 



w 



/\ 



w'", 



4.12.9 



•\ 



4.12.9 



= 





91 6 



V 



/"\ 



^ 



209 



S\ 



209 



= 



26 30 



26 26 



a 



/\ 



A, 



100 



S\ 



203 



= 



54 30 



54 50 



8 



•\ 



8', 



043 



s\ 



043 



= 



approximately 



96 48 



From the measurements marked by an asterisk the axial 

 ratio given on page 62 was calculated. 



