Penjield and Ford — Stibiotantalite. 



63 



The only crystals observed which exhibit a marked hemi- 

 morpliic development are represented in figures 2, 3 and 4. 

 They are orientated according to pyroelectric deportment, the 

 a faces to the front all developing negative electricity on cool- 

 ing, as tested by the carmine-sol phur-lycopodinm mixture 

 suggested by Biirker.* The crystal shown by figure 2 has at 

 the top two ridges and a valley, but no 

 reentrant angle at the sides : The two 

 ridges on cooling develop positive, and the 

 valley negative electricity. This crystal 

 is a polysynthetic twin, and may be taken 

 as a type for illustrating the structure of 

 other crystals. The vertical axis is the 

 twinning axis, and the macropinacoid a the 

 composition face. If it is assumed as in 

 figure 5 that a hemimorphic crystal has the 

 prism g (130) and the dome 77 (209) inter- 

 secting the front end of the «-axis only, 

 a lamella in twin position, figure 6 (as indi- 

 cated by the letters underlined), would 

 give a ridge and a valley at the top, cor- 

 responding to figure 2, but also reentrant 

 angles at the sides, which do not occur 

 on the crystal shown in figure 2. It seems 

 therefore necessary to assume that, in addi- 

 tion to g to _the front, there is a corre- 

 sponding g' (130) behind and, as will be 

 shown, also, an i\' (209) behind, figure 7 : 

 These forms appear in twin position as 

 shown in figure 8, and an interspersed twin 

 lamella, as in figure 9, may then show at 

 the sides the prism g' corresponding in 

 direction with g. The prism lettered g, figure 2, is therefore to 

 be regarded as a composite face, composed partly of g and partly 

 of g'. At the top of such a crystal there may be, as in figures 2 and 

 9, a ridge and a valley, provided that in the twin lamella there 

 occurs in connection with g' not r\' sloping to the front but rj 

 sloping behind. As a matter of fact, only a few of the crystals 



* Annalen der Physik, 1900, I, p. 474. Note. Biirker states that with this 

 mixture carmine goes to positively electrified surfaces and sulphur plus 

 lycopodiuni to negative, which seems contrary to reason, for sulphur should 

 become strongly negatively electrified by agitation and go to a positively 

 electrified surface, the same as with the method of dusting with red oxide 

 of lead and sulphur. As a matter of fact, sulphur does go to the positively 

 electrified pole, but each grain, as may be seen with the microscope, is 

 loaded with a fine dust of carmine and gives a red effect. The lycopodium, 

 on the other hand, goes to the positive pole, and is very free from any 

 admixture of either carmine or sulphur, hence it gives with the sulphur- 

 carmine mixture a strong and sharp contrast of color. 



a 



9 



