152 BELL SYSTEM TECHNICAL JOURNAL 



Epsomite gives a marked response but the crystals are generally small and 

 they do not weather well. There is some possibility, however, that they 

 can be made artificially. 



Boracite gives a marked response, but boracite alters slowly. Its imper- 

 manence may bar it for some uses. 



Stibiotantalite occurs only in thin scales, and the necessary cuts must be 

 made in the most wasteful way. Twinning is prevalent and the composi- 

 tion varies widely. 



Scolecite occurs only as small crystals a few millimeters in diameter and a 

 centimeter or so in length, uniformly twinned. 



lodyrite has been found to be active by other investigators. It is 

 electrically conductive, very soft and not very common. 



Struvite is soft, unstable, and occurs only in small crystals. 



Zunyite occurs only in minute crystals. 



Langbeinite slowly changes its crystal structure. It may be made ar- 

 tificially so may be of some use if it can be kept from alteration. 



Leucophanite and Meliphanite are related minerals. Neither seems to 

 occur in good (i.e., homogeneous and untwinned) crystals of usable size. 



Wurtzite does not appear very active but good crystals were not 

 obtainable. 



Tiemannite crystals were also unobtainable, but fragments of massive 

 tiemannite responded. Crystals might respond more energetically if they 

 were obtainable, but minerals that are too difficult to get would not be of 

 practical use. 



Epistilbite occurs only in small specimens, uniformly twinned. 



The mineral clinohedrite is strongly active but crystals are very rare. 



Cronstedtite and Edingtonite are very weakly active. Crystals of these 

 are very rare. 



