E. MINERALOGY AND GEOLOGY. 219 



E. MINERALOGY AND GEOLOGY. 



THERMO-ELECTRIC PROPERTIES OF MINERALS. 



An interesting series of experiments has been carried on 

 by Hankel upon the thermo-electrical properties of some min- 

 eral species. It is a familiar fact that crystals of tourmaline 

 when subjected to a decided change of temperature develop 

 electricity, and those crystals which are differently formed 

 at the two extremities show at one end positive and at the 

 other negative electricity. Crystals of other species, as, for 

 instance, topaz, which do not possess this hemimorphic 

 character, act differently when subjected to the same con- 

 ditions, as shown by Hankel. The electricity which is de- 

 veloped at both extremities of the same axis is alike for in- 

 stance, positive while the opposite or negative electricity is 

 shown at both extremities of another axis. The investiga- 

 tions just made public have special reference to several spe- 

 cies whose crystals are developed alike in the direction of 

 all lateral axes, but this development is different from that 

 in the vertical direction. For example, crystals of apophyl- 

 lite, when subjected to sudden cooling, develop at the ex- 

 tremities of the vertical axis positive electricity, while later- 

 ally that is, in all the planes of the prisms the electricity 

 is negative. Also crystals of vesuvianite (idocrase) show 

 similar relations, the terminal plane and the pyramidal plane 

 have one character, positive; while the prismatic planes, 

 corresponding to the lateral axes, are negative. Crystals of 

 emerald and beryl have not a very decided electrical char- 

 acter, but here also it is true that the end planes have one 

 electrical character and the lateral planes another. 



It may be added that in other species, as, for instance, 

 prehnite, Hankel has found a large number of positive and 

 negative electrical poles distributed over the surface of a 

 crystal. 



MINUTE WORK IN MINERALOGY. 



As an example of minute work in mineralogy, it may be 

 interesting to note the recent investigations of Baumhauer 



