PHYSICAL PROPERTIES OF MINERALS. 311 



ill axotomous Triphane-spar, in prismatic Zinc-baryte, &c. 

 The opposite extremities of the crystals assume in these 

 species opposite kinds of electricity, and they possess there- 

 fore electric axes. Tetrahedral Boracite shews four electric 

 axes, coinciding with the rhombohedral axes of the hexa- 

 hedron. This difference in the electric phenomena is very 

 often accompanied by a different configuration of the oppo- 

 site terminations of crystals (. ICO.). 



The processes employed in producing and observing the 

 electric phenomena, and the small apparatus required, may 

 be found in many works, both described and illustrated by 

 figures. Perhaps these phenomena will prove in future 

 more useful for the purposes of Natural History, than has 

 hitherto been the case, since, in respect to minerals, they 

 have been too generally considered as mere physical cu- 

 riosities. 



. 212. TASTE. 



Several minerals, solid as well as fluid, produce 

 a sensible taste. Most of the solid ones are taste- 

 less. This difference yields very useful general 

 characters. 



All the Acids and Salts produce some taste. The salts 

 found in nature, commonly not shewing any of the charac- 

 ters required for their exact determination, their taste is 

 almost the only one left to which we possibly may recur ; 

 and for this reason the differences in the kinds of taste have 

 been provided with particular denominations. The follow. 

 ing expressions have been employed : 



1, Astringent for the taste of vitriol; 



2, Sweetish for the taste of alum ; 



3, Saline for the taste of common salt ; 



4, Alkaline for the taste of soda ; 



5, Cooling for the taste of saltpetre ; 



6, Sitter for the taste of epsom salt ; 



