On Crystallography , 361 



Electricity by communication, employed alone, may 

 ^erve to discover the presence of a metal mixed in a con- 

 siderable quantity with a sionc, as takes place with the 

 iron which enters into the composition of jaspers. In 

 order to ascertain this character, we insulate ihe sione on a 

 small stalk, so as to bring it in contact with an electrical 

 conductor, and we judge whether the stone is electrical or 

 not by communication, according as the touch of the fin- 

 ger or the ball of the discharger draws sparks from it. 



The electricity by friction observed comparatively in two 

 different stones may assist in distinguishing them from 

 one another. The cymophane when polished but not cqt^ 

 which presents nearly the same appearance wiih mother-of- 

 pearl feldspar, called moon-stone, ditfers from it by the 

 great facility which it has of being electrified by friction, 

 whereas the same method succeeds but ineiFectually and 

 feebly on feldspar. 



The simplest apparatus for experiments of this kind 

 consists of a small copper needle a h (fig. 76, A) termi- 

 nated hy two bowls and moveable on a pivot. After hav- 

 ing rubbed the mineral several times on a piece of cloth, 

 we must present it to one of the bowls, and we may judge 

 pretty nearly of the strength of the electricity, by the di- 

 stance at which this bowl bes^ins to be attracted. 



With respect to substances electrical by heat, such as 

 the tourmaline, we make use of the same apparatus when 

 we merely wish to ascertain what they are. But it is in- 

 teresting to be afterwards able to determine the parts in 

 which the two electricities reside. To effect this lake a 

 stick of sealing-wax, to the extremity of which a silk 

 thread is attached little better than one eighth of one inch 

 in length : after having rubbed this stick, present by turns 

 jhe two opposite sides of the substance, for example, the 

 two summits of a tourmaline, at a small distance frf)m the 

 silk thread. If the summit which faces the thread be the 

 seat of resinous electricity, there will be repulsion. In the 

 contrary case the thread will be attracted. 



We mav vary this experiment, by placing the stick of 

 wax, after having rubbed it, below one of the two bowls 

 which terminate the needle, at the distance of about a quar- 

 ter of an inch. For the greater simplicity, we may give 

 such a height lo the stand of the needle, that the stick of 

 wax \yhen resiintr by the rubbed extremity on another stick 

 or on a glass tube placed transversely, and by the other ex- 

 .trcmiiy on the table which holds the apparatus, is ar tiie 



distance 



