362 On Crystallography, 



distance required for tlie success of the experiment. In 

 this case, the wax acting on the bowj comrnunicalcs to it 

 an electricitv contrary to its own ; whence it follows that 

 we have inverse cfiecis to the f'oregoino-, f . ^. the side of 

 the stone sohcited by the \ifreous electricitv repels the 

 needle to which we present it, and that which possesses the 

 resinous electricity attracts this needk to it. This niethod 

 is preferable to the first, when the electrical body is very 

 sni;ill, or has but a feeble virtue. 



Fig. 76 B represents the experiment described. We^ 

 there «Jee the tourmahne //' caught by pincers supposed to 

 J)e held in the haiuis of the observer, in such a way that 

 the pole ^ is at a small distance from the bowl a of the 

 needle. C^ is the stick of wax which rests by one of its 

 extremities on a tube of glass U//, and which ads by its 

 part C on the bowl a, in order to produce the vitreous 

 electricity. 



In what has gone before wc have considered the effects 

 of the action exercised on a mineral by another body, so 

 that the former may lie regarded as passive with respect 

 to the latter. What we now call active electricity is that 

 which the mineral excites of itself in the sealintr-wax, by 

 means of friction. In order that rhe experiment ir>ay 

 succeed better, we must, after having heated the stick of 

 wax, flatten it at one end by pressing it on a smooth body. 

 We must afterwards rub this same end with a part of the 

 mineral, which is itself smooth, or at least free from aspe- 

 rities ; then we shall present the wax to the copper needle 

 under which we have placed before-hand another electrified 

 stick of wax, as has been already mentioned. 



Every body, the friction of which thus communicates to 

 wax a certain species of electricity, acquires at the same 

 time the contrary electricity; so that v^e might consider 

 this last electricitv in preference, or, what comes to the 

 same thing, consider the tnineral as being passive with re- 

 spect to tlie wax. But the mineral in which this experi- 

 ment becomes interestins; being conductors of electricity, 

 It is simpler to examine their action upon wax, either be- 

 cause without this expedient we should be obliged to in- 

 sulate them, or because when their vf)lume is somewhat 

 considerable their electricity, by being diflfused over a large 

 surface, would not be sufficiently palpable. 



VV^e have as yet but a small number of substances which 

 excite the vitreous electricity in wax, whereas other sub- 

 stances of an analogous nature produce in it the contrary 



electricity^ 



