of Electricity by Pressure. 205 



Biot, in his Traite de Physique, cites from the manuscripts of 

 that celebrated philosopher the observations upon which he 

 was induced to found this conjecture. 



An experiment made by M. Libes with gummed taffeta 

 seems to accord with this view of the subject. This experi- 

 ment consists in taking a disk of metal, which is held by an 

 insulating handle, and pressing it on gummed taffeta; the 

 taffeta acquires the vitreous electricity, and the disk the 'resi- 

 nous electricity. The effect is the more striking in proportion 

 as the pressure is stronger; but it ceases as soon as the taf- 

 feta has lost that glutinosity which renders its surface easily 

 compressible. If, on the contrary, the metal is rubbed over 

 the taffeta, the metal takes the vitreous, and the taffeta the 

 resinous electricity. Having recently had occasion to repeat 

 the excellent observations of M. Hauy on the electrical pro- 

 perties which simple pressure with the fingers imparts to 

 Iceland spar and to some other mineral substances, I was 

 struck with the different effects produced by the bodies be- 

 tween which they were pressed, accordingly as they were 

 more or less flexible. I wished at first to examine, m these 

 and the preceding experiments, what might be the proper 

 mfluence of the condensation of parts on the development of 

 electricity, not only in minerals, but in other bodies, susceptible, 

 like them, of experiencing this effect. In the course of this 

 examination I have been led to a general result, which seems 

 to promise, one day, to throw light on the yet unknown 

 causes of the development of electricity. This result may 

 be expressed in the following terms : When any two bodies 

 whatsoever, one of which is elastic, are insulated, and pressed 

 one against the other, they remain in two different electric 

 states ; but the excess of contrary electricity which they re- 

 tain, on escaping from the compression, is in proportion as 

 one of the bodies is what is called a bad conductor. The 

 effect thus produced in this latter case, is incomparably more 

 powerful than those arising from simple contact in the expe- 

 riments of Volta. 



The most simple method of obtaining these results consists 

 in forming small disks of the substances with which the ex- 

 jjeriment is to be tried, of the thickness of some millimetres; 

 they are fitted to handles, by which tliey are perfectly insu- 

 lated*. One of these handles is then taken in each hand, and 



the 

 * Each handle is composed of a solid glass tube {Uibe plein en vcrre) 

 covered with lac varnish, and terminated by a wooden knob, which is used 

 in order to avoid the friction of the hand iiijon the glass. The small 

 disks are fixed at the extremity of the tubes with lac. licforc using this 

 instrument, it is advisable to try with the elettroicope whether the handle 



exhibits 



