206 M. Becquerel on the Development 



the substances are pressed, for an instant, one against the 

 other. After withdrawing them from contact, the quantity of 

 electricity acquired by them is ascertained by the electroscope. 

 A single contact is usually sufficient to repel the small disk of 

 the electroscope of Coulomb; but on repeating these contacts, 

 any electroscope whatever may be strongly charged. Some- 

 times the electricity is so strong, that the disk immediately 

 attracts the small light bodies which are presented to it. Let 

 us suppose, for instance, two insulated disks, the one of cork, 

 the other of caoutchouc; after pressure, the latter has acquired 

 the resinous, and the former the vitreous electricity. If we 

 press, in the same manner, the cork on the rind of an orange, 

 both being insulated, the cork acquires the vitreous electricity, 

 and the orange-peel the resinous. Finally, the orange-peel, 

 pressed on the caoutchouc, takes the vitreous electricity, and 

 imparts the resinous to the caoutchouc. 



Pressure exerted upon insulated mineral substances pro- 

 duces analogous effects. Iceland spar, sulphate of lime, 

 fluate of lime, sulphate of barytes, &c., when pressed by the 

 disk of cork, acquire an excess of vitreous electricity, whilst 

 the disk itself contracts an excess of resinous electricity. 

 Disthene and retinasphaltum, on the contrary, have the resin- 

 ous electricity. 



Coal, amber, copper, zinc, silver, &:c., when pressed by the 

 insulated disk of cork, receive an excess of resinous elec- 

 tricity, and the cork receives an excess of the vitreous. 



In all the preceding experiments, the two substances sub- 

 jected to pressure were insulated, in order that the species of 

 electricity acquired by each of them might be separately 

 studied; but, as might be expected, the same effects take 

 place when a single body is insulated, and the other commu- 

 nicates with the common reservoir. The insulated body then 

 acquires by pressure the same kind of electricity as when the 

 body upon which it was pressed was also insulated ; but the 

 electricity acquired by the latter cannot be perceived, since it 

 escapes into the earth. 



For instance, an insulated disk of cork, pressed upon Ice- 

 land spar, fluate of lime, sulphate of lime, &c., acquires the 

 resinous electricity; but when pressed upon copper, zinc, and 

 the other substances, it retains, after the compression, an ex- 

 exhibits any marks of electricit}'. If any appear, the electricity may be 

 expelled by heating the tube in the flame of a taper. In order to ascer- 

 tain to what degree the lac may influence the electric effects of disks by 

 pressure, these disks must be pressed hard upon bodies incapable of giving 

 out much electricity ; it is then perceived whether or not a development 

 of electricity takes place. 



cess 



