22 Mr. Cavallo’s Obfervations , &c. 
only whilft it remains in contact with the rubber; for after 
that, its capacity being diminifhed, the electric fluid endea- 
vours to efcape from it. Thus we may conceive how every 
other part of the glafs acquires the electric fluid, &c. and 
what is faid of the cylinder of an electrical machine may, 
with proper changes, be applied to any other eleCtric and its 
rubber. 
It appears, therefore, that according to this theory, a part 
of the rubber, viz. that which the furface of the glafs cylinder 
enters in turning round, muft ferve to furnifh the eleCtric fluid 
to the glafs, and the upper part muft be poftefled of a negative 
eleClricity capable of inducing a pofitive eleCtricity in the glafs 
contiguous to it. In faCt, this feems to be confirmed by the 
general practice and experience; for that rubber anfwers beft 
for an ufual eleCtrical machine, which can eafily conduCt the 
eleCtric fluid with its under part, and the upper part of which 
is more ready to acquire, and to retain, the negative eleCtricity ; 
hence the rubbers are generally furnifhed with amalgam below, 
and with a piece of iilk above; hence alfo, if the cylinder 
of the machine be turned the contrary way, it will produce 
little or no eleCtricity. 
It often happens, that the part which conducts the fluid, 
and that which acquires the eleCtricity contrary to that of- the 
eleCtric, are not fo difpofed in a rubber as the one above de- 
fcribed ; but it remains always true, that the rubber muft be 
poftefled of thofe two properties, viz. to conduCt the eleCtric 
fluid very readily in one or more parts, and to acquire, as well 
as retain, on other parts, an eleCtricity contrary to that ac- 
quired by the eleCtric that is to be rubbed with it. 
Litde St. Martin’s-Laae. 
Nov. 7 . , 7 s 7 . T. CAVALLO. 
