96 On the Generation of Statical Electricity. 
them and the excited glass, the desired result will be obtained ; 
and the question resolves itself into the proper manner of applying 
the best substance. If an oxidizable metal or amalgam be em- 
ployed, the oxide which is formed will answer this purpose itself, 
to a certain extent, as the oxides of the metals are imperfect con- 
ductors. For this purpose the oxygen of the air is of service. 
The prominent points in such cases will be kept bright by the 
rubbing of the glass. The oxides thus formed, continue to in- 
crease in most cases, particularly with the amalgams, and ulti- 
mately fill up the intervals; then sustaining the pressure of the 
glass, all farther action of the exciting points of the amalgam 
ceases : in such cases, the surfaces of the amalgam require renew- 
al. The oxides, as will be seen hereafter, are but feeble genera- 
tors. ‘Tallow, lard, and substances of a similar nature, answer 
the purpose much better, and hence they were soon discovered to 
be of service in the earlier researches of electricians. The tallow 
or lard being spread over the surface, or mixed up with the amal- 
gam, surrounded each exciting point of the rubber with a non- 
conducting medium, and hence fulfilled the required conditions. 
As, however, these substances readily combine, mechanically, 
with the metallic oxides, forming a black, adhesive mass, which 
collects on the glass, soiling its surface, and is troublesome to re- 
move, bees-wax and shellac were substituted, both of which sub- 
stances, when properly applied, answer the purpose remarkably 
well. Neither of them soils the glass, and what is of much im- 
portance, they give rubbers permanent in their action. 
The question now arises, whether there be not other parts of 
the rubber, besides those surrounding the exciting points, which 
may have an injurious effect? "That portion which precedes the 
first exciting points at the entrance of the glass, obviously can do 
no harm, as the glass is supposed not to be excited when passing 
in their vicinity ; but the case is materially different with the op- 
posite termination of the rubber, which, not being pressed against 
the glass, is highly injurious—abstracting largely from the elec- 
tricity previously generated. This has also been observed by 
electricians, who do not, however, appear to have proposed any 
substantial remedy ; the best hitherto given, apparently, depends 
for its success on the regularity of the pressure ;* and still another 
plan which is liable to the same difficulty.| The silk flap, whose 
* Parlington’s Nat. Phil., p. 151. t Nicholson, Phil. Trans., 1789. 
