THIS rectilineal Direction is observable only as far as the Electricity
can penetrate through unexcited Originally-electrics, and those
perfectly dry; nor is it at all material, whether these Substances are
transparent, as Glass; semidiaphanous, as Porcelain or thin Cakes of
white Wax; or quite opake, as thick woollen Cloth, as well as woven Silk
of various Colours; it is only necessary that they be
Originally-electrics. But the Case is widely different with Regard to
Non-electrics; wherein the Direction, given to the Electricity by the
excited originally-electric, is alter'd as soon as it touches the
Surface of a Non-electric, and is propagated with a Degree of Swiftness
scarcely to be measured in all possible Directions to impregnate the
whole Non-electric Mass in Contact with it, or nearly so, however
different in itself, and which must of Necessity be terminated by an
originally Electric, before the Electricity exerts the least Attraction,
and then this Power is observed first at that Part of the Non-electric
the most remote from the originally-electric. Thus for Example, by an
excited Tube held over it, Leaf Gold will be attracted through Glass,
Cloth, _&c._ held horizontally in the Hand of a Man standing upon the
Floor, and this Attraction is exerted to a considerable Distance. On the
contrary, the rubbed Tube will not attract Leaf Gold or other light
Bodies, however near, through Silver, Tin, the thinnest Board, Paper, or
any other Non-electric, held in the Manner before-mentioned. But if you
rub the Paper over with Wax melted, and by that Means introduce the
originally-electric therein, you observe the Electricity acts in right
Lines, and attracts powerfully. And here I must beg Leave to remind you,
not only of the former Corollary, but of some of the former Experiments
also; by which it appears, that although, to make a Non-electric exert
any Power, we must excite the whole Mass thereof, yet we can excite what
Part, and what only, of an originally-electric we please. Thus we
observe, that Leaf-gold, and the Seed of Cotton-grass, (which grows upon
Boggs and is a very proper Subject for these Inquiries) are attracted
under a Glass Jar made warm[4], and turned Bottom upwards, upon which
are placed Books and several other Non-electrics; and that the Motions
of the light Bodies underneath correspond with the Motions of the Glass
Tube held over them, the Electricity seeming instantaneously to pass
through the Books and the Glass. But this does not happen, till the
Electricity has fully impregnated the Non-electrics, which lie upon the
Glass, which received Electricity is stopped by the Glass, and then
these Non-electrics dart their Power directly through the upper Part of
the Glass after the Manner of Originally-electrics. But if the thinnest
Non-electric, even the finest Paper, as I before mentioned, is held in
the Hand of a Man at the smallest Distance over the Leaf-Gold, and the
Electricity is not stopped, not the least Power will be exerted, and the
Gold will lie still. I must here remark likewise, that this Law of
Electricity is so constant and regular, that I have not found one
Deviation from it; so that even the Quicksilver, spread thin as it
usually is at the Back of a Plate of a Looking-glass, will prevent the
passing through of the electrical Attraction, unless stopped by an
Originally-electric. This Penetration of the electrical Power through
originally-electrics is much greater than has hitherto been imagined,
and has caused the Want of Success to great Numbers of Experiments. I
have been at no small Pains to determine, how far this Power can
penetrate through a dry Originally-electric; and have found by repeated
Trials, that either in a Cake of Wax alone, or of Wax and Resin mixed,
when the Electricity is very powerful, it has passed, I say, in straight
Lines through these Cakes of the Thickness of 2 Inches and 4/10; but I
never could make it act through one of 2 Inches 8/10, for in this it was
perfectly stopped. So that the Cakes commonly made use of to stop the
Electricity, by being too thin, suffer a considerable Quantity of the
electrical Power to pervade them, and be lost in the Floor. I make no
Doubt, if the electrical Power could be more increased, it would
penetrate much further through these Originally-electric Bodies.