ELECT 
oontigtious air. The explofion is, therefore, inade in tliis 
part in preference to any other; and the air thus ex¬ 
ploded is (trongly attracted by the fronting part of the 
other body, and mull fly thither in preference to any 
other point. If, moreover, the fronting part be promi¬ 
nent or pointed, this effcCt will be produced in a fuperior 
degree ; and the current of electrified air, which will 
begin very early, will increafe this difpofition to transfer¬ 
ence in this way by rarefying, the air ; a change which 
the whole courfe of eleCtric phenomena fhews to be 
highly favourable to this transference, although we can¬ 
not perhaps form any very adequate notion how it con¬ 
tributes to this effeCt. This feems to be the reafo'n why 
a great explofion and fnap,' with a copious transference of 
electricity, is generally preceded by a hiding noife like 
the rufhing of wind, which fwells to a maximum in the 
loud fnap itfelf. 
XIV. Some bodies upon being rubbed, are electrified 
pofitively, and others negatively ; and the fame bodies 
are capable of being eleClrified pofitively, or negatively, 
as they are rubbed with different fubftances.—Thus- 
fmooth glafs becomes pofitively eleClrified by being rub¬ 
bed with any fubftance hitherto tried, -except the back 
of a living cat; rough glafs becomes pofitively electrified 
. by being rubbed vvirh dry oiled fiIk, fulphur, and metals; 
negativelv, with woollen cloth, fealing-wax, paper, the 
human hand. White (ilk becomes pofitively electrified, 
by being rubbed with black (ilk, metals, black doth ; 
negatively, with paper, hairs, the hand. Black fi Me- will 
be pofitively eleCtrified with red fealing-wax ; negatively, 
with hare’s (kin, metals, the hand. Sealing-wax will be 
pofitively eleCtrified with the hand, leather; woollen cloth, 
paper, hare’s fkin. Baked wood will be pofitively elec¬ 
trified with filk; negatively, with flannel. If thefe and 
other fubftances, being electrified, be brought near to a 
pith-ball or down-feather, as deferibed Prop. XII. it 
will appear whether they are eleCtrified pofitively or ne¬ 
gatively. 
XV. Bodies infulated, if placed within the influence 
of an eleCtrified body, will be eleCtrified, at the part ad¬ 
jacent to that body, in the manner contrary to that of the 
eleCtrified body.—Bring a conductor (without pointed 
wires) near to the glafs cylinder, whilft the machine is 
working; if the conductor be not infulated, it will be 
negatively eleCtrified till it is brought fo near as to re¬ 
ceive fparks from the cylinder: if the conductor be infu¬ 
lated, it will, in the fame fituation, be eleCtrified nega¬ 
tively, in the parts neareft the cylinder, and pofitively in 
the parts more remote ; as may be feen by bringing an 
excited glafs tube (which is pofitively eleCtrified) near 
to a ball fufpended from the conductor. Let two pith- 
balls be fo fufpended by flaxen threads ^as to be in con¬ 
tact when uneleCtrified ; on being brought near to a body 
eleCtrified pofitively, they will repel each other, being 
eleCtrified negatively : if the balls be fufpended in the 
fame manner by filken threads, they will, in the fame 
fituation, be pofitively eleCtrified. 
Let PC, fig. 21, in the EleCtricity Plate III. be an elec¬ 
trified prime conductor, and A B a metallic body placed 
within its atmofphere, but beyond the ftriking diftance. 
Now, from the principles already explained, it is evident 
that the eleCtrical atmofphere of the prime conduCtor 
muft be pofitive or negative. If it be po/itive, then the ad¬ 
jacent part A of the metallic body A B, will be found to 
be eleCtrified negatively ; the remote part B, will be elec¬ 
trified pofitively; and there will be a certain point D, 
in its natural ftate, or not electrified at all.—If the prime 
conductor be charged with negative electricity, then A 
will be pofitive, B negative, and (till fome point, as D, 
will be found uneleCtrified, which is called the neutral 
point. 
Earl Stanhope has demonftrated, by a confiderable 
number of experiments, that the neutral point D, is the 
fourth point of an harmonical divifion of the line CAB. 
Confequently, the points C, A, and B, being given, the 
V©l. VI. No. 361, 
II I C I T Y. 429 
neutral point D, may be always found. For, by the pro¬ 
portion a (fumed by his lordftiip, as the whole line B C is 
to the part C A, fo is the remote end B D to the middle 
term DA; -therefore, by composition, BC-pCA (BA 
+ 2 AC) : CA :: BD + DA (BA) :: AD. Thus, if 
B A be 40 inches, and C A 36, then A D is equal to 
12^- inches.—From the nature of this proportion, it is 
evident, that the neutral point D can never be farther 
from A than half the diftance between A and B, fup- 
poling the eleCtrified conductor PC to be removed to an 
infinite diftance. It is likewife evident, that the evanef- 
cent pofition of the neutral point D nuift be A, when the 
end A of the metallic body AB comes into contuCt with 
the charged body, PC. 
From the above considerations, lord Stanhope has, 
with great ingenuity, proved by an elaborate mathema¬ 
tical demonftration, illuftrated and confirmed by a great 
variety of experiments, that the denfity of an electrical 
atmofphere fuperinduced upon any body muft be inverfe- 
ly as the fquare of the diftance from the charged body. 
Let a circular plate compofed of rofm and fulphur, or 
of fealing-wax, be negatively eleClrified by rubbing it 
with flannel; whilft it is in this ftate, let a metallic plate 
of the fame form and fize, having a glafs handle faftened 
to its center, be placed, by means of the handle, on the 
eleClrified plate ; then receive a fpark from the metallic 
plate with the finger: after which the metallic plate, 
being removed by the glafs handle, will be found to be 
pofitively eleClrified. This is tiie origin and aClion of 
the electrophorus. 
Let one fide of a plate of glafs be eleCtrified pofitively, 
the other fide will attraCl light bodies, being negatively 
eleClrified.—Let a plate of glafs be placed between two 
metallic plates about two inches in diameter fmaller than 
the plate of glafs, and let the plates be fupported by a 
conductor ; upon pofitively electrifying the upper me¬ 
tallic plate, by means of a wire connected with the prime 
conductor, the fluid, not being able to pafs along the 
glafs, will be accumulated upon the part contiguous to 
the upper metallic plate ; whiift the lower metallic plate, 
being within the eleCtric influence of the upper, will be 
negatively eleCtrified. 
XVI. When any eleCtric fubftance is eleCtrified, it will 
continue in that ftate till fome conductor conveys away 
the accumulated, or reftores the deficient, fluid ; which 
will be done more or lefs rapidly, according to the degree 
of conducting power in the conductor, and the number 
of points in which it touches the electric.—When the 
metallic plate in an eleCtrophorus is electrified, (as fug- 
gefted Prop. XV.) by letting it upon the electric plate, 
touching it with the finger, and feparating it fucceftively, 
many fparks may be obtained, without again exciting the 
eleCtric plate ; for this plate being negatively eleCtrified, 
the metallic plate, on being touched with the hand, be¬ 
comes pofitively eleCtrified, and the eleCtric plate remains 
long in its negative ftate, becaufe, not being a conductor, 
its deficiency will be (lowly fupplied from the air where 
its furface is not covered. See this fully exemplified in 
the explanation and figure of the eleCtrophorus, p.419. 
If a glafs veil'd, a common drinking-glafs, for inftance, 
held in the hand, receive the eleCtric fluid on the infide 
from a wire, or chain, fixed on the conductor, pith-balls, 
placed under the veflel upon a conducting fupporter, will 
continue long in motion.—Let a plate of glafs be eleCtri¬ 
fied in the manner deferibed in Prop. XV. Bccaufe one 
fide of the plate is pofitively eleCtrified, and the other 
negatively, if a communication is made from one metal¬ 
lic plate to the other by means of fome conductor, part 
of tire accumulated fluid will fuddenly pafs to the fide 
which is deficient; upon a fecond application of the plates 
of metal to the glafs, there will be a fecond explofion. 
XVII. If a glafs plane, or cylindrical veflel, coated on 
both fides with tin-foil, or any other conducting fub¬ 
ftance, be charged , that is, pofitively eleCtrified on one 
fide ; and confequently negatively eleCtrified on the other; 
j R a com. 
