the weakejl Natural or Artificial Electricity. xvit 
of the room, or when a chain falls from it upon the table ; 
then if you let the faid conductor in that flats touch the metal 
plate of the condenfer, whilft the electrical machine is in 
adtion, the plate will afterwards give fufficiently ftrong figns 
of eledtricity, which thews the great power this apparatus has 
of drawing and condenfmg the eledtricity. 
27. Fourthly, The ulual way of rubbing divers bodies, and 
then prefenting them to an electrometer in order to examine 
their electricity, is often infufficient, that is, it makes the expe- 
rimenter believe, that a body has not acquired any electricity 
at all, only becaufe the quantity of it is too fmall to affedt an 
electrometer. In this cale it is very advantageous to rub thole 
bodies with the metal plate of our apparatus, which plate for 
this purpofe muft be naked ; for if the plate be afterwards p re- 
lented to an eledtrometer, this will be electrified confiderablv, 
however little eledlricity the rubbed bodies themfelves may 
have acquired. The quality of this eledtricity, viz. whether it be 
politive or negative, may be eafily afcertained, lince the eledtri- 
eity of the metal plate muft be the contrary of that acquired 
by the body rubbed upon it. Mr. cavallo alfo made ufe of 
this method to difcover the eledtricity of certain bodies *. 
But there is a better method, to be ufed in cafe the bodies to 
be examined are not eafily adapted to the metal plate, which 
method neither Mr. cavallo nor others have known. This 
is the following. The metal plate being laid upon the im- 
perfedtly conducting plane, the body to be tried is rubbed 
again ft, or is repeatedly ftroked, upon it ; which done, the 
plate is taken up, and is examined by an eledtrometer. If the 
body tried by this method is a piece of leather, a firing, a 
piece of cloth, or velvet, or other imperfedt conductor of the 
like fort, the plate will be certainly found eledtrified, and in- 
comparably more by this means than if it w^ere ftroked by the 
lame bodies, whilft handing infulated in the air. In fhort, bv 
either of thofe methods you will obtain fome eledtricity from 
fuch bodies as could hardly be expedted to give any, even when 
they are not very dry. Indeed, coals and metals excepted,, 
* Sec his Treadle on Eledtricity, part IV. chap. vi. 
every 
