ON ELECTRICITY IN EQUILIBRIUM. 66'l 



the ground; hence the human body is called a conductor. But when the 

 metallic cylinder, or conductor, of the machine is surrounded only by dry 

 air, and supported by glass, the electric fluid is retained, and its density 

 increased, until it becomes capable of procuring itself a passage, some inches 

 in length, through the air, which is a very imperfect conductor. If a person, 

 connected with the conductor, be placed on a stool with glass legs, the 

 electricity will no longer pass through him to the earth, but may be so ac- 

 cumulated, as to make its way to any neighbouring substance, which is capable 

 of receiving it, exhibiting a luminous appearance, called a spark; and a 

 person or a substance, so placed as to be in contact with nonconductors only, 

 is said to be insulated. When electricity is subtracted from the substance 

 thus insulated, it is said to be negatively electrified, but the sensible effects 

 are nearly the same, except that in some cases the form of the spark is a little 

 different. 



Perfect conductors, when electrified, are in general either overcharged or 

 undercharged with electricity in their most distant parts at the same time; but 

 nonconductors, although they have an equal attraction for the electric fluid, 

 are often differently affected in difterent parts of their substance, even when, 

 those parts are similarly situated in every respect, except that some of them have 

 had their electricity increased or diminished by a foreign cause. This property 

 of nonconductors may be illustrated by means of a cake of resin, or a plate 

 of glass, to which a local electricity may be communicated in any part of 

 its surface, by the contact of an electrified body; and the parts thus electri- 

 fied may afterwards be distinguished from the rest, by the attraction which 

 they exert on any small particles of dust or powder projected near them; the 

 manner, in which the particles arrange themselves on the surface, indicating 

 also in some cases the species of electricity, whether positive or negative, 

 that has been employed ; positive electricity producing an appearance some- 

 what resembling feathers; and negative electricity an arrangement more like 

 spots. The inequality in the distribution of the electric fluid in a noncon- 

 ductor may remain for some hours, or even some days, continually di- 

 minishing till it becomes imperceptible. 



These are the fundamental properties of the electric fluid, and of the dif-^ 

 ferent kinds of matter as connected with that fluid. We are next to examia<i: 



VOL. I. 4iX 



