ON ELECTRICITY IN EQUILIBRIUM. 509 



with it, as well as by many other phenomena ; and this attraction of the 

 fluid of the first body, to the matter of the second, is precisely equal to its 

 repulsion for the quantity of the fluid which naturally belongs to the 

 second, so as to saturate the matter. For the excess or deficiency of the 

 fluid in the first body does not immediately produce either attraction or 

 repulsion, so long as the natural distribution of the fluid in the second body 

 remains unaltered. 



Since also two neutral bodies, the matter which they contain being 

 saturated by the electric fluid, exhibit no attraction for each other, the 

 matter in the first must be repelled by the matter in the second : for 

 its attraction for the fluid of the second would otherwise remain uncom- 

 pensated. We are, however, scarcely justified in classing this mutual 

 repulsion among the fundamental properties of matter ; for useful as these 

 laws are in explaining electrical appearances, they seem to deviate too far 

 from the magnificent simplicity of nature's works, to be admitted as 

 primary consequences of the constitution of matter : they may, however, 

 be considered as modifications of some other more general laws, which are 

 yet wholly unknown to us. 



When the equilibrium of these forces is destroyed, the electric fluid is 

 put in motion ; those bodies which allow the fluid a free passage, are called 

 perfect conductors ; but those which impede its motion more or less, are 

 nonconductors, or imperfect conductors. For example, while the electric 

 fluid is received into the metallic cylinder of an electrical machine, its 

 accumulation may be prevented by the application of the hand to the 

 cylinder which receives it, and it will pass off through the person of the 

 operator to 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 accumulated, 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 different 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 

 n/ay be communicated in any part of its surface, by the contact of an 



