SECT. XXVIII. INDUCTION. 285 



but it is shivered to pieces in an instant if it be a bad conductor 

 or too small to carry off the charge. In that case the physical 

 change is generally a separation of the particles, or expansion 

 from the heat, as in trees, where it turns the moisture into 

 steam, but all these effects are in proportion to the obstacles 

 opposed to the freedom of its course. 



Bodies surrounded by non-conductors are said to be insulated, 

 because when charged the electricity cannot escape. When that 

 is not the case, the electricity is conveyed to the earth : conse- 

 quently it is impossible to accumulate electricity in a conducting 

 substance that is not insulated. There are a great many sub- 

 stances called non-electrics in which electricity is not sensibly 

 developed by friction unless they be insulated, because it is 

 carried off by their conducting power as soon as elicited. Metals, 

 for example, which are said to be non-electrics can be excited, 

 but being conductors they cannot retain this state if in communi- 

 cation with the earth. It is probable that no bodies exist which 

 are either perfect non-electrics or perfect non-conductors. But 

 it is evident that electrics must be non-conductors to a certain 

 degree, otherwise they could not retain their electric state. 



A body charged with electricity, although perfectly insulated, 

 so that all escape of electricity is prevented, tends to produce an 

 electric state of the opposite kind in all bodies in its vicinity. 

 Positive electricity tends to produce negative electricity in a body 

 near to it, and vice versa, the effect being greater as the distance 

 diminishes. This power which electricity possesses of causing an 

 opposite electrical state in its vicinity is called induction. A 

 Leyden jar, for example, or glass jar coated half way up both 

 outside and in with tin foil, when charged with positive electri- 

 city, immediately induces negative electricity on the tin foil 

 outside. Notwithstanding their strong mutual attraction they 

 are prevented from coalescing by the glass, which is a non-con- 

 ductor ; but if the tin inside and out be connected by a conduct- 

 ing wire they instantly unite. When a body in either electric 

 state is presented to a neutral one, its tendency in consequence 

 of the law of induction is to disturb the condition of the neutral 

 body by inducing electricity contrary to its own in the adjacent 

 side, and therefore an electrical state similar to its own in the 

 remote part. Hence the neutrality of the second body is de- 

 stroyed by the action of the first, and the adjacent parts of the 



