ELECTRICITY. 321 



cited to exhibit electricity. The best conductors 

 are the metals and water. 



The equilibrium in the natural electricity of 

 bodies is disturbed most, when an electric and a 

 non-electric are rubbed together. Thus, if glass 

 be rubbed by a piece of flannel, the electricity 

 which is excited will leave the flannel, and be accu- 

 mulated upon the glass, which will then have more 

 than its natural quantity. If then a conducting 

 body, as the finger, be presented to the glass, the 

 superabundant quantity in the glass will pass into 

 the finger, since there is a tendency in all bodies to 

 preserve the equilibrium of the electric fluid. 



It is only during this passage or discharge from 

 one body to another, that electricity is rendered 

 visible and luminous. 



When a body has more than its natural quantity 

 of this fluid, it is said to be positively electrified, and 

 the electricity it contains is called positive elec- 

 tricity ; and when it has less than its natural share, 

 it is said to be negatively electrified. When a con- 

 ductor is so surrounded by non-conductors, that 

 the electric fluid cannot pass from it to the earth, 

 it is said to be insulated, as a piece of metal sup- 

 ported upon a glass pillar, for instance. 



As the quantity of electricity that could be col- 

 lected by exciting rods of glass or sealing-wax is 

 verv small, machines have been contrived for rub- 

 bing together electrics and non-electrics, which are 

 called electrical machines. 



Plate 18. fig. 1. represents an electrical machine 

 of the most simple construction. A Bis a strong 

 board which supports all the parts of the machine, 

 and which is fastened to a steady table by means 

 of one or more iron or brass clamps. The glass 

 cylinder D, quite dry and clean in its inside, is 



vol. I. y 



