ELECTRICITY. 371 



glass, it shows that they are electrified positively, 

 as well as the glass. On the contrary, if it increase 

 their divergency, and attract them, it shows their 

 electricity to be of a kind opposite to that of the 

 glass; that is, negative. And it must be remem- 

 bered, that the electricity of the balls which do 

 not touch, or receive any electricity from the wires 

 of the jar or battery, is always contrary to that 

 with which they are charged; for all bodies placed 

 within the influence of electrified bodies are 

 affected with the contrary electricity. 



In order to ascertain the kind of an exceeding 

 small degree of electricity, it will be convenient to 

 have a very light body ; as a piece of downy feather 

 hanging by a silken thread. This light body, when 

 it is once electrified, either positively or negatively, 

 will retain its virtue a long time, with very little 

 loss. If then, any body, the electricity of which 

 is unknown, be brought to it, the feather will be 

 repelled by it, if it be of the same kind with its 

 own, and attracted if it be the contrary to it. 

 The silk by which it is suspended should be a 

 single thread, as it comes from the worm ; or at 

 least a very few of those threads; and the whole 

 should be as light as possible. 



As glass is apt to attract and condense the mois- 

 ture from the air, (in which case it conducts the 

 electricity over its surface,) it is best to cover with 

 seaUng-xvax, or to varnish over, the glass pillars of 

 the machine, as also all the glass stands, and other 

 articles used for insulating; for when varnished, 

 and especially when covered with sealing-wax in 

 the dry way, they attract the moisture, either not 

 at all, or in a much less degree; and, of course, 

 they insulate much better. 



b b 2 



