Ekctricity. 207 



two kinds of electricity. One of them is a small bar of Iceland 

 spar fixed to the end of a needle or lever, which is then sus- 

 pended by the middle so as to be balanced by a thread of silk. 

 When the spar is pressed between the fingers it becomes posi- 

 tively electric, and then the electricity of another body, how- 

 ever excited, as of a gem by friction, is ascertained by its at- 

 traction or repulsion of the spar. The second instrument is 

 formed of a piece of sealing-wax flattened at one end so that it 

 may stand on a table, and at the other supporting the point of 

 a needle ; a. needle of silver or copper terminated at the ex- 

 tremities by beads, moves on this as on a centre. To charge 

 this apparatus a piece of amber or sealing-wax is to be excited 

 negatively by friction, and then by touching the needle it be- 

 comes similarly electrified, and is then ready to indicate by 

 attraction or repulsion the kind of electricity possessed by 

 another body. 



M. Haiiy has lately remarked upon the extreme permanency 

 of the electrical states of these two apparatuses. His attention 

 was drawn to this circumstance from the perfection of their 

 action during extremely moist weather, and he ■was induced to 

 make a few experiments on the subject. The permanency of 

 the electricity excited on the spar depends on the difficulty of 

 adhesion between it and water. In damp weather no moisture 

 deposits on it, so that electricity given to it is perfectly retained. 

 Even if it be dipped in water and afterwards pressed without 

 wiping, it becomes strongly electric, because no water adheres 

 to its surface to conduct the power away ; and M. Haiiy at last 

 ascertained that immersion in water was not sufficient to re- 

 move electricity previously communicated to it. The per- 

 manency, therefore, of its electric state in the atmosphere, and 

 the value of this property, may easily be conceived. If the water 

 be rubbed on the surface of the crystal so as actually to wet 

 it, then no electricity is generated by pressure, and what may 

 have previously been generated is of course dissipated. 



M. Haiiy has observed also that filiate of lime and the 

 «uclase also acquired electricity by pressure, though not so 



