Experiments and Inductions. 85 



crack in the centre c,but the edges of the crack quite close together, 

 is stuck on to the coated conductor, and a spark taken from a brass 

 ball held as shown in fig. 2. The spark traversed the surface of 

 the shell lac and passed through the crack, as shown by the dotted 

 line. The distance is much greater than if the spark had to pass 

 through the air alone without the shell of lac to assist it. 



c. A glass plate, when hot, was coated with lac, and equal cir- 

 cles of tin-leaf stuck on the centre of both sides. It was held 

 touching a brass ball projecting from the prime conductor. On 

 working the machine, the plate discharged itself by occasional 

 sparks round the edge of the plate; and when the action of the 

 machine was reduced, a corona of light appeared at the edges of 

 the tinfoil, which fluctuated in breadth and intensity in accord- 

 ance with the action of the electric machine, which, if kept mode- 

 rate, did not cause discharge by spark round the edges (fig. 3). 



9. In the still atmosphere of a small room, the cap, wire, and 

 gold-leaves of an electroscope were mounted on an insulating 

 stand and charged. Various non-conducting bodies, such as 

 clear amber, lac, heated crystal, &c, were then successively 

 brought close to the leaves in various positions. Attraction was 

 manifested between the excited gold-leaves and the non-conduct- 

 ing body, which of itself was entirely free from excitement, i. e. 

 it did not attract uncharged gold-leaf. 



a. A stick of clear lac was mounted horizontally on an insu- 

 lating stand so that its conical extremity was level with the 

 gold-leaves. By gently moving the stand, the pointed surface 

 of lac was brought close to the nearest leaf without touching it. 

 When the distance was about ^th of an inch between extremity 

 of leaf and lac-surface, the force kept the leaf suspended and 

 steady as shown in fig. 4. 



b. The charge of electroscope was then changed from positive 

 to negative, and the same point of lac held as before with the 

 same result of attraction — showing that it was not due to excite- 

 ment pertaining to the lac. 



c. The surface of lac brought to touch the leaf; the attraction 

 continued. Removing it gently out of contact, the attraction 

 remained as before. 



d. Sometimes a small speck of gold-leaf separates and adheres 

 to the lac ; repulsion is then manifested at the point where the 

 speck lies, but attraction continues at the other parts. (See 50J.) 



10. a. The mechanical effects of the electric explosion were 

 noted between two balls coated with the soft wax. On dis- 

 charging a Leyden jar between balls thus coated (about j^th of 

 an inch in thickness) and not in contact, the appearance on each 

 was the same, viz. a small circular portion of the coating were 

 thrown off and the surface of the ball exposed, forming a perfect 



