Experiments and Inductions. 95 



forces at the surface opposite to the one excited is a phenomenon 

 that invariably takes place in a few hours after excitement. 



36. Two cakes of sulphur. 4 inches in diameter and *| an inch 

 thick, were luted together at the edges with soft beeswax, then 

 placed upon a strongly excited plate of sulphur of same size that 

 was stuck upon the top of a cylinder of sealing-wax. A brass 

 plate 6 inches in diameter was placed on the top of all and 

 touched (fig. 20). These were allowed to remain in position 

 twenty-four hours. The double plate was then removed and 

 applied to electroscope ; the under surface showed negative, the 

 upper positive. These surfaces were then completely discharged 

 by gas bead-flame ; and the luting being cut, the interior sur- 

 faces of the plates showed no excitement when each was sepa- 

 rately applied to electroscope. Again tried with elapsed time 

 twenty minutes, still no excitement. 



37. An excited cake of sulphur is placed with excited surface- 

 downwards resting on a cake of fine, sealing-wax that is stuck 

 on top of a lac cylinder (fig. 21). At first the force at a centre of 

 top surface was 85° x 8 negative. It then gradually diminished, 

 until in an hour it became zero. After this it gradually became 

 positive; and when eight hours had elapsed the force was mea- 

 sured, 57° positive, and after seventeen hours 50° positive. The 

 sulphur cake was then taken off, and the force at the centre of 

 its excited surface found to be 95° x 8 negative; and at centre 

 of wax cake 95° x 2 positive. 



38. Two cakes of sulphur were each negatively excited on both 

 sides as equally as possible (fig. 22). They were then laid close 

 together so as to form a double cake, and the outside of this 

 double cake completely discharged by holding a small flame on 

 each side simultaneously. The plates were then separated and 

 reversed, the inside surfaces being now the outside of double 

 plate, which was discharged by two small flames as before. The 

 plates were again separated and examined singly at the electro- 

 scope. The sides first discharged were found positive, and the 

 others negative. 



39. A negatively excited cake of sulphur 4 inches in diameter, 

 resting on three sticks of wax and with excited side uppermost ; 

 on this is placed another cake, b, of 2 inches diameter, and on it 

 another, c, of 4 inches diameter (fig. 23); b takes on slowly but 

 continuously an inductive charge positive. In ten minutes there 

 is no sensible effect ; but in two hours it is quite distinct, showing 

 about 50° positive, when adjacent excited surface is 90° x 4 

 negative. The cake c is also induced positive. With b composed 

 of sealing-wax or beeswax, the effect takes place more quickly 

 than with sulphur. 



40. Two 4 -inch cakes of sulphur are excited negative on 



