LESSOiVS IN ELECTRICITY. 



167 



the needle; while cactus, blackthorn, and rose, fell more and more 

 behind the needle in sharpness. Calling, for example, the charge ob- 

 tained from euphorbia 90, that obtained from the needle was 80, and 

 from the rose only 53. 



18,24- 



4f9 



Fig. IT. 



Considering that the electricity is self-repulsive, and that it heaps 

 itself up upon a point in the manner here shown, you will have little 

 difficulty in conceiving that, when the charge of a conductor carrying 

 a point is sufficiently strong, the electricity will finally disperse itself 

 by streaming from the point. 



The following experiments are theoretically important : Attach a 

 stick of sealing-wax to a small plate of tin, so that the stick may stand 

 upright. Heat a needle and insert it into the top of the stick of wax ; 

 on this needle mount a carrot. You have thus an insulated conduct- 

 or. Stick into your carrot at one of its ends a sewing-needle, and 

 hold for an instant your rubbed glass rod in front of this needle with- 

 out touching it. What occurs ? The negative electricity of the car- 

 rot is discharged from the point against the glass rod. Remove the 

 rod, test the carrot: ft is positively electrified. 



And now for another experiment, not so easily made, but still cer- 

 tain to succeed if you are careful. Excite your glass rod, turn your 

 needle away from it, and bring the rod near the other end of the car- 

 rot. What occurs ? The positive electricity is now repelled to the 

 point, from which it will stream into the air. Remove the rod and 

 test the carrot : it is negatively electrified. 



Again, turn the point toward you, and place in front of it a plate 

 of dry glass, wax, resin, shellac, paraffine, gutta-percha, or any other 

 insulator. Pass your rubbed glass tube once downward or upward, 

 the insulating plate being between the excited tube and the point. 

 The point will discharge against the insulating plate, which on trial 

 will be found negatively electrified. These experiments, I may say, 

 were discussed, and differently interpreted by the two philosophers, 

 during an important correspondence between Faraday and Prof. 

 Riess. 1 



1 Philosophical Magazine," vol. xi., 1856. 



