Heat and Static Electricity. 265 



connected with the earth. Above the knob of the jar is the in- 

 candescible platinum wire. As the electricity of the inner coat- 

 ing ( + or — ) is discharged^ that of the outer coating (— or + ) is 

 released, and, traversing the wire of the galvanometer, deflects its 

 needle. The effect may be increased by using a battery of jars 

 as above, or by placing the discharging- wire between the knobs 

 of oppositely electrified jars whose outer coatings are connected 

 with the opposite terminals of the galvanometer. By such means 

 more striking effects can be produced than by the uncondensed 

 electricity of the conductor of the machine. 



§ 46. The assertion that glass is a good conductor of electri- 

 city when red-hot at once suggests a possibility of connecting 

 such a statement with the above experiments of discharge. 



§ 47. Experiment. — Nevertheless this statement is true; for 

 if the end of a glass tube be made red-hot, it may indeed be 

 placed upon the top of a -f or — charged electroscope (being 

 held by the cool end) without discharging it; but if, when so 

 resting, it be touched by a wire earth-connected, the charge is 

 immediately and completely lost. 



§ 48. Experiment. — The end of a stout glass tube is heated 

 red-hot ; on touching with the hot part either conductor of an 

 electric machine, it is found that both + and — electricities 

 may be abundantly conveyed from the conductor to the electro- 

 scope. This faculty continues with both electricities after the 

 glass has ceased to be incandescent at all. It ceases with -f 

 before it ceases with — ; indeed with the latter it is retained 

 almost to a temperature as low as 100° C. 



§ 49. Experiment.- — In examining the discharging-power of 

 red-hot glass some noteworthy facts are observed. When the 

 red-hot glass is insulated, as when it forms the extremity of a long- 

 glass rod, there is a difference in the behaviour of -f and — 

 electricities. With + the hot glass merely acts as an inductive 

 discharger; the leaves temporarily collapse, and return to their 

 divergence when the glass is withdrawn. When the electroscope 

 is charged with — electricity, the first approach of the hot glass 

 causes partial collapse, as by simple induction. On withdrawing 

 the glass, further collapse ensues ; on bringing the glass again 

 near, the same amount of divergence is produced which existed 

 before withdrawal. 



§ 50. On the 4th of January, 1873, at 3-4 p.m., I observed 

 that the difference between the discharging-power for + and — 

 electricity was greatly diminished and sometimes extinguished. 

 There was at the time a severe storm over London, unaccompanied 

 however, as far as I know, by electric atmospheric disturbance. 

 On inquiry at the Observatories of Kew and Greenwich, Mr. 

 Whipple informs me that, for a temporary reason, there is no 



