264 Mr. F, Guthrie on a Relation between 



electricities, have their caps connected by wires with two foil- 

 covered balls standing on insulating stands. A loop-wire of pla- 

 tinum connected with a battery is equidistant from the balls and 

 between them. Whatever the temperature of the wire may be, 

 the leaves fall equally in the two electroscopes; that is, both fall 

 equally quickly with a white-hot wire, and equally more slowly 

 with a red- J lot wire. 



§ 41 . Experiment. — By means of a plano-convex glass lens of 

 6-inch focus, the sun's direct rays may be concentrated so as to 

 ignite paper; when the cap of an electroscope charged with + 

 or — electricity is placed in this focus no sensible discharge 

 ensues. A small blackened brass knob, connected with the elec- 

 troscope and submitted to the heat of the sun similarly concen- 

 trated, does not cause the leaves to collapse. The Leyden jar 

 has its knob polished or blackened; whether the jar be charged 

 with -f or — electricity, no discharge ensues when the sun's 

 rays are concentrated upon it by a lens. 



§ 42. Experiment. — The polished or blackened knob of a Ley- 

 den jar, or the blackened knob connected with an electroscope, 

 is in the principal focus of a spherical metallic mirror 18 inches 

 in diameter. In the focus of a conjugate mirror is placed a 

 white-hot iron ball. The centres of the mirrors are 5 feet 9 

 inches apart, and their foci 3 feet 10 inches. Though in the 

 course of \\ minute the blackened objects become too hot to 

 hold in the hand, though paper is scorched in the receptive 

 focus, neither + nor — electricity of a jar or electroscope is 

 discharged. 



§ 43. Experiments § 41 and § 42 show that a certain proxi- 

 mity must exist between the source of heat and the charged body 

 in order that discharge may take place, and that the discharge is 

 not due only to the intensity of the heat. 



<§ 44. Experiment.- — The edge of the sun's image was made 

 to touch the cap of an electroscope; and an earth- connected iron 

 ball was, so to say, dipped into the image. Whether positively or 

 negatively charged, the electroscope only showed the usual tem- 

 porary inductive release of its electricity. This experiment, to 

 which I attach very great importance in the series under conside- 

 ration, seems to show, in conjunction with §§ 41 &42, that for 

 there to be discharge it must be the inducing body itself which is 

 the source of heat, and that the mere presence of heat of passage 

 geometrically tilling the space between the electrified body and 

 the inducing body is not sufficient for the discharge. 



§ 45. Experiments. — A charged jar is placed on an insulating- 

 support of varnished glass. Its outer coating is connected with 

 one terminal of an astatic galvanometer having a current-con- 

 ducting wire of great length and thinness; the other terminal is 



