IS THE SURFACE ELECTRIFIED? 227 



ception good conductors of electricity, such as the metals. Certain indications led me 

 to make trial of resinous matters, which are non-conductors of electricity. Having 

 made die region about a (Jig. 86) of the air-pump jar very warm, over a spirit lamp, 

 a ring of rosin was spread on it, about the same size as the ring of tin foil which had 

 been formerly there. This ring of rosin was transparent, admitting the light to pass it 

 readily, and at a certain distance appeared of an amber colour. Having arranged the 

 jar as usual, and exposed it to the sun, after a certain length of time well-marked crys- 

 tals were deposited on the perihelion side, on which the rosin was ; these crystals not 

 only came up to the verge of the rosin, and filled also the inner circle, but were found 

 on the rosin itself. 



. Metallic plates of various shapes, and under various circumstances, were ex- 

 posed with a view of causing condensation upon them ; it w ? as not found possible, 

 however, either to cause the formation of aqueous dew or crystalline deposite, except 

 when their temperature was below that of the medium in which they were exposed. 



486. At this stage of the inquiry it becomes important to know whether, along with 

 the rays of light, of heat, and of chemical action, there are not also rays of radiant 

 electricity emitted by the sun. Almost all operations which disturb the equilibrium of 

 light and heat, disturb too that of electricity, and it is well known that upon this fact 

 Dr. HARK founds the explanation of the action of certain voltaic arrangements, espe- 

 cially the calorimotor ; an explanation the correctness of which later researches make 

 more probable. If light, heat, and electricity are set in motion by the force of chemical 

 action, and are often found coexisting, there is nothing improbable in meeting them to- 

 gether in the case before us. It is very true that, as yet, we have not met with any 

 example of electricity under what we understand as a radiant form, but that it consists 

 of undulations of an elastic medium, like the undulations of light and heat, is not to be 

 doubted. The experiments of NOBILI give proof of an interference, analogous to the 

 interference of the rays of light, which has served so well to refer the motions of that 

 fluid to the undulations of an elastic medium ; the analogies of light and heat are every- 

 where kept up, and we look with confidence that they w T ill be extended hereafter to 

 electricitv. 



487. The tendency of the experiments here communicated is to show that certain 

 substances, conductors of electricity, have the faculty of depriving glass of that power by 

 which it causes the condensation of vapours upon it when exposed to the sun ; that 

 deposition will not take place on metallic surfaces, but that certain vitreous and resinous 

 bodies interfere in no manner with the process. The inference appears obvious, that 

 electricity, brought into play in some unusual manner, is the cause of the phenomenon. 



4SS. By the action of the solar ray electricity of high tension can be developed. A 

 copper electrical condenser was taken, the plates of which were about one fortieth of 

 an inch apart, and six inches in diameter ; there was nothing more in their construc- 

 tion than is met with in the usual arrangement. Another condenser was also pro- 

 vided, which was connected with a gold-leaf electrometer, the plates being one inch in 

 diameter, and separated from each other by a verv thin coat of gum-lac varnish. Trials 

 were repeatedly made to discover whether the apparatus w ? as trustworthy. It is a com- 



