IS THE SURFACE ELECTRIFIED] 



mon complaint against instruments intended to indicate low charges of electricity, that 

 they furnish evidence of an accumulation when" none has been communicated; it is 

 necessary, therefore, to examine an instrument to be certain that this objection cannot 

 be preferred against it. Having obtained this preliminary evidence in a satisfactory 

 manner, and having decided the effectual goodness of the instruments in other partic- 

 ulars, the following trial was made. The six-inch condenser was exposed to the sun- 

 beam for one hour, on a clear, bright day ; the charged plate was then parted and ap- 

 plied to the one-inch condenser ; the plates of this being parted, a small but perfectly 

 distinct electric action was obtained. This experiment is not, however, devoid of sources 

 of error, as from the friction occasioned by touching the plate of one condenser with the 

 plate of the other, or the heating action of the ray, which might cause currents of air 

 to brush over it; but it was found, by purposely rubbing one plate of the condenser 

 on the other, that no charge of electricity could be produced, even if the friction 

 were continued during some time ; and on maintaining the temperature of the condenser 

 at the same point to which it was brought by the sunbeam, in order to produce like 

 currents of air, no divergence whatever of the gold leaves was produced. 



489. When the tension of electricity is high, one of the most delicate methods of 

 detecting its presence is by the light it emits in vacuo ; the excitation caused by the 

 tremulous motion of a column of mercury in a barometer tube is rendered visible by 

 the bright light it gives out, when no other method could discover it. On this princi- 

 ple, attempts were made to detect electrical action in the sunbeam, by exposing metallic 

 plates of large dimensions to the ray. and causing any electricity they might gather to 

 give out light in the vacuum ; these trials did not prove satisfactory. 



490. It has been stated (439), that the cloud which rises from phosphorus, when 

 slowly oxidating, is endowed with great mobility; for certain purposes it makes a very- 

 good electroscope. When a piece of phosphorus is shielded from the air by a bell jar, 

 and not exposed to disturbing action of any kind, a fine sheet of vapour rises vertically 

 upward. If, at a distance of several feet, an excited stick of wax be presented, the 

 vapour curls from its path, and leans over to the side of the glass adjacent to the cause 

 of the disturbance. If such a jar be exposed to the sun, a like disturbance is exhib- 

 ited : as soon as the rays fall on it, it seems as though they caused each particle to 

 repel its fellows; the straight column, which before passed to the top of the jar, separ- 

 ates into confused masses, which pass forward to the perihelion side. 



491. No direct proof existing that rays of electricity are emitted by the sun, and as 

 it does not fall within my purpose to discuss their hypothetical action, it may be suffi- 

 cient to give the proof, that if the surface be admitted to be electrified, these deposites 

 should take place. If a receiver be taken, and on any part of its interior surface a 

 glass rod be made to pass, the line which it describes will be stellated with camphor 

 crystals, on exposure to the sun after exhaustion. This curious fact was first observed 

 in the case of an exhausted vessel, which had a small siphon gauge shut up in it, the 

 extremity of which rested against the glass ; by accident the gauge was moved half 

 round the glass, and in a short time after a line of crystals was observed coinciding 

 with the line of motion ; it was found possible afterward to repeat this result at pleas- 

 ure ; the appearances were such as are represented \njig. 90. 



