92 COERELATION OF PHYSICAL FORCES. 



In this case there will still be a molecular change or disinte- 

 gration of the plate ; the portion of it acted on by the dis- 

 charge will present a different appearance from that which is 

 beyond its reach, and a whitish film, somewhat similar to 

 that seen on the mercurialised portions of a daguerreotype, 

 will gradually appear on the portion of the plate affected by 

 the discharge. If the gas be a compound, as carbonic oxide, 

 or a mixture , as oxygen and hydrogen, and consequently contain 

 elements capable of producing oxidation and reduction, then 

 the effect upon the plate will depend upon whether it be pos- 

 itive or negative ; in the former case it will be oxidated, in 

 the latter the oxide, if existing, will be reduced. This effect 

 will also take place in atmospheric air, if it be highly rare- 

 fied, and can hardly be explained otherwise than by a mole- 

 cular polarisation of the compound gas. If, again, the metal 

 be reduced to a small point, and be of such material that the 

 gas cannot act chemically upon it, it can yet be shown to be 

 disintegrated by the electric spark. Thus, let a fine plati- 

 num wire be hermetically sealed in a glass tube, and the ex- 

 tremity of the tube and the wire ground to a flat surface, so as to 

 expose a section only of the wire ; after taking the discharge 

 from this for some time, it will be found that the platinum 

 wire is worn away, and that its termination is sensibly below 

 the level of the glass. If the discharges from such a plati- 

 num wire be taken in gas contained in a narrow tube, a cloud 

 or film consisting of a deposit of platinum will be seen on 

 the part of the tube surrounding the point. 



Another curious effect which, in addition to the above, I 

 have detected in the electrical discharge in attenuated media, 

 is that when passing between terminals of a certain form, as 

 from a wire placed at right angles to a polished plate, the dis- 

 charge possesses certain phases or fits of an alternate character, 

 so that, instead of impressing an uniform mark on a polished 

 plate, a series of concentric rings is formed. 



Priestley observed that, after the discharge of a Leyden 



