LIGHT AND ELECTRIFICATION. 423 



subject, and it is over soon to attempt to forecast its 

 probable development ; nevertheless a partial attempt may 

 be made for what it is worth. 



The new radiation appears to differ from ordinary 

 ultra-violet radiation only in the matter of wave-length. 

 Its wave-length is probably extremely short, not vastly 

 greater than the size of atoms, and all its other known 

 properties and peculiarities will follow from that according 

 to known theories of dispersion, especially the electro- 

 magnetic one of von Helmholtz. 



Now this X radiation, when it falls upon an electrified 

 surface, discharges it, somewhat in the same fashion that 

 ultra-violet light does ; but whereas light discharges 

 electricity solely, or at any rate chiefly, of the negative sign, 

 this X radiation discharges both positive and negative ; 

 and indeed it seems to act by converting the gas or other 

 insulating material near a charged body into a conductor. 

 This it probably does by dissociating the substance into 

 charged atoms which are then free to act as carriers, and 

 speedily convey to a distance the charge of the electrified 

 body by journeys along the lines of force. 



It may be that ultra-violet light acts in somewhat the 

 same way, but not in exactly the same way. The air is 

 transparent to ultra-violet light, it is not perfectly trans- 

 parent to X rays. 



There is no difficulty in supposing that the X rays 

 dissociate some ingredient of the atmosphere, but there is 

 great difficulty in supposing ordinary ultra-violet light to be 

 able to do so. What the ultra-violet light chiefly does is 

 to promote or to create the conditions necessary for the 

 ready interchange of electric charge between gas and 

 solid ; and that this is so is practically proved by the great 

 importance of the nature of the solid surface, as well as of 

 the gas in contact with it. The gas seems indeed of 

 secondary importance, but the cleanness and oxiclisability 

 of the solid is essential to a rapid and ready discharge with 

 ordinary light from the visible spectrum. High ultra- 

 violet light can act indeed over a wider range, and where- 

 as light of long wave-length can only discharge negative 



