424 SCIENCE PROGRESS. 



electricity, it is probable that light of extremely short wave- 

 length can discharge positive also, and from surfaces not 

 specially clean nor oxidisable. 



The X rays seem to go farther in the same direction ; 

 that is to say, their activity does not appear to depend much 

 upon the nature of the surface, nor do they seem to discrimi- 

 nate much between positive and negative electrification. 



We may surmise, then, that long-wave light is effective 

 in promoting discharge only when dissociated or incipiently 

 dissociated atoms are already present in the neighbourhood 

 of the surface. It is otherwise known that strongly electro- 

 positive substances, like clean sodium or zinc, are surrounded 

 by a number of electro-negative (chiefly oxygen) atoms, 

 straining to get at it. And, similarly, a negatively charged 

 surface may be surrounded by a number of straining posi- 

 tive atoms. Under these circumstances it is not difficult to 

 picture the result of impinging waves of light, and of the 

 electrical oscillations which they must necessarily set up, as 

 resulting in an interchange of electricity between the surface 

 and the gas which otherwise might not have occurred. 



When positive electricity has thus been received by the 

 metal from the air under the action of light, detached nega- 

 tive ions will be left in the atmosphere, and these will be 

 repelled by the body if kept negatively electrified, and so 

 may constitute a kind of feeble cathode ray. 



Thus it appears as if there were a sort of reciprocal 

 action ; the impact of light on a negatively electrified sur- 

 face results in the production of something akin to cathode 

 rays, and the impact of cathode rays upon a positively elec- 

 trified surface results in something akin to light. 



Another instance of reciprocity has also been observed. 

 Certain substances exposed to X rays fluoresce strongly, 

 that is, emit light which in some cases persists an appreciable 

 time, and some of these substances when made to fluoresce 

 by exposure to light begin to emit X rays and continue to 

 emit them for long after, as has been observed by M. 

 Becquerel. 



There is one matter dealt with in the last article which 

 requires more cautious handling than it then received, and 



