900 



SCIENCE. 



[N.S. Vol. IV. No. 103. 



at times the positive surface charge, or even 

 to reverse it. The negatively charged par- 

 ticles of the gas will thus be driven away, 

 while others will come to take their places, 

 and in so doing will impart additional posi- 

 tive electricity to the metal. The fact that 

 strongly electro-negative gases, such as H 

 and CO2, give an especially marked photo- 

 electric action seems to support this view. 

 On the other hand, as is pointed out by 

 Elster and Geitel, this explanation seems to 

 require that all action should cease when 

 the light rays fall normally on the surface, 

 a conclusion which is not borne out by ex- 

 periment. 



CONCLUSION. 



It is clear from what precedes that a 

 thoroughly satisfactory explanation of the 

 discharging action of light has not yet 

 been found, and that many questions 

 concerning the phenomenon remain to be 

 settled by further experimental investiga- 

 tion. That interest in the subject is not 

 diminishing is evidenced by the number of 

 papers that have only recently been pub- 

 lished. A list is given below of those that 

 have come to my attention too late to 

 enable their results to be included in this 

 article. 



Elster and Geitel. The influence of light on the form 

 of the discharge from a Holtz machine. Wied. Ann. 

 57, p. 401, 1896. 



Elster and Geitel. On a photo-electric after effect of 

 cathode rays. Wied. Ann. 59, p. 417. 1896. 



Warhurg. The action of light on the spark dis- 

 charge. Wied. Ann. 59, p. 1. 1896. 



Klemencie. A lecture experiment to illustrate the 

 mutual influence of two spark gaps. Wied. Ann. 

 59, p. 63. 



Swyngedauw. Action of ultra-violet light upon ex- 

 plosive, static and dynamic potentials. C. E. 122, 

 p. 131, 1896. Beibl. 20, 660. 



Dufour, Dutoit and Hofer. Dissipation of elec- 

 tricity under the action of light. Arch, de Gen6ve, 

 34, p. 294, 1895. 



Simon. On the influence of waves of high refrangi- 



hility upon the electrical conductivity of rarified 

 gases. Wiener Berichte, 104, p. 565, 1895. 



Batelli and Gariasso. On the dissipation of electro- 

 static charges by ultra-violet rays. Nuovo Cimento 

 III., p. 321, 1896. 



Sella and 3Iaiorana. Action of Eontgen raysand of 

 ultra-violet light on the disruptive discharges in air. 

 Nuovo Cimento III., p. 238, 1896. 



The bearing of the phenomena here de- 

 scribed upon the question of the nature of 

 the X-rays is doubtless of especial interest 

 at the present time. The fact that the 

 X-rays are capable of producing a rapid 

 dissipation of electricity from charged bod- 

 ies has been urged in support of the view 

 that these rays are similar in kind to ultra- 

 violet light, but of excessively small wave- 

 length. The argument loses some of its 

 force when we recall the fact that ultra-vio- 

 let rays are capable of dissipating a negative 

 charge only, while the action of the X-rays 

 is practically the same for both charges. 

 This difference between the two phenomena 

 does not in itself offer a fatal objection to 

 the argument, but recent experiments of 

 Batelli and Garbasso (see above reference) 

 indicate a much more fundamental differ- 

 erence between the two effects. It has 

 been found that air which has been exposed 

 to X-rays retains the power of discharging 

 electrified bodies even when no longer ex- 

 posed to those rays. No indication of such 

 an after effect has been found in the case of 

 ultra-violet light. The work of Batelli and 

 Garbasso was undertaken with the special 

 object of testing this point, and their results 

 show that no trace of an after effect can 

 be detected with the rays from an arc lamp, 

 even when the direct discharging action is 

 as great as that produced by X-rays. If 

 these results are confirmed I think we 

 shall be forced to the conclusion that the 

 discharging action of ultra-violet light dif- 

 fers essentially from the similar effect pro- 

 duced by the Eontgen rays. 



Ernest Mebritt. 

 CoENELL University. 



