Decembek 11, 1896.] 



SCIENCE. 



857 



This is in contradiction to the conclusion of 

 Hertz, who found that the most active rays 

 are from the arc itself. 



14. Illumination of the positive terminal, 

 or of the spark gap between the two termi- 

 nals, was absolutely without effect. In 

 order to increase the sparking distance the 

 ultra-violet rays must fall upon the brightly 

 polished negative terminal. 



15. On experimenting at different air 

 pressures it was found that the greatest 

 effect was obtained at a pressure of about 

 35 cm. The results obtained with ordinary 

 air, and with air that had been carefully 

 dried, were practically identical. The 

 phenomenon was also observed when the 

 spark took place in hydrogen, the results 

 being only slightly different from those in 

 air. In carbon dioxide, however, the sen- 

 sitiveness of the spark was noticeably in- 

 creased. Other rays, also, in addition to 

 the extreme ultra-violet, were found to be 

 effective. " With CO^ the active rays lie be- 

 tween the lines O and K of the visible 

 spectrum." The influence of these rays 

 could also be observed when the spark oc- 

 curred in air, but in much less degree. 



16. Wiedemann and Ebert give a brief 

 discussion of a possible theory of the action 

 based upon '■ absorption by the gases con- 

 densed upon the electrodes.' This is sup- 

 posed to facilitate the formation of cathode 

 rays by a process analogous to resonance. 



17. A paper by Hallwachs,* which ap- 

 peared at the same time as that of Wiede- 

 mann and Ebert, describes work along a 

 somewhat different line, and indicates the 

 distinct gain which may result from the re- 

 moval of unessential and complicating con- 

 ditions. In attempting to reduce the ex- 

 periments of Hertz to a simpler form, 

 Hallwachs was led to believe that the effect 

 was due to an increased tendency for the 

 discharge of negative electricity, resulting 

 in some way from the action of ultra-violet 



* Wied. Ann. 33, p. 301, 1888. 



rays. Such an effect, he argued, might be 

 present even when no spark discharge oc- 

 curred. The spark gap was therefore dis- 

 pensed with. A disk of polished zinc, 8 

 cm. in diameter, was suspended from an 

 insulated support and connected with a 

 gold-leaf electroscope. The two were then 

 charged negatively. When the zinc plate 

 was exposed to rays from an arc lamp its 

 charge was found to be rapidly dissipated. 

 During this experiment the electroscope 

 itself was screened from the rays of the 

 lamp, while in front of the zinc plate was 

 placed a large zinc screen containing a win- 

 dow of gypsum. This substance was found 

 to absorb the rays only slightly. 



18. The effect was observed only when 

 the plate was negatively charged. 



19. Hallwachs was able to show that the 

 active rays are absorbed, reflected and refracted 

 in the same manner as had been determined 

 by Hertz. Everything seemed to show that 

 the increased rapidity of discharge observed 

 with negatively charged zinc was merely 

 another manifestation of the Hertz effect. 

 The fact that the extreme ultra-violet rays 

 are most effective was definitely proven. 



20. It was by no means evident in the 

 case of Hallwachs' first experiment whether 

 the action occurred at the charged surface 

 or in the surrounding medium. To test 

 this point two plates were set up parallel to 

 one another and at a distance of 3 cm. apart. 

 Each was charged negatively and connected 

 with an electroscope. When the rays passed 

 between the plates and parallel to the sur- 

 face scarcely any effect was observed. But 

 if the plates were turned, so that one of 

 them received rays from the lamp, its charge 

 immediately began to fall. It therefore ap- 

 pears that the action occurs at the charged 

 surface itself, and that if the medium exerts 

 any influence it is of only secondary im- 

 portance. An experiment of Bichat,* who 



* C. E. 107, p. 557, 1888. Wiedemann's Beiblatter 

 13, 39. 



