DEVELOPMENTS IN ELECTEOMAGNETISM BLOCH. 241 



has already been considerably studied and shows very different char- 

 acteristics than those at first attributed to it by Lenard. It seems 

 to be produced exclusively by the Schumann or extreme ultra-violet 

 rays of wave length less than 0.180 jjl. These rays will not pass 

 through air although they will through fluorite and partly through 

 quartz. It produces small ions of both signs, neutral centers, large 

 ions, and ozone. It is extremely sensitive to minute traces of im- 

 purities in the gas, traces which can not be detected by other means. 

 It can be distinguished from the Hertz effect and become very much 

 greater. All these conclusions are drawn from the researches of 

 Hughes,^ Cannegieter,^ Lenard and Eamsauer,^ and Leon and Eugene 

 Bloch.* The latter have shown also that the radiation transmitted 

 by quartz and coming from a mercury arc ionizes the air feebly in 

 the neighborhood of the arc and seems consequently to emit a small 

 amount of Schumann rays. In place of the usual source of Schu- 

 mann rays, a hydrogen tube furnished with quartz windows, Lenard 

 and Ramsauer used a very powerful spark between electrodes of 

 aluminum. Then the ionization takes place even through air and 

 quartz and the experimenters attribute it to rays of wave length 

 less than 0.1 [;.|x, the smallest ultra-violet rays known and which were 

 discovered by Lymann. As no measure of these wave lengths were 

 made, it seems as probable that the effect is due to ordinary Schu- 

 mann rays which have been partially transmitted by media generally 

 opaque to them because of the great original intensity of the light. 

 This question remains to be studied as well as the I^enard effect in 

 general the knowledge of which is yet very limited despite the great 

 number of interesting problems connected with it. 



1 Hughes, Proc. Cambr., vol. 15, p. 483, 1910. 



- Cannegietcr, Proc. Amst., p. 1114, 1911. 



^Lenard and Ramsauer, Sitzungsber. Heidelberg, 1910-1911. 



* Leon and Eugene Bloch, Comptes Rendu.s, vol. 155, pp. 903, 107(5, 1912. 



44863°— SM 1913 16 



