176 SCIENCE PROGRESS. 



a quartz prism established the fact that it was the ultra- 

 violet rays which were specially active. In fact if a thin 

 sheet of glass were interposed, so as to cut off the extreme 

 and invisible ultra-violet rays, the action ceased ; though a 

 layer of water was transparent to the action, except when 

 certain salts were dissolved in it ; while coal gas was remark- 

 ably opaque, and stopped the action when interposed in a 

 sufficient stratum between the two sparks. He further 

 modified the experiment by taking the sparks between 

 different metals in different gases, and in air at different 

 pressures. He thus found that in rarefied air the effect 

 was exaggerated, and that an atmosphere of hydrogen was 

 rather better than one of air. He also found that the region 

 of the spark terminals near the negative pole or cathode was 

 the most sensitive part of a spark gap, so that, in fact, the 

 main photo-electric influence depended on whether the 

 region near the cathode surface was or was not illuminated. 

 The subject was taken up by a number of experi- 

 menters, among whom Wiedemann and Ebert specially 

 examined the effect in different gases, and established the 

 fact that the illumination operated only on the cathode 

 surface or negative pole, and that illumination of the anode, 

 or of the air between the terminals, had little or no influence 

 on the discharge. Supplementing this, Hallwachs, and 

 later Hoor, discovered that a freshly cleaned metallic sur- 

 face lost a negative charge quickly under the action of light, 

 but retained a positive charge ; while Hallwachs and Righi 

 independently and simultaneously discovered that such a 

 freshly cleaned metallic surface, when exposed to light, not 

 only discharged any negative electricity it might possess 

 but was able actually to acquire a slight positive charge. 

 Lenard and Wolf also made the important observation that 

 a negatively charged or an uncharged metal seemed to 

 evaporate, or throw off something which might be called 

 "dust," under the action of light. Other investigators into 

 the subject who must be mentioned are Stoletow and 

 Branly, but probably none have worked at it more ener- 

 getically and successfully than the well-known coadjutors of 

 Wolfenbiittel, Messrs. Elster and Geitel, who have dis- 



