894 



SCIENCE. 



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



tube, the pressure being in tbe neigbbor- 

 3iood of 1 mm. Under these conditions the 

 surface remained permanently unaltered. 

 Such an active electric cell was extremely 

 sensitive. Diffuse light was effective even 

 through the glass sides of the tube ; no 

 quartz window or similar arrangement was 

 at all necessary. The surface is sensitive 

 even to rather long waves, for a slight effect 

 was observed with the sodium flame. 



84. In 1891 Elster and Geitel* again 

 took up the investigation of the actino-elec- 

 tric behavior of sodium and potassium. In- 

 stead of amalgams, the clean unoxydized 

 surfaces of the metals themselves were 

 used. Such surfaces were prepared in 

 vacuo by a method described at length in 

 the article cited. The apparatus and con- 

 nections are shown diagrammatically in 

 Fig. 2. The metal, M, was kept perma- 



-jr- B 



Fig. 2. 



Ei^vtTu 



nently connected with the negative pole of 

 a ' Zamboni ' battery, B ( a dry battery. See 

 Wied. Elect. I.: 272) , the positive pole of the 

 battery being grounded. The potential of 

 the negative pole was then measured by an 

 Exner electroscope. ( E in figure. ) A 

 platinum electrode, P, above the surface of 

 the metal was kept grounded and served to 

 <3arry away the charge dissipated from M. 



* Wied. Ann. 43, p. 225. 



85. *Surfaces of pure potassium were 

 found excessively sensitive. It was impos- 

 sible to maintain a negative charge at all 

 in a room lighted by daylight/ An oil 

 lamp 10 cm. distance reduced the potential 

 from 212 volts to zero. Even at a distance 

 of 6 m. its effect was noticeable. Magne- 

 sium light at the latter distance reduced 

 the potential 60%. Even the rays from a 

 Bunsen burner were effective, as was also 

 the fluorescent light from luminous paint. 

 A noticeable effect was observed with moon- 

 light. 



86. The effect of various absorbing media 

 was next studied. Finally, the apparatus 

 was placed in the solar spectrum formed by 

 a glass prism. The maximum effect was 

 found in the blue, but the influence was 

 noticeable as far as the red, and far into 

 the ultra-violet. It' is suggested that this 

 apparatus may be used to advantage in 

 showing ( for lecture experiments ) the ex- 

 istence of rays beyond the violet. 



87. On charging the potassium positively 

 Elster and Geitel in some cases observed 

 what appeared to be an increased rate of dis- 

 sipation oi positive electricity.* They show 

 conclusively, however, that this result is due 

 to the action of diffuse rays upon the (now) 

 negative platinum electrode, P (see Fig. 2), 

 and are strongly of the opinion that other 

 observers who have published accounts of 

 an action on positive charges have been 

 misled by some similar disturbance. 



88. Pure sodium surfaces were found 

 somewhat less active than potassium. Ex- 

 periments upon various amalgams gave re- 

 sults showing that the order of sensitiveness 

 is as follows : Rb, K, IS'a, Li, Mg, Tl, Zn. 

 Pure K and Na are more sensitive than any 

 of the amalgams. 



89. After a number of papers dealing with 

 atmospheric electricity and the meteoro- 

 logical aspect of the negative discharge 

 phenomena, Elster and Geitel again took 



*1. c, p. 236. 



