December 18, 1896.] 



SCIENCE. 



891 



simple one. An aqueous solution of fuchsin, 

 for example, is very senitive to the action 

 of the ultra-violet rays ; but an alcoholic so- 

 lution of the same concentration is entirely 

 inactive. Yet the same rays are absorbed 

 by the two solutions, the absorption being 

 more marked with the alcoholic solution. 

 A cell containing the latter, when placed 

 in the path of the rays, destroyed all action 

 on the aqueous solution. The author sug- 

 gests that the case is analogous to such as 

 occur with fluorescent substances, where 

 the solvent has great influence upon the in- 

 tensity of the fluorescence. 



64. Hallwachs next made an attempt to 

 investigate the effect of different wave- 

 lengths in the neighborhood of ultra-violet 

 absorption bands. A solution of fuchsin 

 showed a strong band between X = .250 ,a 

 and )- = .275 /x. The solution was tested 

 in a spectrum formed with a quartz prism. 

 But the effect was so weak as to make re- 

 liable results impossible. 



An approximation to monochromatic 

 rays was obtained by using suitable absorb- 

 ing media. But it then appeared that the 

 most active rays were in the extreme ultra- 

 violet, beyond the absorption bands. The 

 effect in the neighborhood of the bands 

 was too small to enable any conclusion to 

 be drawn. 



65. During his experiments Hallwachs 

 found that some of the less active liquids 

 showed some slight effect when -positively 

 charged. He was inclined to believe, how- 

 ever, that this effect was accidental, and 

 due to the discharge of the induced negative 

 electricity on neighboring bodies. 



LATEK WORK OF WIEDEMANN AND EBERT. 



66. At about this time further investiga- 

 tions were undertaken by Wiedemann and 

 Ebert* upon the spark discharge as in^ 

 fluenced by ultra-violet rays. The method 



* Wied. Ann. 35, p. 209. Before Hallwachs and 

 Lenard and AVolf. 



was practically the same as that first used 

 by these writers.* 



Various metals were first investigated. 

 The electrodes were made of the same form 

 in each case, and great care was shown in 

 obtaining the same intensity and quality of 

 illumination. It was found that Pt was by 

 far the most sensitive of all the metals 

 tried. A telephone, placed in series with 

 the spark gap, showed an especially notice- 

 able change in the character and pitch of 

 the sound when a spark gap of about 2 to 

 3 mm. was illuminated. The effect in other 

 metals is very much less intense. 



In the order of decreasing sensitiveness, 

 the metals are as follows : Zn, Cu,*Fe, Al, 

 Pd, Ag. 



67. The sparking distance most favorable 

 for showing the effect was found to be dif- 

 ferent with different metals. The change 

 in the appearance of the spark, under the 

 influence of ultra-violet rays, is discussed in 

 Wiedemann and Ebert's paper at some 

 length (p. 213 1. c). 



68. Experiments with liquids showed that 

 strongly absorbing solutions were most ac- 

 tive. The most sensitive liquid was found 

 to be a solution of Nigrosin. Water showed 

 only a very slight effect, which, however, 

 was greatly increased by impurities. 



69. At low pressures the effect of illumi- 

 nation was practically nil. (But a slight re- 

 duction of pressure increases the effect. 

 See Hertz, and Wiedemann and Ebert 1st 

 paper.) " As soon as the pressure condi- 

 tions are such as to cause a considerable 

 development of cathode rays, the influence 

 of illumination can no longer be noticed " 

 (p. 217). 



70. Wiedemann & Ebert call attention 

 to the fact that, since their experiments 

 were made upon disruptive discharges, 

 some caution must be used in applying 

 these results to cases of continuous dis- 

 charge, such as those investigated by Hall- 



*Wied. Ann. 33, p. 211. See ?i?i 11-16. 



