896 



SCIENCE. 



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



metal case, S, with an insulated wire lead- 

 ing outside to an electrometer. Just above 

 D was a sheet of iron wire gauze, G. The 

 substance to be tested was placed in the 

 dish, D (in powder form), and the gauze, G, 



Figs. 



was kept at a positive potential of 300 

 volts. The quartz window, Q, being covered 

 by a metal plate, D was grounded for an in- 

 stant. The ground was then broken and 

 light allowed to shine upon D. As the in- 

 duced negative charge was dissipated, the 

 electrometer showed an increasing deflec- 

 tion. This arrangement was found much 

 more sensitive than that used before. 



95. Among the minerals tested, certain 

 varieties of fluorescing fluorspar were found 

 most sensitive. The most efiective rays 

 were not the ultra-violet, but the blue. The 

 sensitiveness of fluorite was not increased 

 by diminished air pressure ; and was not 

 destroyed by wetting the surface with 

 water. 



96. Fluorite was rendered insensitive by 

 being heated to redness ; and its power of 

 phosphorescing was also destroyed by this 

 means. Elster and Geitel suggest that the 

 phosphorescence and actino-electric activity 

 are due to the presence of some organic 

 impurity which is removed by heating. 



Various other materials were tested and 



found to be slightly sensitive. Freshly 

 fractured surfaces were more active than 

 old surfaces. 



EFFECT OF A MAGNETIC FIELD ON ACTINO- 

 BLECTRIC PHENOMENA. 



97. In the course of his low pressure ex- 

 periments in 1890,* Eighi found that the 

 positive charge acquired under the influ- 

 ence of illumination is lessened when a 

 magnet is so placed as to make the lines of 

 force parallel to the surface. The particles 

 which carry the charge ' appear to meet with 

 opposition in their motion through the 

 magnetic field, so that the actino-electric 

 action is weakened.' 



98. The influence of the magnetic field 

 in diminishing the effect of illumination 

 was discovered independently by Elster and 

 Geitel, f who were led to investigate the 

 matter through a consideration of some- 

 what similar effects in the case of the spon- 

 taneous discharge between incandescent and 

 cooler bodies . J Amalgamated zinc was used 

 as the active metal and the spark of an in- 

 duction coil as a source of light. At pres- 

 sures below 5 mm. the influence of illumin- 

 ation could be in some cases entirely de- 

 stroyed by a magnetic fleld. In place of air 

 H, CO2, and O were also tried. CO2 was 

 found especially sensitive, both to the actino- 

 electric effect and to the restraining influ- 

 ence of the magnetic field, but much lower 

 pressures were necessary to exhibit the lat- 

 ter action. (Maximum at .005 mm.) These 

 experiments were successfully repeated with 

 daylight, sodium amalgam being used in- 

 stead of zinc. 



99. At the close of the article just cited § 

 Elster and Geitel discuss at some length the 

 probable cause of the phenomena. While 

 accepting some form of convection as the 



* Ace. dei Lincei 6, p. 81. Ace. di Bologna 10, p. 

 85. Beibl. 14, p. 1167. 



t Wied. Ann. 41, p. 166. 1890. 

 % Wied. Ann. 37, p. 315 ; 38, p. 27. 

 |l.c., p. 174. 



