LIGHT AND ELECTRIFICATION. 177 



covered many remarkable facts in connection with it. It 

 was not till the beginning of 1894 that the subject was 

 seriously taken up by the writer. 



This brief outline of some of the work that has been 

 done in this subject since its discovery in 1887 must serve 

 instead of a proper historical sketch, because we want to 

 pass from the consideration of the investigators to a study 

 of the facts themselves. 



EXPERIMENTAL DETAILS. 



A demonstration of the fundamental fact can be made 

 with very simple appliances, but unless they are properly 

 arranged it is easy to fail. Several experimenters have 

 indeed doubted the fact because they were unable to repeat 

 it. No method can be simpler than the one indicated in 

 the first few lines of this article ; and with a well-insulated 

 electroscope, whose rate of leakage can be timed, the only 

 difficulty is the obtaining of a suitable light. Sunlight on 

 a mountain top is certainly effective, but sunlight on a 

 plain is much weaker, and in a British town (perhaps even 

 in the British Isles) it has hardly, any power at all; the 

 active rays are all filtered out by the gross and polluted 

 atmosphere usually existing above such places. Visible 

 brightness of illumination is not what is wanted, the most 

 effective rays are invisible ; for instance, Hertz observed 

 that the oxyhydrogen flame was just as effective before the 

 lime was introduced into it as after, and that it is the 

 bottom blue part of a candle flame which possesses any 

 power at all ; hence the light from a brush discharge or 

 from the aureole surrounding an induction coil spark is 

 very much more effective than, for instance, the lime 

 light or even the magnesium light. An arc-light is, how- 

 ever, the most powerfully active source, especially if it is 

 made extra long, and still more if a little zinc is placed in 

 the positive crater for volatilisation. Hertz himself noticed 

 that the violet flame between the poles was more active 

 than the intensely bright carbons themselves. In using 

 the arc-lamp it must be understood that is to be naked. 

 There are to be no lenses in the lantern, or if there are, 



