Vacuum Gold-leaf Electroscopes in Rarefied Gases. 755 



"upon the gold leaves, they instantly diverged, although not 

 to the extent of the divergence obtained in previous experi- 

 ments, when the lower ends of the leaves were often two or 

 more centimetres apart. When an opaque screen was inter- 

 posed between the source of light and the electroscope, the 

 leaves quickly collapsed. If the screen was passed backwards 

 and forwards across the path of the rays, the leaves 

 alternately diverged and fell together again. Even the 

 movements of a string, of a pendulum swinging across the 

 beam of light could be followed, by observing the slight 

 kicks of the leaves, which were produced when the light was 

 momentarily cut off by the string. Screens of glass,, or other 

 transparent substances, have practically no effect in cutting 

 off the radiations which affect the gold leaves. The leaves 

 responded to the movements of a shutter in front of the 

 Xernst lamp when the light was made to pass through a 

 plate glass door, just as if the door were not there. 



Screens of various materials were interposed between the 

 electroscope and the Xernst lamp. When the light was trans- 

 mitted through coloured glasses the divergence of the leaves 

 was diminished, ruby glass almost halving the deflexion. 

 The effective radiation appears to pass with the greatest 

 freedom through several sheets of white, or coloured paper, 

 so long as the screen is at all transparent to light. A parallel- 

 sided plate of ice was made, by sawing a slice off a large 

 block of ice and then rubbing its surfaces down on a sheet 

 of plate glass until they were quite parallel and true. The 

 sheet of ice allowed the radiations which affect the gold 

 leaves to pass through with scarcely any perceptible dimi- 

 nution in intensity. No effect, however, could be detected 

 when the Xernst lamp was replaced by a spirit-flame, the 

 sheet of ice still being present ; although the light from a 

 candle or match would produce the effects observed with 

 the Xernst lamp. This was also found to be the case, when 

 another vacuum electroscope was completely immersed in 

 a glass vessel of alum solution, and the Xernst lamp, and 

 the various flames, brought near to the gold leaves. The 

 Xernst lamp * and the flames of a candle or match produce 

 their full effects ; but the influence of the spirit-flame, as far 

 as can be detected by visual observation of the gold leaves, is 

 completely stopped. Screens of lead, copper, zinc, etc., 



* Care must be taken when the Xernst lamp is used as a source of 

 light in these experiments. The metallic holder of the lamp is frequently 

 electritied, and it ought to be " earthed " by means of a suitably con- 

 nected wire. It also seems as if the Xernst filament were perpetually 

 shedding off an emanation, which will electrify an ordinary electroscope 

 placed at a distance of several inches from it. This electrification is 

 resinous. 



3 E 2 



