1896 - 97 .] Conductive Effect in Air by Rontgen Rays . 41 3 : 
allow of its being fixed at different distances from the tinfoil- 
coated end-wall of the box facing it. 
The distance between the disc and the tinfoil was at first 4 cms. 
The arc lamp was distant about 20 cms. from the box. The light 
from it shone through a slit in the tinfoil covering the side of the 
box perpendicular to the surface of the oxidised copper. The slit 
was 4 cms. long, 1 cm. broad. Its length was first placed parallel 
to the copper surface, so that the light admitted by it shone in tho 
space between the two metals in such a way as not to illuminate 
either directly. It was found (1) that the ultra-violet-light-zero’ 
did not deviate from the metallic zero when the sheet of light 
passed between the two metals ; (2) that a negative charge given 
to the insulated oxidised copper was not discharged ; and (3) that 
a positive charge was removed very slowly— about 4 scale divi- 
sions per minute from a charge of 197 scale divisions from the 
metallic zero. 
When the length of the slit was placed perpendicular to the sur- 
face, so that a small portion of both metals, as well as the interven- 
ing air, was illuminated, it was found that the reading deviated 
about -f 1 scale division per minute from the metallic zero. The 
oxidised copper was charged positively and negatively. Discharge 
took place at about 4 scale divisions per minute from a charge of 
+ 202 scale divisions ; and 3 scale divisions per minute from a 
charge - 246 scale divisions : the charge reckoned from the metal- 
lic zero in each case. 
The slit was then so arranged as to allow the light to shine on 
