258 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
most by light, is ultimately most repelled, whilst the bromide follows next in sen- 
sitiveness. As might be expected, the insensitive iodide is less acted on by light ; 
the washed sensitive iodide is little affected by the magnesium light, and never 
attains any great sensitiveness to the mechanical action of radiation. The most 
sensitive silver iodide has its capacity for molecular change almost exhausted by the 
first exposure to the li ght of burning magnesium, and after the sudden jump from 
11 '6 to 22*6, is comparatively little affected by subsequent exposure to daylight or 
sunlight. 
236. Table XI. — Selenium, Crystalline and Vitreous. 
Lampblack ( standard disk ) . . 
Selenium disk, crystalline, No. 1 
„ „ „ No. 2 
„ „ vitreous . . . 
No screen. 
W ater screen 
interposed (5 millims.). 
100-0 100-0 
211 18-6 
19-1 170 
15-0 
The selenium disks were suspended direct in the apparatus, without being attached 
to mica or pith disks as were the previous substances experimented upon. They 
are 17 millims. diameter, and about 0 - 5 millim. thick. The disks were kindly prepared 
for me by Mr. It. J. Moss, who, in conjunction with Mr. H. N. Draper, has been for 
some time investigating the action of light on selenium in different physical states.* 
Disks of vitreous selenium were first prepared ; they were heated for one hour to 
100° C., and then for one hour to 210° C., which left them in the condition in which 
Mr. Moss and Mr. Draper find selenium most sensitive to light action. Two disks of 
crystalline selenium thus prepared were experimented with ; and at the same time 
a third disk of selenium, in its ordinary vitreous condition, was exposed in the appa- 
ratus for purposes of comparison. 
Mr. Moss finds t that exposure of selenium in a Sprengel vacuum causes it to 
acquire a film of mercury selenide, which increases its conducting power for elec- 
tricity. As it was likely this alteration might cause the molecular disturbance on the 
surface of the selenium, under the influence of radiation, to vary, extra care was 
taken to keep the vapour of mercury from penetrating to the selenium by interposing 
between the pump and the apparatus additional tubes filled with sulphur, copper, and 
closely packed gold leaf (355). 
When the crystalline selenium was under experiment, an effect already noticed in 
previous cases was very marked. On cutting off the light, and watching the index ray 
in its passage across the graduated scale, it was observed that the ray did not readily 
return to zero. Experimenting many times in rapid succession, this sluggishness 
could be accumulated. In other instances in which this action has been noticed, I 
* ‘ Proceedings of the Royal Irish Academy,’ vol. i., ser. ii. (Sci.), p. 529. 
t 1 Proceedings of the Royal Society,’ May 11, 1876, vol. xxv. p. 22. 
