MR. W. CROOKES ON REPULSION RESULTING PROM RADIATION. 
255 
them moving so as to touch where they overlap in the centre. At the centre of each 
beam is a mirror, by means of which a luminous index is reflected to a scale, so as to 
render evident any movement of the beams. The instrument being in adjustment, light 
from a candle is allowed to fall on the large plate of platinum at the a end of beam b. 
Almost instantly the motion of the index ray of light from the mirror c shows that the 
smaller plate of platinum attached to the other beam, a e, is driven backwards, in the 
same way as it would be had the large plate in front of it been transparent instead of 
opaque. The beam a d is prevented from pushing the hinder plate at a by the stop at d. 
233. The action on the palladium saturated with hydrogen is lower than might be 
expected. The saturation was effected by making the metal the negative electrode of 
a three-cell Grove’s battery, decomposing water acidulated with sulphuric acid. After 
action had gone on for about half an hour, the palladium was washed, dried with a 
cloth, and introduced into the apparatus. During exhaustion it is possible that a 
little hydrogen might have escaped from the metal, but the loss could not have been 
great, as the rate of exhaustion was not materially reduced. When the proper 
exhaustion was reached, no escape of the gas from the metal was noticed, the gauge 
remaining stationary. It was thought possible that, on allowing radiation to fall on 
the palladium-hydrogen plate in the vacuum, the repulsion might be accompanied by 
a liberation of hydrogen. I could not, however, detect any action of this kind, either 
by a depression of the gauge or by a diminution in the sensitiveness of the other disks 
to the radiation, which would have occurred had hydrogen escaped into the vacuum. 
Moreover, it is probable that had the action of incident radiation been accompanied 
by a liberation of hydrogen, the repulsion would have been stronger than it actually 
was. On testing the palladium plate after removing it from the apparatus, it was 
found to be abundantly charged with hydrogen.* 
234. Table X. — Silver Salts. 
Lampblack ( standard disk) 
Silver bromide 
.Silver chloride 
. Silver iodide, insensitive to light .... 
„ „ sensitive to light, washed 
„ ,, „ „ not washed 
The silver salts were prepared by double decomposition in a room feebly lighted by 
orange light. They were well washed on the filter, and then divided into two por- 
tions; to one was added a dilute solution of the precipitating salt (potassium iodide or 
bromide, or sodium chloride), and to the other a dilute solution of silver nitrate. 
* This persistence of the palladium-hydrogen alloy was discovered by Graham. See Proc. Roy. Soc., 
June 11, 1868, yol. xvi. p. 422 • or Collected Works, p. 284. 
No screen. 
Water screen 
interposed (5 milliins . ). 
o 
o 
100:0 
100-0 
31-0 
31\6 
20-9 
14-7 
20-9 
9:2 
18-1 
20-0 
11-6 
1-5 
