3G8 
ME. W. CEOOKES ON EEPT7LSION EESULTING EEOM EADIATION. 
several observations (which, however, were not so concordant as I should have liked) 
were — 
Standard Black. 
Rock-salt. 
Reduced to Black =100. 
Black. 
Rock-salt. 
131 
4 
100 
3 
Standard Black. 
Alum. 
Reduced to Black = 100. 
Black. 
Alum. 
120 
6 
100 
5 
There was no action on either the alum or the rock-salt when a screen of water or of 
alum was interposed. 
204. In consequence of some experiments tried by Professors Tait and Dewar, and 
published in ‘ Nature, ’ July 15th, 1875, I fitted up a more sensitive apparatus for the 
purpose of carefully examining the action of radiation on alum, rock-salt, and glass. 
The apparatus was similar to the one described in par. 186, there being, however, no 
window at either end. To the •horizontal beam suspended by the glass fibre were 
attached a plate of alum at one end and a plate of rock-salt at the other. Each plate 
was perfectly polished and transparent, and measured 12'5 by 13-5 millims., and was 
1’5 millim. thick. The deflection was produced by a candle placed opposite the 
crystalline plate under examination, and it was measured in the usual way by a reflected 
ray of light. The following were the results : — 
Distance of candle 
from plate. 
Deflection observed. 
Reduced to alum =100. 
Alum. 
Rock-salt. 
Alum. 
Rock-salt. 
150 millims 
21 
17 
100 
81 
22 
17 
100 
77-3 
95 99 
24 
17 
100 
71 
100 „ 
48 
30 
100 
62-5 
” 
43 
26 
100 
60-4 
The action on the alum was found to increase at each observation. A glance at the 
plate showed the reason. When it was first put in it was perfectly smooth and trans- 
parent. Before it had been long in the vacuum efflorescence commenced, and when 
the first observation was taken the surface of the alum plate was dotted over with small 
white specks, which increased in size and number as the experiments were continued. 
The opacity thus caused was apparently sufficient to account for the increased action of 
radiation upon the alum plate. 
The pump was kept going the whole time, so that the water vapour evolved from 
