ME. W. CEOOKES ON REPULSION RESULTING PROM RADIATION. 
25 7 
spends itself in chemical action, and causes a molecular change to take place in the 
structure of the silver iodide. When no latent change is produced, mechanical 
action is represented by 20 ’9 ; on slight latent change, the mechanical action is 18T ; 
but when strong latent disturbance in the molecular structure of the silver iodide 
is caused by the incident radiation, the mechanical action sinks to 11 '6. The amount of 
repulsion behind a water screen, as shown in the second column, corroborates this 
reasoning as far as the non-sensitive and the most sensitive varieties of silver 
iodide, but it fails in the intermediate case. For this anomalous action I am unable 
to account. 
235. The same series of silver salts which are given in Table X. were now subjected 
to further experiments. Without removing them from the apparatus, or interfering 
with the adjustment, the disks were first exposed for one minute to the light of burning 
magnesium, and then faint sunlight was reflected on to them for two minutes ; owing to 
the direction along which the ray had to pass, three reflections were necessary. After 
each of these exposures to light, the apparatus was completely closed up and kept in 
darkness till the molecular disturbance induced by the light had passed off. Experi- 
ments were then tried with a standard candle in the usual way, and the amount of 
repulsion of each disk recorded. Lastly, air was let in, the disks were carefully lifted 
out of the tube by the flat glass fibre on which they were cemented (222) and exposed 
to bright daylight for half an hour ; all the disks except the insensitive silver iodide 
were visibly darkened. They were then re-introduced into the apparatus, the whole 
was exhausted, and the experiment repeated. The results of these experiments are 
given in the following table : — 
Table Xa. — Silver Salts after Exposure to Light. 
Lamjphlack ( standard ) 
Silver bromide 
„ chloride 
„ iodide, insensitive .... 
„ „ sensitive, washed . 
„ „ sensitive, not washed 
Original 
repulsion 
before 
100-0 
31-0 
20-9 
20-9 
18-1 
11-6 
Repulsion by candle light after exposure to 
Magnesium 
light. 
100-0 
33-0 
21-0 
19-9 
19-0 
22-6 
100-0 
35-1 
24-3 
20-0 
23-2 
29-8 
Daylight. 
W ithout 
screen. 
100-0 
58-7 
69-8 
23-0 
27-3 
38-5 
Behind 
water screen. 
ioo-o 
88-8 
98-1 
12-3 
40-1 
75 - 5 
This table shows very clearly how readily a change in the state of the surface is 
detected by an increased amount of repulsion under the influence of radiation. With 
the exception of the insensitive silver iodide in the second column (probably due 
to unavoidable errors of experiment), every exposure to light has left the silver 
surface in a condition more sensitive to radiation. The silver chloride, which darkens 
2 L 
MDCCCLXXVIII. 
