IOWA ACADEMY OP SCIENCE 
181 
This seems to prove that the dark resistance was lowered by the sand-blast, and 
also that the sensitiveness was lowered the same as by heating the cell. 
I must add however that I have only once made this experiment owing to lack 
of time.” 
It may be noted here that after the fifteen-day interval, cell 31b increased 
its resistance by 100 per cent, while in the same time 32b increased by only 
43 per cent. Tt is probable that the amount of the increase by the latter was 
for both cells due to temperature change, but the additional apparent increase 
was no doubt merely a recovery from the sand-blasting effect. 
THE SLOW CHANGE FOLLOWING RUPTURE. 
It was after I noted that in Giltay’s experiments the selenium apparently 
recovered from the sand-blasting effect that I looked for a recovery from the 
filing action. The conductivity did decrease very slowly in the different tests 
that were made.' The following table gives a sample of one set of observations: 
1 ' 
.O^lT^gms. 
9:25 
A. M. 
2.5 
9 :30 
18. 
9:45 
15. 
12:00 
14.4 
3 :00 
P. M. 
13. 
86 34/1 
2 
3 
.0098 gms. 
.011 gms. 
5:00 
12. 
215 
18/1 
Jan. 
14, 12:00 
8.6 at 
23°C 
Jan. 
15, 
7.4 at 
22°C 
Jan. 
17. 
6.5 at 
22°C 
Jan. 
18, 
6.3 at 
22°C 
Jan. 
23, 
6.0 at 
21°C 
Jan. 
24, 10:40 
11:00 
3.0 
Jan. 
25, 
2.5 
53 
21/1 
Jan. 
26, 
1.8 at 
20°C 
Jan. 
29, 
1.7 at 
21°C 
Jan. 
30, 
1.4 at 
19°C 
Feb. 
11, 
1.3 at 
22°C 
49 
38/1 
Feb. 
16, 8:00 
8:05 
1.4 
10:30 
1.1 
3:00 P. M. 
.75 
Feb. 
17, 
.5 
12 
24 /I 
Feb. 
20, 
.45 
Mar. 
11, 
.2 at 
26°C 
8.8 
44 /I 
Apr. 
3, 
.16 at 
24..5 
7.9 
48/1 
Apr. 
29, 
.14 at 
24“C 
7.8 
56/1 
The foremost fact that is clear from a study of the table is that the filing 
changed the electrical properties of the selenium. After the first removal of 
the selenium by the file the conductivity increased as a result from 2.5 to 18.0, 
and that in the light from 86 to 215. The treatment however was given in the 
diffuse light of the room and consequently a small part of the increased con- 
ductivity in the light must be explained as due to incomplete recovery from 
the light. But the increased conductivity in the light can not be explained 
otherwise than on the basis of altered properties of the selenium. The amount 
of the increase of conductivity seems to diminish after the first filing. 
In all instances it is clear that there is a slow decrease in the conductivity 
after filing, much slower than any change in selenium by any other known 
