332 
IOWA ACADEMY OF SCIENCE 
T-T 
1 2 
T-T 
3 2 
T-T 
4 2 
T 
1 
T 
2 
T 
3 
T+T/2 
1 2 
c 
k/k 
2 
X Const 
R 
State of 
Selenium 
1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
10.9 
— 3.3 
6.7 
27.2 
16.3 
13.0 
21.8 
27 
.34 
2.48 
dark 
10.9 
- 3.4 
6.3 
27.6 
16.7 
13.3 
22.1 
67 
.35 
light 
12.5 
- 4.1 
3.7 
31.8 
19.3 
15.2 
25.6 
33 
.37 
2.12 
dark 
12.8 
— 4.2 
3.7 
32.1 
19.3 
15.1 
25.7 
70 
.37 
light 
19.4 
— 7.1 
— 2.3 
44.7 
25.3 
18.2 
35.0 
41 
.39 
1.75 
dark 
19.5 
— 7.2 
— 2.6 
45.1 
25.6 
18.4 
35.4 
72 
.39 
light 
27.9 
— 9.8 
^ 7.7 
58.6 
30.7 
20.9 
44.7 
45 
.36 
1.67 
dark 
28.2 
— 10.4 
— 7.7 
58.9 
30.7 
20.3 
44.8 
75 
.38 
light 
46.1 
— 17.5 
— 19.8 
88.9 
42.8 
25.3 
65.9 
53 
.39 
1.55 
dark 
46.4 
— 17.5 
— 19.2 
88.6 
42.2 
24.7 
65.4 
82 
.38 
light 
61.7 
— 23.8 
— 28.8 
113.5 
51.8 
28.0 
82.7 
92 
.39 
1.28 
light 
61.7 
— 24.4 
— 29.4 
114.1 
52.4 
28.0 
82.3 
72 
.40 
dark 
73.8 
— 29.1 
— 37.4 
134.2 
60.4 
31.3 
97.3 
72 
.40 

dark 
Conclusion . — Contrary to the experiments described above, by 
Bellati and Lussana, I have failed to detect any increase in the 
thermal conductivity of selenium as a result of exposure to 
light. At any rate while the electrical conductivity increased 
nearly 300 per cent., the thermal conductivity increased, if at 
all, less than 4 per cent. This does not seriously disturb the 
accepted electron theories, but makes us modify them in con- 
nection with the element selenium. It may be that the number 
of free electrons in the selenium is much smaller than the num- 
ber in ordinary good conductors, and so that even if a large 
number of them are made free, there are still too few of them 
to have much effect on the thermal conductivity. In other words 
the thermal conduction is by means of the atoms and mole- 
cules. On the other hand it may be quite possible that there 
are no free electrons in the selenium in the sense of being com- 
pletely free, but that they are unstable in the atom, and that the 
action of light makes them more unstable. The electrical con- 
duction takes place then because the electric field can draw 
the electrons out of the atom. There being no field in the case 
of the thermal conduction, the electrons would stay in the atom, 
and hence there would be no change in the thermal conductivity 
with the increased illumination. 
Physics Laboratory, 
University of Iowa. 
