EMISSION VELOCITIES OF PHOTO-ELECTRONS. 
225 
observed for the elements investigated in this research. Lindemann* has given a 
formula which connects certain constants of the elements with the maximum of the 
selective effect. Except in the case of Li, the agreement with experiment is fairly 
good. The formula is ^ gj.g 
where a is the atomic volume and n the valency. The application of this formula to 
the elements used in this research is given in Table V. It is recognised that the 
process of extrapolation to elements of valency greater than 2, has no experimental 
justification. 
Table V. 
Element. 
Atomic 
weight. 
Atomic 
volume. 
Valency. 
^raai. 
L 
Vo. 
Ca 
40-1 
2 . 5-4 
2 
A 2330 
3-17 X 10-16 
2-57 volts 
Mg 
24-3 
14-0 
2 
A 1730 
3-39 
3-08 
Cd 
112-4 
13-0 
2 
A 1670 
3-66 
3-49 
Zn 
65-4 
9-2 
2 
A 1400 
3-79 
3-77 
Pb 
207-1 
18-1 
4 
A 1390 
3 - 55 
3-42 
Bi 
208-0 
21-2 
5 
A 1340 
. 3-63 
3-37 
Sb 
120-2 
18-1 
5 
A 1240 
3-69 
3-60 
As 
75-0 
13-1 
5 
A 1060 
£2=3-7 
£2:4-5 
Se 
79-2 
17-6 
6 
AlllO 
£-4-8 
O2 
16-0 
12-6 
6 
A 940 
£2:8-0 
The calculated value of the maximum of the selective effect in Ca is well within 
the range of wave-lengths used in this research, and that for Mg is not very far 
outside. Yet there was nothing abnormal in the emission velocities from Ca 
compared with those from other metals. 
In looking for some connection with the selective effect, a remarkable relation 
between the values of k and and the atomic volume was noticed (Table V). In 
any set of elements of the same valency there is a regular increase in h and with 
decreasing atomic volume. As we pass from one valency to another, the relation is 
discontinuous, the discontinuity always being in the same direction. This may be 
expressed by saying that if each atomic volume were multiplied by a suitable fraction 
which becomes smaller with increasing valency, then the values of k and would 
change continuously with the product of the atomic volume and a factor depending 
upon the valency. 
There is no clear connection between the changes in k and Vy with changes in the 
atomic weight. 
10. Summary. —(l) The maximum velocity of photo-electrons from the surfaces of 
a number of elements prepared by distillation in vacuo has been measured. 
* Lindemann, ‘Verb. d. D. Phys. Ges.,’ XIIL, p. 482, 1911. 
VOL. CCXII—A. 2 G 
