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IONISATION, DIFFUSION, ROTATION 
257 
The chief points in which they improve on our crude discussion are as 
follows— 
(a) The weight factor q in the ionisation formula has been attended 
to; and the formulae have been adapted to treat symmetrical groups of 
electrons. The reason for the latter modification is that at the jpth ionisa 
tion it is usually not one particular electron which is marked down as the 
next to be detached but one of a symmetrical group. 
( b) A so-called “electrostatic correction” is introduced. In (174T) ifj 
represents the energy required to detach the electron in the actual circum 
stances of the atom in the star. But we usually prefer to regard i/iasa 
constant of the atom determined by experiment and theory. In that case 
an electrostatic correction must be applied on account of the disturbance 
of the ion by the ions and electrons around it; the ion tends to surround 
itself with negative charge since it repels other positive charges from its 
vicinity. This shields the outer part of the field of the nucleus and therefore 
less work is done in removing a bound electron to infinity. 
(c) Attention is paid to the careful calculation of successive ionisation 
potentials, following the method of D. R. Hartree. By X ray experiments 
we can only find the ionisation potentials for removing an electron from 
an orbit in a complete atom; this must be supplemented by theory to 
obtain the ionisation potential of an ion from which the outer electrons 
have already been removed. The following table for iron* giving the 
energy of removal of the _pth electron in volts and in Angstroms will show 
the nature of the results— 
Table 33. 
Successive Ionisation Potentials for Iron. 
V 
Volts 
A 
> 
Volts 
A 
V 
Volts 
A 
26 
9150 
1-35 
19 
1350 
9-16 
12 
300 
4M 
25 
8650 
1-43 
18 
1250 
9-89 
11 
280 
44-1 
24 
2010 
615 
17 
1150 
10-72 
10 
250 
49-5 
23 
1880 
6-57 
16 
480 
25-8 
9 
220 
56-1 
22 
1730 
7-15 
15 
435 
28-4 
8 
150 
82 
21 
1590 
7-77 
14 
390 
31-7 
7-2 
(80) 
(150) 
20 
1490 
8-29 
13 
350 
35-3 
1 
8-15 
1520 
(d) Due account is taken of overlapping of successive ionisations. 
(e) The terms representing excited states of the atom are includedf. 
* Hartree, Proc. Camb. Phil. Soc. 22, p. 473 (1924). Similar tables for oxygen 
and silver are there given. Tables for zinc and bromine are given in Fowler and 
Guggenheim’s paper. 
t Mr Fowler informs me that the treatment of excitation is incomplete and he 
hopes to improve it. When account is taken of atoms with more than one excited 
electron the molecular weight in stars of small mass will probably be increased. 
e 17 
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