48 SECTIONAL ADDRESSES. 
have frequencies given by v=(n, p,)—(m, s)” and v=(n, px) —(m, d) 
where a has the values 1, 2,8. In all about 54 wave-lengths have been 
allocated into places in these series. In this connection it is of import- 
ance to note that Thorsen does not seem to have been able to assign 
any of the wave-lengths in the lead spectrum to a related principal series. 
Following up this work, Grotrian** has recently pointed out that 
of the wave-lengths known to be selectively absorbed by non-luminous 
lead vapour,” the prominent ones A=2833 A, A=2170 A, A=20538.8 A, 
and A=3683 A were not included in the series formulated by Thorsen. 
He has been able to show, further, that they can be included in a more 
extended scheme of first and second subordinate series that includes, 
in addition to those of Thorsen, two others that have for their highest 
frequencies v= (2, »,)=59826cem ‘andy=2, p,; = 51677 em’. According 
to this scheme A=2833 A would have the frequency v= (2, ,) —(2,s), 
A=2053 A the frequency v= (2,p,)—(3,s),A=2170 A the frequency 
v=(2, »,)— (8, d.), and A= 3683 A the frequency (2, p,;)—(Q, s). These 
results lead at once to definite conclusions regarding the outermost orbit 
in the normal atoms of lead. Since all the wave-lengths absorbed 
are members of subordinate series, it follows that the electron last 
acquired by a neutral atom of lead must be bound in an orbit for which 
the subordinate quantum number k has the value 2.‘ This leads to the 
conclusion that the scheme of orbits for lead will include two of the 
6, type and two of the 6, type. 
From the frequencies 
v= (2, p,) — (2, s) =35296em™ and v=(2, p,) =59826em 
it follows that the resonance and ionisation potentials of lead should 
be respectively 4.385 v. and 7.4 v. As Foote and Mohler have found 
by the method of electronic impact these critical potentials to be 1.26 v. 
and 7.93 v., it will be seen that while the values for the resonance 
potentials show no agreement, there is a fair agreement in the case 
of the values of the ionisation potentials. 
As very little is known about the series spectra of tin** and german- 
ium, one cannot as yet write with precision about the outermost orbits of 
the normal atoms of these elements. Considerations of periodicity make 
it highly probable that they will be of the same type as those of lead. 
This would mean that tin should have its two outermost electrons 
bound in the normal atoms of equivalent 5, orbits, and the normal 
atoms of germanium their two outermost electrons bound in 4, orbits. 
The results obtained with the series spectra of lead will no doubt lead 
immediately to the organisation of the spectral lines of tin and ger- 
manium into series. 
Though but little has been published about the series spectra of 
neutral atoms of silicon, Fowler reports that he has been able to show 
that the are spectrum of this element includes a number of related 
20 According to Thorsen 7 has the effective value 2 in this formula. 
21 Grotrian, Die Naturwissenschaften, Heft 13, March 30, 1923. 
22 McLennan and Zumstein, Proc. Roy. Soc. of Canada, Section III., 
vol. xXiv., p. 9, 1920. 
_ *3 The writer has been able to show recently that the spectrum of tin 
includes series of the same type as those of lead. + ito] ero teas 
