A.—_MATHEMATICS AND PHYSICS. AT 
spectral series that would enable us to caleulate the resonance and 
ionisation potentials for this group of elements must be of the type 
v=(n,7™) — (m, x) and not of the type Y=(n, 6) — (m, ™). Moreover, 
this makes it clear that the series of wave-lengths that should be selectively 
absorbed by vapours in the normal state of the elements of the 
aluminium group would be the first and second subordinate ones repre- 
sented by V=(n, 7%) —(m, 6) and v=(n, ™) — (m, 8). 
Recent experiments by Carroll '* and by Grotrian,** as well as earlier 
ones by Wood and Guthrie’’ and by the writer,’* show that the wave- 
lengths most readily absorbed by non-luminous thallium vapour all 
belong to the sharp or diffuse subordinate series in the spectrum of this 
element. — With indium vapour Grotrian has obtained similar results. 
With aluminium, as with thallium, the wave-lengths absorbed by the 
comparatively cool vapour that surrounds an electric are in the metal 
belong to the sharp and diffuse subordinate series. 
From these results it is clear that the evidence furnished by spectral 
data amply confirms the view put forward by Bohr that in the case 
of the heavier elements of the aluminium group at least, the electron 
last acquired by the neutral atom of the respective elements is bound 
in an orbit of the n, type. In the case of the light element boron, 
the series data available are so meagre that it is not possible as yet to 
affirm that the same law applies. 
From the known values of the frequency v=(n, 7,) in the spectra 
of the elements aluminium, gallium, indium, and thallium, it follows 
that the resonance potentials for these elements are respectively 3.12 v., 
3.08 v., 3.00 v., and 3.26 v., and that the ionisation potentials are 
respectively 5.94 v., 5.96 v., 5.75 v., and 6.07 v. 
For thallium, the only element of this group as yet investigated 
by the method of electronic impact, Foote and Mohler found the reson- 
ance and ionisation potentials to have the respective values 1.07 v. 
and 7.3 v. The agreement, it will be seen, is not very close. It should 
be noted, however, that Foote and Mohler, in giving their results, 
indicate that they should be considered to be approximate only. 
Electronic Orbits of the Atoms of the Lead-Tin Group. 
It will be seen that the scheme of orbits given in Table I. makes 
no provision for the elements of the Lead-Tin group. The reason for 
this is that hitherto but little spectroscopic data have been available 
for these elements. Besides, the development of the quantum theory 
does not appear to have been sufficiently advanced to include the atoms 
of these elements within the scope of its application. Progress with 
these elements is, however, now possible owing to the fact that 
Thorsen *® has been able to organise a part of the spectrum of lead 
into a triplet set of first and second subordinate series. These series 
‘ 
Carroll, Proc. Roy. Soc., Series A, vol. 103, p. 334, May 1923. 
16 Grotrian, Zeit. fiir Phys., Bd. 12, p. 218, 1923. 
17 Wood and Guthrie, Ast. Phys. Jl., vol. 29, p. 211, 1909. 
' iy McLennan, Young and Ireton, Proc. Roy. Soc. of Canada, Section III., 
mt, L919: 
1® Thorsen, Die Naturwissenschaften, Heft 5, Feb. 2, 1923. Recent experi- 
ents by Thorsen lead to the value of 9,18'v, for the ionisation potential of gold. 
1923 F 
