AA SECTIONAL ADDRESSES. 
vigorously with other atoms, one would expect orthohelium also to be 
capable of entering into chemical combinations. From this it would 
appear that molecular helium originates in atoms that have undergone 
a transition into the metastable state. As to the atoms of parhelium, 
there appears to be no warrant of this or any other character for sup- 
posing that they can participate in any kind of chemical union. 
It is probable that orthohelium, if obtainable in sufficient amounts, 
may be found to be more easily liquefied and solidified than parhelium. 
It would, however, in the liquid or solid state be highly explosive. This 
will be seen from the data in Table II. A study of the band spectrum 
of helium or of its compounds at low temperatures would be interesting 
for what it might reveal regarding the origin of the spectrum of nebule. 
The views just presented have gained strong support from Frank and 
Knipping’s experiments on the excitation potentials of helium atoms 
by electronic bombardment, and by Lyman’s recent work on the 
extreme ultra-violet spectrum of helium, in which it has been shown 
that radiation of the wave-lengths 600.5 A, 584.4 A, 537.1 A, 522.3 A, 
and 515.7 A are absorbed by helium in its normai state. The scheme"? 
set forth in Fig. 7 and the data collated in Table IT. are self-explanatory, 
and show how on the view just put forward the radiation whose wave- 
lengths were measured by Lyman can originate, and how the excitation 
potentials observed by Frank and Knipping can be realised. 
According to this scheme electrons with a speed corresponding to a 
potential of 19.75 volts will be able to transform parhelium into ortho- 
helium, and those with speeds corresponding to 20.55 volts and 
91.2 volts will be able to lift the electrons from 1, S orbits to 2, S and 
2, P orbits respectively. Under bombardment by electrons with speeds 
the equivalent of 24.5 volts the helium atoms will be ionised. The 
scheme shown in Fig. 7 also indicates how the series spectrum of 
orthohelium originates. 
The considerations set forth above would seem to clear up some of 
the difficulties that have hitherto been encountered in realising a satis- 
factory model of the helium atom, and in reaching an explanation of 
the origin of the radiation that atoms of helium can emit. The com- 
plete solution of the problem, however, has received a set-back from 
the results of an investigation recently carried out by Kramers,** for 
according to his calculations the ionisation potential of the crossed 
orbit model comes out 3.8 volts less than the experimentally determined 
value. His calculations also show that in a mechanical sense the 
crossed orbit model cannot be considered to be a stable one. Although 
real progress has been made, it cannot be said that finality has been 
reached in the determination of the form of a completely satisfactory 
model of the atom of so simple an element as helium. 
A somewhat novel aspect of the problem has recently been empha- 
sised by Silberstein.** | He assumes the crossed orbit model of the 
helium atom to be capable of taking up a number of stationary states 
10 (frotrian, Die Naturwissenschaften, Heft 17, p. 321, 1923. 
11 H, A. Kramers, Zeit. fiir Phys., vol. 13, p. 339. 1923. 
12 Silberstein, Nature, Ap. 28, p. 567, 1923, and July 14, p. 53, 1923. 
