A.—MATHEMATICS AND PHYSICS. 43 
circular orbits with planes inclined at an angle to each other. Bohr 
assumes this angle to be 120°, and on account of the interaction between 
the two electrons the two orbits are supposed to be slowly turning 
about a fixed momentum axis inthe atom. A diagrammatic representa- 
tion of this model is shown in Fig. 6. 
Such a model, however, will not account for the whole of the 
spectrum of helium, which is known to consist of two complete but 
separate sets of series, the one being made up of single lines and the 
other of doublets. An important feature of the spectrum of helium, 
too, is that it contains no lines that are the result of combinations 
between spectral terms belonging to one of the sets of series and those 
belonging to the other. The explanation put forward is that while 
helium in its normal state exists in the form of atoms with crossed 
orbits, designated by the name parhelium, it can also exist in a meta- 
stable form, known as orthohelium, as well. In the latter state the 
electronic orbits are supposed to be in the same plane with the electrons 
revolving in the same direction. In the most stable form of ortho- 
F1G. 6.—Helium Model. 
helium oue of the electrons is supposed to move in a 1, orbit and the 
second in a 2, orbit. The singlet series in the spectrum of helium are 
assigned to parhelium and the doublet series to orthohelium. 
If parhelium be bombarded by electrons it appears to be possible to 
transform its atoms into the metastable form, but once the atoms are 
in the latter state it does not seem to be possible for them to revert 
directly to the normal form by means of a simple transition accom- 
panied by the emission of radiation. They can only do so by a process 
analogous to a chemical reaction involving interaction with atoms of 
other elements. 
The fact that helium, under certain conditions, can be made to emit 
a band spectrum in addition to its line spectrum connotes the possi- 
bility of helium existing in the molecular form. Since helium in the 
form of orthohelium has its outer electron in a 2, orbit, the atoms of 
orthohelium in so far as chemical combination is concerned occupy a 
position analogous to atoms of lithium, which also possess a 2, orbit 
in their normal state. As this feature enables lithium to react 
