54 SECTIONAL ADDRESSES. 
field. The atoms were caught on a glass plate placed immediately 
behind the pole, and it was found that they were deposited in two 
distinct sharply defined layers, indicating that the atoms were sorted 
out into two distinct and separate beams. ‘The positions of the bands 
on the plate showed that one of the beams was attracted by the pole 
and the other repelled by it, the attraction being slightly the greater in 
intensity. No evidence was obtained of an undeflected beam. From 
these results it was concluded that all the silver atoms in the stream of 
vapour possessed a definite magnetic moment, and that while the atoms 
were passing through the magnetic field their magnetic axes had two 
distinct orientations in space. 
By assuming the correctness of this interpretation, Gerlach and 
Stern found from measurements on the various magnitudes involved 
in the phenomenon that within the limits of error of their experiments 
the magnetic moment of the normal atom of silver in the gaseous 
state was that of one Bohr magneton. 
Bohr, also, has drawn attention to another possible illustration of 
the principle of the quantisation of orbits in space. It is known that 
all the rare gases do not exhibit the property of paramagnetism. From 
this fact the conclusion has been drawn that the atoms of these gases 
in their normal condition do not possess any angular momentum. 
According to the quantum theory, however, this conclusion may not be 
warranted, for we have seen that for an atom which has a finite angular 
momentum and, consequently, possesses a magnetic moment, the 
theory prescribes certain definite directions for the axis of momentum 
relative to a magnetic field in which the atom may be situated. If 
we assume that the atoms of the rare gases in a magnetic field can place 
themselves with their momentum axes perpendicular to the magnetic 
field, it follows that they could appear to be diamagnetic, and all indica- 
tion of paramagnetism on their part would be absent. In this connec- 
tion I may point out that Bohr has made the suggestion that evidence 
in support of the validity of this view is derivable from the results of 
an analysis, on the basis of the quantum theory, of the anomalous 
Zeeman effect shown by the rare gases. 
One point that may be worthy of notice in dealing with phenomena 
associated with the principle of space quantisation is that the permitted 
orientations depend only on the values of the quantum number involved, 
and not on the magnitude of the magnetic field applied. 
Orbits characterised by certain definite values of the quantum number 
should take up their permitted orientations in weak magnetic fields as 
well as in strong ones, provided the time allowed for the process to 
take place was ample, and provided suitable pressures were used and 
disturbances arising from the presence of contaminating gases were 
eliminated. Such conditions as these have recently been realised by - 
Gerlach and Schutz,**° and they have been able to obtain with sodium 
vapour at low pressures in the absence of foreign gases remarkably 
striking manifestations of the magnetic rotation of the plane of 
45 Gerlach and Schutz, Die Naturwissenschaften, vol. 11, Heft 28, p. 638, 
1923. 
