A.—_MATHEMATICS AND PHYSICS. 58 
St. Meyer,*’ Curie,’ and Soné” that the atom ions of potassium and 
chlorine and the atoms of argon are diamagnetic. 
Since these ions and the atoms of argon contain the same number 
of electrons, and since the electrons in all three are supposed to be 
bound in orbits of the same type and of the same area, one would 
expect them to show identical diamagnetic properties. The experi- 
mental results, however, do not appear to support this view. While 
the specific magnetic susceptibility of argon has been shown 
by Soné to have the value 5.86x10-°, that of the singly-charged nega- 
tive atom ions of chlorine and of the singly-charged positive atom ions 
of potassium have been found from observations on the magnetic pro- 
perties of potassium chloride to be equal approximately to 0.55x10-*, 
i.e. the diamagnetic susceptibility of argon is about ten times that of 
the ions of potassium and chlorine. As Pauli** has shown that this 
high value of the diamagnetic susceptibility of argon leads on certain 
simple assumptions to a value for the moment of inertia of the atoms 
of argon about ten times too great, it would appear that the discrepancy 
arises in connection with the evaluation of the diamagnetic susceptibility 
of argon. Although all the experimental work involved appears to 
have been carefully done, it is evident that the mvestigation of the dia- 
magnetic properties of these elements will have to be carried further 
before the matter is finally cleared up. 
Quantisation in Space. 
One of the most surprising and interesting developments of the 
quantum theory is that which shows that quantum numbers determine 
not only the size and form of the electronic Keplerian orbits in atoms, 
but also the orientation of these orbits in space with regard to a favoured 
direction such as that provided by an intra-atomic or by an external 
magnetic or electric field of force. For any-arbitrary value of the 
azimuthal quantum number k, the simple theory shows that there 
are exactly k+1 quantum positions of the orbital plane characterised 
by whole numbers. For example, if k=1 the normal to the orbit may 
be either parallel to the direction of the controlling field or at right 
angles to it. If k=2 the normal to the orbit may take up in addition 
to these two positions a third one, in which the normal to the orbit 
makes an angle of 60° with the field. For higher values of the quantum 
number k, the possible orientations of the corresponding orbits become 
regularly more numerous. 
A striking confirmation of this theory is afforded by the very 
beautiful experiments of Gerlach and Stern.** In these a stream of 
atoms of vaporised silver was allowed to flow past a wedge-shaped 
pole of an electromagnet which provided a radial non-uniform magnetic 
40 St. Meyer, Ann. der Phys., vol. 69, p. 239, 1899. 
41 Curie, Jl. de Phys., 3, 8. 4, p. 204, 1895. 
42 Soné, Téhoku Univ. Sc. Rep., vol. 8, pp. 115-167, Dec. 1919, and Phil. 
Mag., vol. 39, p. 305, March 1920. 
43 Pauli, Zeit. fiir Phys., Bd. Heft 2, p. 201, 1920. 
44 Gerlach and Stern, Zeit. fiir Phys., vol. 7, p. 249, 1921: vol. 8, p. 110, 
1921; vol. 9, p. 349 and p. 353, 1922. 
