431 
relations between melting point, expansion etc. Bat no definite 
statement of the periodicity of coëfficiënt of linear expansion of 
elements with their atomic weights is available. 
Grüneisen [Ann. Phys. 1910 (IV), 33, 33 — 64] has found that the 
observed expansion of metals by heat is, in general, in close agree- 
ment with that required by Thiesen’s (Ber. Deut. Phys. Ges. 1908, 
6, 947) exponential formula l 2 — / 1 =.y(7 7 2 1 + e — T^+ e ) and this agree- 
ment is particularly good at low temperatures. The experimental 
given e is a periodic function of the atomic weight of themetaland 
that its maximum values are reached when the atomic volumes are 
at their respective minima. It has now been found out that the 
coëfficiënt of linear expansion of elements is also a periodic property. 
By plotting the values given in Lanüolt and Börnstein’s Tabellen 
well defined periodic curves are obtained. The alkali metals lie on 
the topmost points, whilst S, Se and Te lie on a straight line on 
the ascending portions of the curve. 
Cu, Ag and Gold, as well as As, Sb and Bi occur in similar 
positions in the minima. 
Molecular Magneüc Rotation. 
If we imagine a layer of unit length of any substance placed in 
a magnetic field of unit intensity and traversed by a beam of homo- 
