456 



It seems to inc to be nitlier indicated to derive tlie amount of 

 temperature energy (cf. Su|)i)l. N°. 30^/ § 1), which the molecules 

 possess at a temperature T on account of their rotations, from a 

 consideration of the equilibrium of the system of rotating molecules 

 with black radiation of that temperature, and for the latter to regard 

 Planck's formula as given, in the way as is done for the translatory 

 motion in the gas in Suppl. N". 30a. Einstein and Stern also start 

 from the consideration of this equilibrium. Addition of the zero- 

 point-energy forms then a new hypothesis which is justified by the 

 results ^). 



Instead cf resolving the molecular rotatory motion of a gas con- 

 sidered as a whole into the rotations of the separate molecules one 

 can also, imitating Jeans and Debije, resolve it into a system of 

 stationary waves. The elementary motions which come to appearance 

 in that way entirely correspond in properties with that of a Pjanck's 

 oscillator with definite frequency. It seems to me that the conside- 

 ration of the equilibrium between black radiation and molecular 

 rotatory motion in a way corresponding to that followed in Suppl. 

 N". 30a for the translatory motion, leads to the result that to each 

 of such principal modes of vibration in the system Planck's formula 

 has to be applied. This is done in ^ 2. 



1) The result obtained by Ehrenfest I.e., that with his distribution of rotation 

 frequencies without the hypothesis of a zero-point energy a good representation 

 of the results obtained by EucKEx at the lowest temperatures can be given, is 

 remarkable, hi the mean time in the observations no indication of the not mono- 

 lone course of the specific heat, which is found by him at higher temperatures, 

 is present. On the other hand a further investigation based on the assumption of a 

 zero-point energy is justified by the indications for its existence which have since 

 appeared in other fields also: cf. M. Planck, i.e. p. 455 note 3, for the molecular 

 translatory motion in gases: H. Ka.merltngh Onnes and W. H. Keesom, Communi- 

 cation to the WoLFSKEHL congress, Götlingen 1913, W. H. Keesom, Suppl. N". 30a 

 (May '13), for the free electrons in metals: W. H. ICeesom, Suppl. N^. 30^ (May '13), 

 concerning the deviations from Curie's law for paramagnetic substances: E. Ooster- 

 HUis, Suppl. W. 31 (June '13), concerning the spontaneous magnetization of 

 ferromagnetic substances : this communication I, concerning the dependence on 

 temperature of the molecular magnetic moment of ferromagnetic substances above 

 the Curie point: this communication II, to which as perhaps pointing in the same 

 direction, although in the mean time less definitely, the following can be added: 

 the behaviour of the dielectric constant of aethylaether (cf. a following paper), the 

 deviation at low temperatures from J. Becquerel's law concerning the width of 

 some absorption bands (cf. J. Becquerel and H. Kamerlingh Onnes, Comm. 

 W. 103 § 5, Febr. '08), the behaviour of the viscosity coefficient of gases at low 

 temperatures, W. H. Keesom, Suppl. N . 25 § 6 (Sept. '12), H. Kamerlingh 

 Onnes, C. Dorsman and Sophus Weber, (Jomm. W. 134 a, h, c (March, 

 April '13). 



