440 



has been explained by Einstein and Stern in a satisfactory manner 

 by the assumption of a zero-point energy. 



2. The assumption of a zero-point energy for the translational 

 motion of gas-molecides appears to be also required ^). 



3. Keesom^) has shown that the quanta-theory with the additional 

 assumption of a zero-point energy is of great importance in the 

 theory of free electrons in metals and removes a number of diffi- 

 culties inherent in the equipartition-theory. 



4. In conclusion it is shown in this paper, that the assumption 

 of a zero-point energy for the rotations finds a strong support in 

 the observations on the susceptibility of pai'amagnetic substances; it 

 appears that by means of it the majority of the deviations from 

 Curie's law observed at low temperatures ^), which seem to be 

 reducible to three seemingly largely different types may be correlated '') 

 and quantitatively explained in a satisfactory manner. 



Physics. — ''The vapour pressures of Jiydrogen from, the boiling 

 point down to near the triple-point.'' By Prof. H. Kamerlingh 

 Onnes and Dr. W. H. Keesom. Communication N". 137f/ 

 {On the measurement of very low temperatures. XXIIl) from 

 the Physical Laboratory at Leiden. (Communicated by Prof. 

 H. Kamerlingh Onnes). 



(Communicated in the meeting of June 28, 1913). 



§ 1. The vapour pressures of hydrogen between the boiling point 

 and the triple-point have been determined by Dewar ^) and by 

 Travers and Jaquerod "). The value, which was obtained at 



1) Communication to the Wolfskehl-Congress at Göttingen by H. Kamerlingh 

 Onnes and W. H. Keesom. Comp. also W. H. Keesom. Comm. Suppl. N^. 30a. 



2) W. H. Keesom. Comm. Suppl. N'\ 30&. 



•^) The abnormal behaviour of anhydrous ferrous sulphate which has a maximum 

 in its susceptibility is here left out of account. 



4) The theory given in this paper confirms the supposition by Kamerlingh 

 Onnes and Perkier mentioned in § 1 , that as regards paramagnetism corresponding 

 temperatures exist, which must be taken proportional to a definite temperature, 

 characteristic of eacli substance; on the above theory the temperature of maximum- 

 susceptibility chosen by the authors for the purpose is to be replaced by the 



"internal" temperature - -y^, which may be calculated from the zero-point energy. 



5j ,1. Dewar. Proc. Roy. Soc. London, A. 76 (1905), p. 836. 



6, M. W. Travers and A. Jaquerod. Phil. Trans. (A) 200 (1902), p. 155. 



