441 



Leiden for the heat of vapoiization of liydrogen at the boiling i)oiiil ^), 

 differed, however, appreciably from the values which follow with 

 the aid of the Ct.apeyron-Ct.ausuts formula from (he measurements 

 of the vapour pressures mentioned above, taking into account the 

 compressibility of hydrogen vapour according to the measurements 

 of Kameklingh Onnes and W. J. dk Haas"). A deviation between 

 the AvoGADRO scale of temperatures fixed by the aid of the gas 

 thermometer, and the Kelvin scale of temperatures') of such an 

 amount as would be indicated by the difference between these results 

 may be considered improbable ''). It seemed therefore of interest 

 to undertake a new determination of the v^apour pressures of hydrogen 

 in the region mentioned. 



§ 2. The measurements were made with the aid of a vapour 

 pressure apparatus as described by Kameklingh Onnp^s and Braak 

 in Coram. No 107a (May 1908) PI. I apparatus A.. 



The temperatures were measured ^) with the aid of the platinum 

 resistance thermometer ") Ptf. We could avail ourselves of a more 

 recent comparison (May 1913) of Ftj with the hydrogen thermo- 

 meter performed at 6 points, regularly distributed over the range 

 from the boiling point to near the triple-point, by Kamerlingh Onnes 

 and HoLST in their research concerning the comparison of the scales 

 of the hydrogen and the helium thermometers. The corrections^) to 



1) For a preliminary report see: W. H. Keesom, Handelingen 13de Nat. en 

 Geneesk. Gongr. 1911, p. 181. According to a later calibration of the ampère- 

 and voltmeter, which were used, with the aid of a water calorimeter the values 

 given there are to be diminished by an amount of SO/qo- Hence the value found 

 with the smallest velocity of vaporization for the heat of vaporization of hydrogen 

 at the boiling point (more accurately at an average pressure of 751,5 mm.) becomes 

 110,2 ca]i5. The vapour pressures according to Dewar lead to the value 106, 

 whereas those according to Travers and Jaquerod give a still smaller value. 



2) H. Kamerlingh Onnes and W. J. de Haas. Gomm. N^. 127c (May 1912). 



3) Gf. H. Kamerlingh Onnes and W. H. Keesom. Math. Enz. V 10, Leiden. Gomm- 

 Suppl. N". 23, § 82a. 



*) This view is supported by the fact, that Keesom, Suppl. N". 30a § 4, has 

 found that the application of the quantum-theory with the introduction of the, 

 zero-point energy to the molecular translatory motion for a gas at such a density 

 that the dimensions and the mutual attraction of the molecules need not to be 

 considered, leads to an appreciable divergence from the equation p = IiT,v at 

 extremely low temperatures only. 



5) We gladly record our cordial thanks to Mr. G. Holst for the aid afforded 

 to us by performing these temperature measurements. 



6) The bath was kept constant within Vijoo of a degree. 



^) The corrections given here differ somewhat from those given in the Dutch 

 edition of this paper, in accordance with the results of the research of Kamerlingh 

 Onnes and Holst mentioned in the text. 



