507 



the thermal molecular pressure will be needed, if values of B are 

 to be derived iVom the comparison of thermometers with different 

 initial pressure. The same is true with respect to possible correc- 

 tions for deviations, as predicted by the theory of quanta. 



7. Approxiiiuite formula fov the vapour-pressure of helium. We 

 did not succeed in representing our observations by Nernst's vapour- 

 pressure formula, treated as interpolation-formula. 



The Bose-Rankine form ') 



1 



1 



Ig /\-m.Ug = A-\-B--^ C-,^^D- 



1 



gave with 



4= + 3.7290, £=-7.9780, 



the results shown in Table V 



C'=- 0,13628, 



7>=-f 4.3634 



Even with tins formula containing four constants the observations 

 appear lo agree only very imperfectly. 



Physics. — Methods and apparatus used in the cryogenic hd)Oratory. 

 X VI. The neon-cycle. By H. Kamerlingh Onnes. (Comm. 

 147c fi'oni the Physical Ijaboratory at Leiden). 



(Cominimicaled in the Meeliiig of .kine 2ü, 1915). 



1. Introduction. In several accurate investigations on the law 

 of dependence on the temperature of the properties of substances 

 the difficulty is encountered, when going below 55° K., that not 

 till '20° K. is reached liquid baths of the desired constancy are again 

 available. The gap between 55° K. and 20° K. in a range which other- 



1) G. A. Crommelin, Comm. N". 138c. 

 Proceedings Royal Acad. Amsterdam. Vol. XVllI. 



33 



