Physics. — ''On the medsurement of loio temperatures. XXVII. 

 Vapour-pressures of hydrogen in the neighbourhood of the 

 boiling point and between the boiling point and the critical 

 temperature.'' By P. G. Cath and H. Kamerlingh Onnes. 

 (Communication No. 152« from tlie Pliysical Laboratory at 

 Leiden). 



(Communicated in the meeting of June 30, 1917.) 



§ J. Introduction. Having obtained in the hydrogen-vapour cryostat ^) 

 the means of keeping temperatures constant for a considerable time 

 between the boiling point of hydrogen and the melting point of oxygen, 

 we were able to carry out a long desired determination of the 

 vapour-pressure of hydrogen above the boiling point. Our investiga- 

 tion on this subject was in connection with the determination of the 

 critical point of hydrogen and extends as far as this point. Accordingly 

 a few of our results falling in the immediate vicinity of the critical 

 temperature have already been published in the paper on the latter 

 subject. ') 



In order to obtain a connection with the measurements of 

 Kamerlingh Onnes and Keesom of the vapour-pressure below the 

 boiling point, a few determinations were made in the neighbourhood 

 of this point by means of the cryostat with liquid hydrogen and 

 the vapour-pressure apparatus which were also used by the authors 

 mentioned. 



As we shall see in ^ 5 the agreement at the boiling point of 

 hydrogen is not so close as we thought we might expect, considering 

 the degree of accuracy of the measurements in both cases; conse- 

 quently a renewed investigation with a view to establishing the 

 boiling point of hydrogen with the accuracy required remains much 

 to be desired. 



1) H. Kamerlingh Onnes, Coram. No. 151a. These Proceedings XIX (2) p. 1049. 



2) H. Kamerlingh Onnes, C.A. Crommelin and P. G. Cath, Comm. No. 151c These 

 Proc. XX(1) p. 178. The present communication we have included in the series "On 

 the measurement of low temperatures", as it forms an immediate continuation of 

 Comm. XXIII of that series. It is, however, also closely connected with the series 

 "isothermals of di-atomic substances etc.", of which the communication just quoted 

 froms NO. XIX. 



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