Physics. — "Isothennals of di-atomic substances and their bmary 

 mixtures. XIX. A preliminary determination of the critical 

 point of hydrogen'". By H. Kamerlingh Onnes, C. A. Crommelin 

 and P. G. Cath (üommiinication W. 151c from the Physical 

 Laboratory at Leiden). 



(Communicated in the meeting of June 24, 1916). 



1. Introduction. Apparatus and method. 



The results so far obtained by different observers in the determi- 

 nation of the critical data for hydrogen show a wide divergence. 

 Dewar^) found 71. = 29° K, 30° K and 32° K respectively, /?a; = 15 

 atm., Olzewski's *) latest values were 7\. = 32°. 3 K and pk = 13 atm., 

 BuLLE ") found 7^= 31°.4 K and /;jt = ll atm. It will appear from 

 this communication that all these results differ very considerably *) 

 from those which we consider to be the correct ones, viz. 

 7\. = 33°.2K, pic =12.8 atm. In view of the great difficulty of 

 the problem this is not to be wondered at. 



We ourselves had repeatedly been compelled to defer the accurate 

 investigation regarding the critical condition of hydrogen, because 

 we were unable to get over the diflficulty of keeping temperatures 

 such as those, at which the experiments had to be carried out, 

 sufficiently constant. As soon as this difficulty had been conquered 

 by the construction of our hydrogen-vapour cryostat ') we were 

 enabled to avail ourselves of this material improvement in order to 

 arrive at a first determination of the critical temperature of neon ') 

 and also a more accurate measurement of the critical data of hydrogen. 



1) J. Dewar. inaug. Adress Brit. Ass. Adv. Sc Belfast 1902. 

 3) K. Olszewski, Ann. d. Phys. (4) 17 (1905) pg. 986, Ann. de chimie et de 

 phys. 8 (1906) pg. 198. 



5) F BuLLE, Physik. ZS. 14 (1918) pg. 860. 



Bulle's results differ a good deal more from ours than Olszewski's much 

 earlier results. 



'*) Wilh the exception of Dewar's j^k and ours (Note added in the translation). 

 In fact the weaker point in Olszewski's work will have been probably the determi- 

 nation of the temperature. 



6) H. Kamerlikgh Onnes, Proc. XIX (2) p. 1049. Gomm. N'\ 151a. 



6) H. Xameblingh Onnes, C. A. Crommelin and P. G Cath, Proc. XIX (2) p. 1058. 

 1917. Gom.m. N". 1516. 



