245 



lieat of the ''saturated electron vapour". As according to our tlieoi-y 

 tlie density of the saturated electron vapour becomes constant at 

 low^ temperature, a there reduces to the specific heat at constant 

 volume. Paying attention to the units used we therefore obtain for d, 

 expressed in Joules, at those low temperatures 



, "=!«-' 4.-^" (^) 



or 



^— 10--:.— .6^». T'F^ /g) 



Again it is easily verified that the Kelvin's relation is satisfied. 

 Hence the TnoMsoN-effect also approaches to at 2^ = 0, and 

 this according to T^ 



Physics. — "Lvt/ieniials of di-atomic substances and their binary 

 mixtures XIII. Liquid-densities of hydrogen between the boiling 

 point and the triple point ; contraction of hydrogen on freezing " 

 By Prof. H. Kamerlingh Onnes and Dr. C. A. Crommelin. 

 Comm. N". i37« from the Physical Laboratory at Leiden. 



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



Densities of liquid hydrogen have so far only been determined by 

 Dewar ^), As a more accurate knowledge of this quantity is of 

 importance for the equation of state the following measurements 

 were undertaken. They were made with the same apparatus as 

 served for argon. ^) 



The temperature was determined by measuring the vapour pressure 

 of the hydrogen in the cryostat. The sensibility of this method is 

 very great, as 1 mm. difference in vapour pressure corresponds to 

 0°.Ü04 at the boibng point and to 0°.02 at the triple point. The 

 measurement does not therefore require great care being taken. As 

 shown by measurements by Kamerlingh Onnes and Keesom, the 

 vapour pressure may be determined by reading the pressure in the 

 cryostat itself, instead of in a special vapour pressure tube filled 

 with pure hydrogen and placed in the cryostat, if an accuracy of 

 0°.02 or 0°.03 is considered sufficient. 



1) J. Dewar, Proc. R. S. (A) 73 (1904) p. 251. 



2) E. Mathias, H. Kamerlingh Onnes and G. A. Crommelin, Proc. Oct. and 

 Dec. 1912 and Jan. 1913 Comra. N". 131a. 



