179 



On the whole the same apparatus were used for both substances; 

 as regards these ^ we may therefore refer to the communication on 

 neon just quoted. Only tlie pressures were measured in a different 

 way. The readings on the closed manometer Mg„, which was used 

 in the case of neon, do not begin before 20 atm. and the critical pressure 

 of hydrogen being smaller than that, we had to use the open 

 standard-gauge of the laboratory ^) for the puipose. 



The hydrogen which had been purified by distillation was free 

 from all admixture as shown by the heterogeneous parts of the 

 isothermals in the pressure-density diagram being straight lines 

 running parallel to the density-axis (comp. tig. 1). At the same time 

 we wish our results to be considered as preliminary ones. In the 

 first place because a larger number of observations than we have 

 made (comp. fig. 1) will be needed to smooth out certain small 

 irregularities which our observations still show and thus draw with 

 sufficient certainty the set of isothermals which have to serve the 

 purpose of settling in a perfectly satisfactory manner physical 

 constants of so fundamental a character as the critical data of hydrogen. 

 But even more because in our opinion it is necessary for definite 

 determinations to be able to follow the critical phenomena by eye 

 which we have not been able to do as vet, no more than in the 



p(atn 



t 



100 



xoo 



JOO 



6(H> 



Fig. 1. 



1) H. Kamerlingh Onnes, Proc. I p. 213. Comra. N». 44. 



