181 



parts of the isothermals are accurately parallel to tiie axis of 

 abscissae proves on the one hand that the hydrogen was absolutely 

 pure and on the other that the temperature could be kept constant 

 to .01 of a depjree '). It proved possible to connect the points which 

 give the beginning and the completion of the liquefaction by means 

 of an ordinary parabola. The pressure corresponding to the top of 

 this parabola was taken as the critical pressure. The temperature 

 corresponding to the critical pressure thus found was obtained by 

 extrapolation from the measurements of the vapour-pressnre at the 

 temperatures .immediately below the critical temperatures. 



Beside the observations just mentioned on the isothermals which 

 show a heterogeneous part, an isothermal will be found plotted in 

 the figure which was determined at a slightly higher temperature 

 and where a similar part parallel to the density-axis (heterogeneous 

 condition) is no longer found. The tangent to the point of inflexion 

 is inclined to the axis of densities and this isothermal thus appears 

 to belong to a temperature higher than the critical temperature. It 

 is clear, that by means of this figure an upper and a lower limit 

 may be fixed for the critical pressure and a probable value of the 

 critical pressure may be established, from which in their turn follow 

 an upper and a lower limit and a probable value of the critical 

 temperature. The degree of certainty with which the latter deter- 

 mination may be effected is fairly satisfactory. For hydrogen the 

 two limiting values just mentioned as regards the temperature differ 

 by less than .4 of a degree. The figure further shows that in the 

 manner described the temperature itself may be considered to be 

 determinable with a certainty of .1 of a degree. 



^ 2. Results. 



In the following table of vapour-pressures the absolute temperature 

 as well as that on the centigrade scale have been corrected to the 

 KELViN-scale by means of the table of corrections published on 

 former occasions ''). 



The pressure is given in international ') cms mercury and in 



^) To make this result possible the distribution of the temperature must also 

 have been a very uniform one throughout the experimental space. We are glad 

 to express once more our thanks to Mr. J. M. Burgers, phil. caud. for his careful 

 assistance in the regulation of the temperatures. 



2) H. Kamerlingh Onnes, Proc, X, p. 589. Gomm. N". 102Ö, and H.Kamerlingh 

 Onnes and G. Holst, Proc. XVII, 1, pg. 501. Gomm. N". 141a. 



*) In the original paper the pressures jwere given in local cms mercury and in 

 international atmospheres. [Note added in the translation]. 



