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Physics. — "Isotherms of di-atomic gases and their binary mixtures. 

 VI. Isotherms of hydrogen between -104° C. and— 217° C." 

 By Prof. H. Kamerlingh Onnbs and C. Braak. Coram. N°. 97" 

 from the Physical Laboratory at Leiden. 



(Communicated in the meeting of December 29th 190G). 



§ 1. Introduction. 



The investigation treated in this Communication forms part of the 

 investigation on the equation of slate of hydrogen, which lias been 

 in progress at Leiden for many years. J ) 



With that part of our measurements 2 ) which we now deem fit for 

 publication, we have more directly carried on the work that H. H. 

 Francis Hyndman had already done with one of us (K. 0.) before 

 1904, so that, though all the observations, one for this, another for 

 that reason, but always for the purpose of reaching the desired 

 accuracy (which, we may add, was increased in the course of the 

 investigation) have been repeated, an important share of the final 

 success of the measurements is due to the said investigator. 



The results obtained by us furnish data for applying the correction 

 of the readings of the hydrogen thermometer to the absolute scale 

 experimentally (see the following comniunical ion), and for determining 

 the deviation between the net of isotherms of hydrogen and that of 

 the mean reduced equation of state (see Comm. N°. 71, June 1901 

 and Comm. N°. 74, Arch. Néerl. 1901) *). The points determined in 



1 ) In Comm. N\ 69 (March 1901), where the apparatus have been described which 

 were used in this investigation, the Communications referring to this subject, have 

 been mentioned. Since then the isotherm for 20° C. to 60 atms. was given in 

 Comm. N°. 70 (May and June 1901) with the accuracy of which the open standard- 

 manometer (Comm. N'. 44 Oct. 1898) and the closed standard manometers (Comm. 

 N°. 50 June 1899) admit, which investigation is carried on in Comm. N°. 78 

 (March 1902) for the isotherms of hydrogen at 20° C. and 0° C, which have been 

 determined with the apparatus that have also been used for this investigation. 

 The suitability of these apparatus for accurate determinations of isotherms has 

 been shown in Comm. N'. 78, and is confirmed by this Communication for low 

 temperatures. Several communications e. g. N ,s . 83, 84, 94& and 94/, further 85 

 and 95^', finally N os . 89, 93 and 95 are more or less in connection with this 

 investigation, the great importance of which, if accurately carried out, is demon- 

 strated in Suppl. N°. 9. 



2 ) We soon hope to publish the results of measurements at higher pressures 

 and lower temperatures, and also those of supplementary determinations at lower 

 pressures. 



' A ) Definitive values for the virial coefficients B and C (§ 12 contains only provi- 

 sional values) from which the difference with those according to the reduced 

 equation may follow, are given in the following communication. 



