( 776 ) 



present, just as when it (1896) was first mentioned as the basis of 

 the measurement of low temperatures at Leiden in the first com- 

 munication (N". 27; on this subject, still the most suitable temperature- 

 scale to determine low temperatures down to — 259° unequivocally 

 with numerical values, which come nearer to the absolute scale than 

 those on any other scale. It is therefore of great importance to 

 know the corrections with which wo pass from the normal hydrogen- 

 scale to the absolute one. 



As is known they may be calculated tor a certain range of tem- 

 peratures, when the equation of state for this region of temperature 

 has been determined al about normal density. Up to now we had 

 to be satistied for that calculation for the hydrogen thermometer 

 below 0° with equations of state of hydrogen obtained in a theore- 

 tical way. Berthelot ') derives them by means of the law of the 

 corresponding states from experimentally determined data of other 

 substances in the same region of reduced temperature. Callendar 2 ) 

 modifies van dek Waals' equation of stale so as to render it adapted 

 to represent the results of the experiments of' Joule — Kelvin for air 

 and nitrogen as well as those for hydrogen between 0° and 100°, 

 and supposes that a same form of equation holds also for hydrogen 

 outside this region. Chiefly this comes to the same thing as the 

 application of the law of the corresponding slates, albeit to a limited 

 group of substances. Though such theoretic corrections as have been 

 given by Berthelot and Callendar are a welcome expedient to help 

 us in default of other data '), yet an experimental determination of 

 these corrections remains necessary. 



We have obtained them in this research by using the isotherms 

 of hydrogen between —104° C. and —217° C. given in Comm. N°. 97°. 



1 ) Sur les thermomètres a gaz,Travaux et Mémoires du Bureau International, T. XIII. 



2 ) Phil. Mag. [6] 5, 1903. 



s ) Wroblewski's determinations of isotherms at the boiling point of ethylene 

 and oxygen are not accurate enough for this purpose. In the results found for 

 the last temperature this is immediately apparent from the irregular situation of 

 the points on the isotherm. The values obtained at the boiling-point of ethylene 

 give more harmonious results. And yet a correction on the absolute scale would 

 follow from them which has the wrong sign, viz. — 0°.07. 



At the temperature of liquid air Travers has determined the difference of the 

 hydrogen thermometer of constant volume and constant pressure, from which we 

 may also derive the corrections to the absolute scale for these temperatures. It is 

 obvious that this derivation cannot be very trustworthy. 



Further it is now possible (see ï? 1 of Comm. N°. ( J7") to derive data on the 

 expansion of hydrogen at low temperatures from the determinations of Witkowski ; 

 they will be discussed in a following communication. 



