TEMPERATURE AND THE PROPERTIES OF GASES 45 



from actual isotherms by Kamerlingh Onnes and Braak for 

 hydrogen and helium are to be taken with much greater con- 

 fidence than those obtained by wide extrapolation of experi- 

 mental values or from theoretical considerations, using some 

 equation of state. In each case the differences between real 

 and absolute may be expressed by means of a series, if the 

 observations are sufficiently numerous and accurate to allow 

 the coefficients to be obtained. For hydrogen this is the case, 

 and in a series of the form 



(12) 



A/ = a f- b 



100 



VIOO/ \IOO/ MOO/ 



the coefficients have the following values in the range + ioo° C. 

 to — 217-4° C. 



a = - 0-0143307 

 b = + 0*00669 1 6 

 c — + 0-0049175 

 d — + 0*0027297. 



Similar differences can be obtained for other gases by 

 correspondingly careful measurements. 



The following table gives values calculated from the above 

 equation for hydrogen and from experimental isotherms for 

 helium, where, however, the values have been interpolated in 

 the experimental range. Those in square brackets are extra- 

 polated. 



Table II. Corrections to Absolute Scale, International 



Thermometer 



These values appear to be as accurate as it is possible to 

 obtain them at the present time, except by a direct measurement 

 of the values of B and C at the temperature concerned, which 

 is naturally more likely to give a correct value. 



