TEMPERATURE AND THE PROPERTIES OF GASES 43 



points at about 127 K., 20 K., and 5 K. respectively, as they 

 practically cover the range of exact measurements 0= 1 to 0= 10. 

 For the first gas the values of Amagat are used, for the second 

 those of Onnes and Braak, for the third those of Onnes, and in 

 each case the empirical expression of actual results by means of 

 equation (6) will be used, as these coincide with the actual 

 isotherms within the limits of experimental error. The 

 hydrogen results are the most important on account of their 

 accuracy and the wide range of temperature covered, so that 

 both the Kelvin zero and the variations from the Kelvin scale 

 can be obtained from the same set of measurements. 



There are two types of standard thermometers used — those at 

 constant volume and constant pressure ; but as the latter is less 

 simple, and in most cases the corrections are larger, the constant 

 volume thermometer is used more frequently, excepting at high 

 temperatures. With an initial pressure of 760 mm. or 1 atmo- 

 sphere at zero C. it is known as the normal hydrogen, helium, or 

 other gas thermometer as the case may be, and with an initial 

 zero pressure of 1,000 mm. as the international thermometer. 

 With these small densities all terms above the third in equation 

 (6) become vanishingly small, and even the third has very small 

 influence, so that obtaining the corrections at these pressures 

 resolves itself into the problem of measuring the value of B as 

 accurately as possible. 



The value of the absolute zero is usually obtained by 

 correcting the pressure coefficient at one of the standard 

 temperatures mentioned above to a zero density by the aid of 

 the second and third terms of equation (6), which gives the 

 deviations from the perfect gas state of equation (2). This 

 deduction depends on the assumption that at limitingly low 

 pressures any gas will be in a state where its deviations from 

 the Boyle-GayLussac-Avogadro law expressed by equation (2) 

 may be neglected. With a constant volume thermometer the 

 pressure coefficient is the change of pressure with a given 

 known interval of temperature, which is usually taken to be 

 zero C. to ioo° C, as these points are obtainable with very great 

 accuracy, or rather the exact value of the boiling point of water 

 (although usually not exactly ioo° C.) is easily determinable at 

 the time of the experiment. Hence the pressure coefficient 



t- t — T ' an< ^ ^ we 0Dtam ^is relation with equation (1) 



