( 606 ) 



fmiotion, observations of ^realcr arciiracy arc roqiiircMl over an area 

 wliieli comprises llic eritical slalc and also appi'oaclies it siiriiciently. 

 These observations mnsl be acr-nrate to witiiin \ 5„„o, fis is nsnal 

 in the Leiden laboratorv in the investigations of hi- and nionatomie 

 substances and their binary mixtures, while the nature and] the 

 quantity of the separable admixtures ought to be known to Vioooo 

 of the whole mass ^). 



§ 3. Our conclusion about the existence of a disturbance function 

 in the equation of state in the neighbourhood of the critical point 

 liquid-gas is based on the following data which may be arranged 

 into three groups. 



a. In Comm. N". 74 (Arch. Néerl. (2) 6 (1901) livre jub. Bosscha 

 p. 874) has been pointed out that Amagat's obsei'vations of the 

 isothermals of carbon dioxide near the critical point show systematic 

 deviations from the values derived fron) the special undisturbed 

 equation of state. This equation of state was derived from the empiric 

 equation of state introduced in Oomm. N". 71, June '01, b}' choosing 

 the virial coeflicients so (Comm. N". 74 § 4) that the agreement with 

 the observations over the whole area of observations is as good as 

 possible while the agreement with the general reduced equation 

 of state at a reduced temperature lying far outside the area of 

 observation was retained. 



We get a similar series of observations if we compare the obser- 

 vations of carbon dioxide in the neighbourhood of the critical point — 

 described in Comm. N". 88 (Jan. '04) — with the special undisturbed 

 equation of state, while using the reduced virial coefficients V s. 1 

 (Comm. N". 74, p. 884) and the critical temperature and pressure 

 found in Comm. N". 88. 



It really appeared in Suppl. N". 14, Jan. '07 (Kamerlingh Onnes 

 and Miss Jolles) that the o-itical quantities, derived according to 

 V/öy ^0, ''V/dy2 = from the special undisturbed equation of state 

 V s. 1, show great deviations from those derived experimentally. 



A similar difference was found by Amagat (Journ. de phys. (3) 8 

 (1899) p. 353) when he derived the densities of saturated liquid and 

 vapour from the equation of state (containing 10 constants) formed 

 by him for carbon dioxide. The curve which represents the densities 

 calculated thus as a function of the temperature, at lower tempera- 



1) For such an investigation carbon dioxide would be fittest owing to liie com- 

 parably small ditliculties in preparing it perfectly pure and keeping its temperature 

 sufficiently constant, and also because much is already known about its equation 

 of slate over a large area. 



