2J5 



Physics. — '* Vapour pressures of suhstances 0/ loio critical tem- 

 perature at loio reduced temperatures. I. Vapour pressures of 

 carbon dio.vide between — 160° C. and — 183° C. By H. 

 Ka.mkklingm Onnks and Sophus Wkber. Comniiinication NM37^ 

 from (lie IMivsiciil Laboratory at Leiden. (Communicated by 

 Prof. 11. KAMKRLiN(in Onnes.) 



(Conunuiiicated in the mt'ctin^' of June 28, 1913). 



§ 1. Introduction. Tlie knowledge of the vapour pressures of 

 substances of low critical temperature at low reduced temperatures 

 lias recently obtained special importance'). The gradual transforma- 

 tion shown by the reduced equations of state for various substances, 

 when ordered according to their critical temperature, manifests itself 

 in particular in the gradual transformation of the reduced vapour 

 pressure curves, and tlie deviations from the law of corresponding 

 states, which show themselves clearly in this transformation and are 

 connected with Nkhnst's heat-theorem, have acquired increased im- 

 portajice by the connection with Planck's theory of energy-quanta. 

 We have therefore undertaken a series of determinations of vapour 

 pressures for substances of low critical temperature at lower tem- 

 peratures than in previous determinations. The measurement of the 

 very low vapour pressures with which we are concerned in this 

 case forms a separate branch of research. In measuring the pressure 

 in a space at ordinary temperature, connected by a tube with (he 

 space, where (he vapou.r and li(|uid or solid are in equilibrium 

 at a known low temperature, correct results can onl}^ be arrived 

 at, if due regard is paid to the theory of the thermal molecular 

 pressure. 



Knudsen '^), who has developed this theory, has shown that, when 

 a temperature gradient exists along a closed tube containing a gas 

 in equilibrium, there will in general be a pressure difference between 

 (he ends of the tube, such that the higher pressure is observed where 

 the temperature is higher. The magnitude of the ditferenee depends 

 on various circumstances, specially on the ratio of the radius of 

 the tube R to the free path of the gas molecules X. In the limiting 



1) For the lilcr;itLuc and a discussion of llic questions referred to here, see: H. 

 Kamerlingh Onnks and W. H. Keesüm: Die Znslandsgleichung. Eiic. d. math. 

 Wissensch. V. 10. Leiden Gomm. Suppl. N". 23. 



~) Martin Knudskn: Ann. d. i'hys Bd. 31 p. 205 1910. 

 >> )) » )• -J^ )' t)83 ,, 



