( 288 ) 



Physics. — • „Measurements on ihe system of isothermal lines near 

 the plaitpoint^ and especially on the process of the retrograde 

 condensation of a mixture of carbonic acid and, hydrof/en^\ 

 By Dr. J. Vekschaffelt. (Comraunieatioii W. 45 from 

 the Physical Laboratory at Leiden by Prof. TT. Kamer- 

 ling n Onnes). 



(Road in tlio Meetinn- of Dpcoiiil)Pr 24"' 189S). 

 § 1- 



In one of liis first comniunications on the process of the retrograde 

 condensation Kuenen has given some measurements, from which the 

 course of the ratio of the volume of the liquid-pliasc to that of tlie 

 vapour-phase along one definite .r-line (van der Waals' y/surface) 

 may be deduced. It was desirable to get a general view of the 

 process of condensation along different *-lines on one and the same 

 «//-surface (especially between the ;(• at the critical plaitpoint and the 

 ,r at tlie critical point of contact) or, what comes to the same in 

 this case, to get to know, for a definite composition of the mixture, 

 the process of the condensation along lines belonging to i//-surf:xces 

 for different temperatures, near and especially between the tempera- 

 ture of the plaitpoint and that of the critical point of contact for 

 this definite iv. 



An opportunity for making measurements on this subject was 

 offered by a more ample investigation on isothermals of mixtures of 

 carbonic acid and hydrogen which was undertaken by myself^). 



§ 2. Method. 



The pure hydrogen was prepared by means of the apparatus used 

 in filling the hydrogen-thermometers for the measurement of low 

 temperatures, and formerly described ^). 



The pure carbonic acid was obtained from the commercial liquid 

 gas, following the method first applied by Kuenen ^) and now re- 

 gularly used in the Physical Laboratory at Leiden. The way of 

 preparing the mixture and the apparatus used were mainly the same 

 as those formerly used bij Kuenen in a similar investigation (see 

 the annexed figure). 



') Compare Kamerlingh Onnes, Versl. Kon. Akad., Dec. 1894, pg. 179. 

 2) Kamerlingh Onnes, Versl. Kon. Akad., May 30tb 1896; June 27tli 1896. Coram 

 I'r. the Phys. Lab. of Leyden, N". 27. 

 ^) Diss. Leiden, 1892. 



