( 678 ) 



where the onter surfaces are connected by the copper box used 

 for the vacuum jacket of Comm. N". 85. This jacket remains 

 free from dew deposit, so that the divisions on the tube can be 

 clearly read. Above the liquid surface the vacuum tube is lengthened 

 by an ordinary tube of about 50 cm, so that the solution is protected 

 from the atmosphere by a layer of cold air. 



The apparatus was e.g. placed once at about 25 meters distance 

 from the refrigerator, in another room. It would be less suitable 

 to convey the methylchloride itself over this long distance owing to 

 the increased danger from fire. 



The calcium chloride solution had a specific density of 1.28, the 

 vacuum under which the combined pumps worked was set so as to 

 produce a temperature of — 45° C. in the refrigerator and remained 

 satisfactorily constant. The small pump moves about 2 liters of 

 solution per minute. 



In order to keep temperatures below — 20° C. constant at long 

 distances by a methylchloride circulation, it will be necessary to 

 have a refrigerator with a greater cooling surface. 



Physics. — Communication N" 86 from te Physical Laboratory 

 at Leiden. "The representation of the continuity of the liquid 

 and gaseous conditions on the one hand and the various solid 

 aggregations on the other by the entropy-volume-energy surf ace 

 of GiBBs" (By Prof. H. Kamerlingh Onnes and Dr. H. Happel). 



(Communicated in the meeting of June 27, 1903\ 



§ 1. The meaning of the following research will be best made 

 clear by showing its relation to the former communications (66, 71, 

 74) from one of us (H. K. 0.). Like these it arises from the cer- 

 tainty that it is increasingly necessary to represent the experimental 

 values for an equation of state from a general point of view. 



In the first place this research is connected with that of W. 66 ') 

 on the reduced n, «, -y (rj = entropy, b = energy, v = volume) Gibbs' 

 surface formed after van der Waals' original equation of state. 



The drawings of N". 66 show that a ridge appears on the side 

 of small volumes on the Gibbs' surface given there, by shifting the 

 constant isotherm (fig. 1) of simple form (I.e. PI. I and II), derived 

 from the above mentioned equation of state, along a vertical directrix. 



1) Kamerlingh Onnes, Die reducirten GiBBs'schen Flachen. Vol. jub. Lorentz, 

 Archiv. Need. Sér. II, T. V. p. 665—678. Leiden, Comm. n^. 66. 



