. 451 



Prism and interferometer were inonnted on the piers cemented 

 to the large brick pier of the laboratory . The tnbes are entirely 

 disconnected from the interferometer and mounted on a large iron 

 [ girder; this girder is placed npon piers of freestone cemerited to 

 large plates of fi'eestone fixed to Ihe wooden laboratory floors. In 

 this manner the adjustment of the interferometer cannot be disturbed by 

 vibrations proceeding from the tid)es. At the right of the horizontal 

 projection the four large valves may be seen, by turning which the 

 current was made to flow in either direction through the tube systems. 



The monntings containing the glass plates by which the tubes 

 are closed are not given in the Plate. One of these mountings con- 

 taining the plane parallel plates of glass is drawn to scale in Fig. 3 

 at one half of the natni-al size. The four plates of glass are by 

 HiLGER, the^ are circular of 24 m.m. diameter and JO m.m. thick; 

 in a second series of observations plates 7 m.m. thick have been 

 used. The accuracy of parallelism of the plates is excellent; they 

 are indeed cut from echelon plates. The general plan adopted for 

 the construction of the j)late mountings is this: one can only be 

 sure that no change will occur in the position of the plates during 

 the course of an experiment, if this position is entirely definite. In 

 order to attain this the glass plate rests upon the inner, accurately 

 grinded, surface of the brass piece d. This piece d fits accurately 

 into the conical inner part of a piece />, itself rigidly screwed to 

 the tube a. Parts d and h are connected by means of the counter 

 nut c. The glassplate is held against d by the nut e. There is no 

 objection to the presence at the inside between e and d of rings of 

 hard india-rubber and of brass. {To he continued). 



Physics. — "A nefv relation between the critical quantities, and on 

 the unity of all substances in their thermic behaviour." (Con- 

 clusion). By Dr. J. J. van Laar. (Communicated by Prof. H. A. 



LORENTZ). 



(Communicated in the meeting of April 24, 1914). 



By way of supplement we shall add the calculation of three more 

 isotherms below the critical temperature, for which (loc. cit.) data 

 are known from the unsaturated vapoui* region. If the |i-values above 

 Tk were somewhat too high on the w^hole, now we shall find values 

 which are much too krio, lower even than /?„, and therefore impos» 

 sible. These deviating \alues can only be explained, when \\ilhlo\v 

 temperatures and large volumes a.'^sociation in ihe vaiwur is assumed. 



