1321 
in the Physical Laboratory, had proved already that the substance 
we had under consideration is paramagnetic. 
For the moment these principal points may suffice. Perhaps we 
will discuss these questions when publishing further experimen- 
tal results. 
We gladly use the occasion to thank Prof. van Rompuren for his 
kindness in putting his preparation at our disposal. 
Summary. 
1. A new method is described to measure with the aid of ther- 
mopile and galvanometer the extinction of liquid-erystalline sub- 
stances. 
2. The very different influence on the extinction of a vertical 
(longitudinal) and a horizontal (transversal) magnetic field is traced. 
3. An explanation of the observed phenomena is drawn in outline 
whereby the principal supposition is that the wall of glass directs 
the particles in planes parallel, directs then according the lines of 
force. 
Physical Laboratory, Institute for Theoretical Physics. 
Ttrecht, October 1916. 
Physics. — “The clustering tendency of the molecules at the critical 
point’. By Prof. L. S. Ornstein. (Communicated by Prof. 
H. A. Lorentz). 
(Communicated in the meeting of May 27, 1916). 
In a former communication by Dr. F. ZERNIKE and the author *) the 
arrangement of the molecules in space using a new method of pro- 
bability is deseribed, more accurately than this was possible in the 
considerations of von SMoLvcHowsKr and Einstein; by this method 
it was also possible to calculate the opalescence at the critical point 
itself, which was impossible with the formulas of Kwersom-EINSTEIN. 
We introduced a function /, defined in the following way. Suppose that 
space is divided into a great number of elements of volumedV,dV,...dV, 
etc. The numbers »,,»,,r, etc. may represent the deviations of the 
average number of molecules in these elements. Then, if the devia- 
tions in all the surrounding elements of the working sphere are 
gwen (r‚r‚ etc), the average deviation in the element dV, may be 
represented by: 
1) Accidental deviations of density and the opalescence at the critical point of a 
single substance. These Proc, XVII, p. 793, 1914, 
