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§ 3. Attempt at a theory of the phenomena. 
Finely we want to make a few remarks containing aprovisional 
elementary explanation of the observed phenomena, reserving to give 
afterwards a more mathematical theory. 
We suppose that the molecules of para-azoxyanisol, as may be 
expected from the chemical constitution are of an oblong form, and 
that therefore a magnetic field will try to place their longitudinal 
axes parallel to the field. 
Further we suppose the particles to undergo a directing couple 
from the glass-wall, so that the wall tries to direct them parallel to 
itself. The forces proceeding from the wall extend like all mole- 
cular influences only to a very small distance from the wall. 
Let us admit further, that the particles influence each other, *) 
in such a way that particles try to turn their axes reciprocally 
parallel. | 
The result of the influences is that with given temperature and 
pressure two phases in equilibrium are possible, one of them being 
very sensible to a cause of outward direction, and the other not 
being so. 
In the first (the liquid crystalline) phase, there will appear in con- 
sequence of the orientating influence of the molecules on each other 
regions, wherein the axes of the molecules are grouped around a 
direction of preference. In several parts of this phase the directions of 
preference will be divided accidentically, and as a consequence of 
irregular refraction such an unarranged condition will be opalescent. 
In the other phase the effects of the molecules cannot cause 
suchlike directed regions and therefore there is no extinction. We 
hope to come back to the thermodynamics of these phases as well 
theoretically as experimentally. 
Besides the forces mentioned above the influence of the molecular 
motion, which is always opposed to the directing effect has to be 
taken into consideration. In the following we will not go into the details 
of the optical problem of extinction, but we will use in our consi- 
deration the plausible supposition that the extinction will be the 
smaller according as the arrangement is more regular. 
Let us begin our explication at the virginal state. By the influ- 
1) These latter influences cause a clustering tendency. For, if a molecule is at 
a certain point with a given direction of axis, this fact will influence the proba- 
bility of the direction of the axis of a neighbouring molecule. Consequently the 
problem with which we are occupied at present shows an analogy with the problem 
of the clustering tendency in the neigbourhood of the critical point, treated by 
Zernike and one of us, 
