1196 
8. The diffraction tells us something of the size of the erystals 
and by this means possibly of the temperature at which they have 
been formed: “with falling temperature the size of the erystals dimi- 
nishes” 5. In that way the halo-colours, which have been too much 
neglected, may possibly contribute to a better knowledge of the 
higher atmosphere. 
VI. Conclusions. 
The above investigation seems to me to justify the following 
conclusions : 
1. The simple refraction-theory cannot explain the halo-phenomena 
completely, in particular as regards the great variety of the colours. 
2. The diffraction-theory gives a simple explanation of the colours 
which appear and allows special conclusions to be drawn regarding 
the influence of the size and the shape of the crystals. It alone 
gives the ordinary circle its correct place of 22°. 
3. The rings which have been observed in the neighbourhood of 
22° are secondary diffraction-rings: their radii are not constant. 
4. The diffraction-theory will probably be able to afford a better 
insight into the formation of the circumzenithie arc. 
5. It is necessary that the colours be accurately recorded by 
each observer in order to permit a further testing of the theory and 
a complete deduction of the origin of the observed phenomenon. 
Chemistry. — <“Jn-, mono- and divariant equilibria’. XVI. By 
Prof. F. A. H. ScHREINEMAKERS. . 
(Communicated in the meeting of March 31, 1917). 
The regions in the P,T-diagram. 
In communication VIII we have already briefly discussed those 
regions; now we shall consider them more in detail. When the 
equilibrium 
| genet ee ge ET ee a a EN 
consists of components, then it is generally divariant; consequently 
it is generally represented in the P,7-diagram by a region. We 
shall consider this region # in its whole extensity, viz. without 
taking into consideration that some parts may become metastable 
by the occurrence of other phases. 
With a definite equilibrium /# we may distinguish: 
1) Penner, |. c. p. 289, 
