1350 
the same for S,,, viz. (G", + 5), then the metastable unary melting 
point of S,,, (Su, L',), the corresponding point for S,,, (Su, +L"), 
and finally at a still higher pressure the unary transition point, 
(Sus + Sa): 
OBSERVATION. 
As is known it often occurs that thongh the vapour tension curves 
of two different modifications do not intersect below their melting 
point temperatures, the melting point curves of these two states do 
yield a point of intersection. In this case the system is monotropic 
under the vapour pressure, but enantiotropic under the melting pressure. 
As it is illogical to apply the terms monotropic and enantiotropic 
only to the case that the substance is under the vapour pressure, 
it is expedient to state when mentioning these phenomena, under 
what circumstances the system is considered to be. And just as we 
can now speak of monotropic and enantiotropic for a system that 
is under constant pressure, the same denominations can also be 
applied when the temperature is thought to be constant, as this has, 
moreover, been repeatedly done in this communication. 
Anorg. Chem. Labor. of the University. 
Amsterdam, Jan. 19, 1916. 
Physics. — “The Symmetry of the Röntgen-patterns of Tetragonal 
Crystals”. By Prof. H. Haga and Prof. F. M. Jaraer. 
(Communicated in the meeting of February 26, 1916). 
§ 1. For the purpose of further completing our experiments on 
the specific symmetry of the diffraction-images, which can be obtained 
by radiating through crystals by means of Rén7TGEN-rays, we publish 
in the following paper the results, which were obtained by us in 
the study of tetragonal crystals. 
It appeared to be rather difficult to study an object of all seven 
classes of the tetragonal system, while many of the hitherto known 
representations of the mentioned symmetry-classes could hardly be 
obtained in such a degree of perfection, as is required for this 
kind of experiments. Moreover, from the tetragonal-bisphenoidical 
class no representatives are hitherto known *) with certainty. 
') It is not yet certain, whether the compound: 2 CaO. Al,0; . SiO,, mentioned 
by Weypere (Anz d. Akad. d Wiss. in Krakau, 611 — 616) (1906), may indeed be 
considered to be a representative of this symmetry-class. 
