: 830 
lamellae; at best it may be an axis of pseudo-symmetry of this 
structure. It is, therefore, by no means improbable, that finally 
submicroscopical twinformation between the lamellar units, composing 
the pseudo-symmetrical crystal, has to be considered as the primary 
cause of the anomalies formerly observed in the Rönraer-diffraction- 
images of such crystals. 
But then the question arises, why such a twinning-process happens 
oftener in one principal direction of intergrowth, than in the other 
equivalent ones? This question must arise, however, because in 
such crystals one has to deal not with a relatively small number 
of super-imposed lamellae in each direction, but with an extremely 
great number of them. It might be supposed that there were special 
influences during the growth of the crystal from its mother-liquor, 
which caused such a directing and preferential action in this respect: 
but it is at the moment difficult to guess, of what nature those 
influences really may be. Perhaps a factor of some importance 
therein may have been the /eat-effect during the crystallisation, 
which causes convection-, and concentration-currents to appear in 
the environing liquor, corresponding in their turn to greater or 
smaller changes of the viscosity of the solution in those directions. 
It is well known, that the degree of viscosity of a medium plays 
an important rôle with respect to the occurrence of twins, and 
generally in such a way, that an increase of the viscosity appears 
most favourable to the occurrence of twin-formations. It is not 
improbable that influences of this kind may in the end appear to 
favour also the twinning of the very thin submicroscopical lamellae, 
of which the crystal is built up in one special direction. 
Perhaps systematicul investigations on the phenomena of erystal- 
lisation of such pseudo-symmetrical erystals under variable, but 
well-determined external circumstances, may in not too distant a 
future bring us better evidence on this subject. 
Laboratory for Inorganic and Physical 
Chemistry of the University. 
Groningen, April 1920. 
