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of symmetry probably by twinning, — we found in all cases an agree- 
ment between the actually observed and the theoretically predicted 
symmetry of the RÖNTGEN-patterns of these tetragonal crystals. The 
principal correctness of the diffraction-theory is thus proved here 
sufficiently for tetragonal crystals also. 
Furthermore it could be shown, that penta-erythrite cannot be of 
ditetragonal-pyramidal symmetry, as hitherto assumed, but that 
it can probably be only tetragonal-pyramidal. 
Laboratories for Physics and for Inorganic 
and Physical Chemistry of the University. 
Groningen, February 1916. 
Mineralogy. — “On Rintgen-patterns of Lsomorphous Crystals.” 
By Prof. F. M. Jaeger and Prof. H. Haga. 
(Communicated in the meeting of February 26, 1916). 
§ 1. In the following paper we wish to discuss the results, which 
were obtained by us in radiating through the similarly orientated 
planparallel plates cut from crystals, which are related to each other 
as direct “isomorphous” ones. To avoid unnecessary complications, 
we used for this purpose crystals of inorganic substances of relatively 
simple chemical composition, in whose analogously built molecules 
every time one single species of atoms was substituted by another 
closely related species. Thus in comparing one term of a series 
with another one, only one single of the structural elements is 
substituted by another one. 
§ 2. The isomorphous compounds, whose Röntgen-patterns were 
compared with each other, were the following: 
a. The series of the rhombic-bipyramidal, typically pseudo-hexagonal 
carbonates of calcium, strontium, barium and lead, which are found 
in nature as the minerals: arragontte, strontianite, witherite and cerussite. 
b. The carbonates of calcium antl manganese, which are found 
as the ditrigonal-scalenohedrical minerals calcite and rhodochrosite*) 
in beautiful crystals. 
c. The series of the rhombic-bipyramidal su/phates of the metals 
calcium, strontium, barium and lead, which are known in mineralogy 
as the minerals an/ydrite, coelestine, baryte and anglesite. 
1) Or dialogite. 
