576 The American Naturalist. [June, 
standing of the differences between polymerism, polymorphism, and 
pseudo-symmetry. The first he regards as purely chemical, and as 
not affecting in any way the arrangement of particles in crystals. It 
has to do merely with the molecules, while polymorphism has to do 
with the crystal particles.3 The molecules unite according to certain 
definite laws, yielding the crystal particles, which may exist even in 
solution, when they may reveal themselves by their effect upon 
polarized light. The crystal particles are in their turn disposed 
regularly to form the crystal. When this disposition is identical with 
that of the molecules in the crystal particles a symmetrical body results. 
When the arrangement of the particles is different from that of their 
constituent molecules the body is pseudo-symmetrical. The various 
forms of a polymorphic body belong to the first class, which is divided 
into two groups. In the first belong those substances in which @ 
change in crystallization occurs without a corresponding change in 
homogeneity. In the second class belong those bodies which, when 
heated, break up into a large number of individuals of a different 
grade of symmetry from the original individual, without definite 
contours and without definite arrangement. To explain the first class 
the author supposes the molecules in the crystal particles to undergo a 
rearrangement coincident with that of the particles themselves. This 
is called direct polymorphism. Indirect polymorphism is the term 
- applied to the change the second class of substances undergo. In 
these the symmetry of the particles changes, while that of the aggre- 
gate of particles remains unchanged. To the first class only is the 
term polymorphism strictly applicable. The second class belong 
rather to the paramorphic bodies. The second part of the paper 
contains results of observations on polymorphic substances, among 
which are the bichromates of rubidium, potassium, and ammonium, and 
the sulphates of sodium and of lithium. O. Lehmann," in a very brief 
communication, states his view of crystal structure, which is quite — 
different from that of Wyrouboff. He declares that the essential 
condition of a crystal is not the regular arrangement of the molecules 
in the particles, but it is the construction of. these molecules. He also 
reiterates the statement made in his ‘‘ Molecular Physik ’’ to the effect 
that no chemical substance can crystallize in more than one way- 
Allotriomorphic and polymorphic substances are different chemically- 
In a second paper! he gives brief descriptions of eight additional 
a o AMERICAN NATURALIST, Feb., 1890, p. 174. 
Zeits. f. Kryst., XNIIL., 1890, p. 456. 
BIb., p. 464. 
