1892.] Mineralogy and Petrography. 699 
S. Dak., Messrs. Culver" and Hobbs find that the reddish to yellowish- 
brown diallage is strongly pleochroic. 
Mineralogical News.— General COrystallographic.—W yrouboff, 
in the continuation of his crystallographic study of closely related 
double salts of sulphuric, selenic and chromic acids, has reached some 
exceedingly interesting results bearing upon isomorphism. After care- 
fully measuring the crystals of ten of these compounds and comparing 
their optical properties, he finds that while several of them are crys- 
tallographically similar these same compounds possess quite different 
optical characteristics. Fe K,(SO,),, Mn K,(SO,), and Mn K,(SeQ,), 
are optically as well as morphologically similar. Since the optical 
properties of crystals change when they are subjected to changes of 
temperature it follows that these properties are dependent upon the 
arrangement of the molecules—upon the character of the crystal net- 
work. Isomorphous bodies are those that possess identical networks, 
consequently isomorphous bodies are those that are similar morpholog- 
ically and at the same time optically, and in which the changes suffered 
under similarly changed conditions are similar. The magnesian sul- 
phates with seven molecules of water are good examples of a truly 
isomorphous group. There is another kind of isomorphism embracing 
those bodies in which the morphological properties are similar but the 
optical ones different. In such bodies, since the arrangements of the 
molecules in the two intermingled substances are different, there should 
be evidence in these of optical anomalies, which are not apparent in 
the simple compounds, and this is found frequently to be the case. A 
further conclusion drawn by the author from his experiments is to the 
effect that while in general, substances whose chemical composition is 
analogous have similar crystalline forms, it does not necessarily follow 
that isomorphous bodies possess analogous compositions; they need 
merely to be built upon the same plan, possess identically arranged 
networks. Many of the views put forth in the paper are novel, and 
some of them are rather startling. We shall look forward with much 
interest to their discussion by German mineralogists. The relation 
between symmetry and the chemical composition of crystals continues 
to attract the attention of mineralogists theoretically inclined. Fock" 
now suggests that the method by which the problem is to be attacked 
is through the aid of stereochemistry. He assumes that the erystal 
particles have the same symmetry as the crystal individual, and seeks 
Trans. Wis. Acad. Sci., viii, 1891, p. 206. 
NZeits. f. Kryst., xx, p: 76. 
50 
