306 Mr Haidinger’s Comparative Viezv of the Series 
14} Aq and Mg 8^4-14} Aq) possess prismatic forms, the first 
dependent upon a scalene four-sided pyramid P=:127° ^7' ; 
45'; 78« 5', the other upon P=127'» 22'; 126« 48'; 78® 7': 
the forms of the two isomorphous species, of rhombohedral 
Corundum, and of rhombohedral Iron-ore (A1 and Pe) are 
derivable from acute rhombohedrons, the first from llzz86® 
6', the second from Rz=:85® 58', &c. But, however striking 
this coincidence may be, as the differences may lie between the 
limits of errors of observation, yet there are many instances 
of isomorphous combinations, where the angles vary beyond 
these limits, and indicate that it Avould be too precipitate to as- 
sume a perfect equality even for the other. But it will not ad- 
mit of a doubt, that there really are certain analogies in the forms 
of crystallisation among certain compounds of analogous substan- 
ces. Thus, the carbonates of lime, of iron, of manganese, of 
zinc, of lime and iron, of lime and magnesia, and others, affect 
rhombohedral forms, and are cleavable parallel to the faces of 
a finite, obtuse rhombohedron of that system ; on the other 
hand, the carbonates of lime, of lead, of barytes, of strontites, 
exhibit as remarkable analogies in the prismatic forms which 
they present, and which extend not only to the relative measures 
of their angles, and the occurrence of very nearly the same sim- 
ple forms, but also to the peculiar mode of regular composition, 
so very frequently found in every one of these substances. If 
we abstract from the chemical relations, the analogies of the 
forms still remain, and their consideration becomes a pure crys- 
tallographic problem : it will even be necessary to carry the 
comparison farther than could be indicated by the chemical ana- 
lysis, and include, for instance, in the comparative consideration 
of the above mentioned prismatic forms, also those of prismatic 
Ir^n-pyrites, of prismatic Melane-glance (brittle silver-ore), of 
prismatic Corundum (chrysoberyl), and others, which present 
very hearly the same peculiarities in their series of crystallisa- 
tion. In the study of these analogies, though they have been 
observed by the earliest crystallographers, yet an extensive field 
is still open to the inquiry of naturalists, and cannot but yield 
highly interesting results, when carried on with the circumspec- 
tion which it requires. 
