654 —  — Scientific Proceedings, Royal Dublin Society. . a 
of the same order as the chemical differences between the substances 
compared. In the systems of low symmetry, these differences are 
not generally of such a simple character as those which appear 
when a body is subjected to a single linear distortion, they are 
traceable to slight differences in the dimensions in different 
directions of the space-lattices formed by the most symmetrical 
singular points or centres in the respective isomorphs.’ They 
are ordinarily most evident in a certain principal zone, whilst — 
between the corresponding angles of other zones nearly perpen- 
dicular thereto practically-complete identity subsists.’ 
It is generally the case that the similarity of form is greater , 
the nearer the compared bodies are related chemically. 
As we should expect from the similarity of form, and the 1 
capacity to crystallize together, isomorphous crystals commonly — 
have similar cleavage. This similarity can scarcely be appealed 
to as throwing any additional light on the matter, but it is per- 
haps significant that the correspondence is not particularly great 
in some cases, é.g., when we compare aragonite, strontianite, ceru- — 
site, and witherite ; the occasional occurrence of such divergences 
is to be anticipated if the constituents which differ occupy definite 
situations in the structure. The more or less considerable dissimi- 
larity of the etched figures on isomorphous crystals* would seem — 
to point the same way; for it is easy to see, and indeed to show 
experimentally, that if two similarly arranged homogeneous struc- 
: 
: 
tures be built up of different mixed materials in such a way as to — 
give great differences of the same property, cohesion, pyroelec- 
tricity, or some other, in different directions in the same structure, 
the materials can be so selected and the common arrangement — 
have such a configuration as to give widely-diverse sets of direc- 
tional relations in the two systems, notwithstanding that the 
arrangement is the same in both. 
A very similar line of argument may be adopted with reference 
1 See note 1, p. 651. 
2 Fock’s ‘‘ Hinleitung in die chemische Krystallographie,’” p. 65; or Pope’s trans- 
lation of same, p. 92. 
3 A case of great dissimilarity is given in Pope’s translation of Fock’s ‘‘ Chemische 
Krystallographie,’’ p. 102. 
Cases in which the dissimilarity of the etched figures is such as to place the crystals 
in a different class of symmetry are referred to presently. 
