Relation 
among sy- 
stems of 
crystalliza- 
tion. 
Relation 
between 
form and 
density 
&e. * 
170 SEVENTH REPORT—1837. 
17. Remarks on the Table of Isodimorphous Groups. —One 
of the most striking facts exhibited by this table is the existence 
of an intimate relation between certain forms not mutually de- 
rivable ;—between the several systems of crystallization. That 
these systems are zatural is proved both by geometrical consi- 
derations, and by the fact that the same substance crystallized 
in forms belonging to different systems possesses different phy- 
sical properties (13), yet the isodimorphous groups show that 
there is a relation, not accidental but constant between crystals 
of a given dimension in one system and crystals of a given di- 
mension in another system. ‘Thus in the first group the 
Rhomboid of 105° to 107° is related to the Rt. Rh. Prism of 116° to 118° 
Regular Octohedron —————_ Rt. Rh. Prism of 139° in the second. 
Do. é ——————. Rhomboid__ of 71°30 in the third. 
Square Prism —————_ Rt. Rh. Prism of 91°10 in the fourth. 
18. The form of the crystal is dependent on the form and ar- 
rangement of the crystalline molecules; instead however of © 
necessarily agreeing in form with either of those observed in the 
crystal, the phenomena of dimorphism show that they probably 
differ from both, and by their union in the direction of one or 
other of two axes of attraction of nearly equal force build up 
one or other of the observed crystalline forms. If the connec- 
tion between the system of crystallization indicated by the table — 
be really of this kind; if forms constantly related in dimension, 
but belonging to different systems, may be formed by the collo-— 
cation of molecules of one constant form, it is not impossible 
that this relation may hereafter be expressed analytically ; that 
more general formulze may be obtained involving the properties 
of two or more systems, and by means of which the form and 
dimensions of the molecules may be deduced from those of the 
dimorphous crystals which are made up of them, and which we 
can measure. 
19. While tracing the connection of the forms of dimorphous 
bodies we are naturally led to inquire if any relation be obser- 
vable between the form assumed and the physical properties — 
which accompany it. Our data are still too few and imperfect 
to enable us to give any satisfactory answer to this inquiry. 
In regard to density, the observations recorded in Table I. 
would indicate that the same substance— 
Sulphur in the form of a Rh. Octohed. is more dense than in that of an Ob. Rh. Prism. 
Carbon Reg. Octohed. Rhomboid. 
Bisulphuret of Iron ; 
and } Reg. Octohed. —_—— Rt. Rh. Prism. — 
Arsenious Acid 
Carbonate of Lime 
and Rt. Rh. Pr. —— Rhomboid. 
Baryto Calcite 
