BY PRESSURE IN THE MOLECULES OF BODIES. 
93 
double refraction, which can only be produced by axes of compression in the 
equator of a prolate spheroid, excludes the supposition, that the ultimate mole- 
cules are spherical particles converted by the forces which unite them into 
those oblate and prolate spheroids, by means of which, according - to the views 
of Huygens, all the varieties of rhombohedrons may be formed* ; for if this 
were the case, the obtuse rhombohedrons should possess one positive axis, and 
the acute ones one negative axis of double refraction. We are constrained 
therefore to suppose that in rhombohedral crystals the molecules have the 
form of an oblate spheroid, with its axes so related, that the change superin- 
duced upon it by the forces of aggregation determines the exact form of the 
combination. In carbonate of lime for example, where the precise inclination 
of the faces of the rhombohedron can be produced only by oblate spheroids 
whose polar is to their equatorial axis as 1 to 2.8204, we may suppose that 
the spheroids were originally more oblate, and that the forces by which they 
receive the doubly refracting structure dilated them in the direction of the 
smaller axis, so as to produce a spheroid having its axis as 1 to 2.8204. Hence 
if we could suppose the molecules placed together without any forces which 
would alter their form, they would compose a rhombohedron with a greater 
angle and having no double refraction. But when they are combined by the 
attractive forces of crystallization, they compose a rhombohedron of 105°, pos- 
sessing negative double refraction. 
In this view of the subject, the form of the ultimate molecules of crystals 
existing separately, may be regarded as determining within certain limits the 
primitive form to which they belong ; while the doubly refracting structure 
and the precise form of the crystal are simultaneously produced by the action 
of the forces of aggregation. 
These views receive a remarkable illustration from a new doubly refracting 
structure, which I discovered many years ago in chabasie, and which will form 
the subject of a separate communication. In certain specimens of this mi- 
neral, the molecules compose a regular central crystal, developing the pheno- 
mena of regular double refraction ; but in consequence of some change in the 
state of the solution, the molecules not only begin to form a hemitrope crystal 
* See Huygens’s Traite de la Lumiere, chap. v. and the Edinburgh Journal of Science, No. xviii. 
pp. 311, 314. 
