On Double Refraction. 301 
When the three axes A, B, C are all equal, the three rectangular 
compressions, produced by the aggregation of the molecules, will 
destroy one another at every point of the molecule, and the body 
which they compose will have no double refraction, and cleavages of 
equal facility. Hence all crystals in which it is known by cleavage 
that the particles cohere with equal force in three rectangular direc- 
tions, have actually no double refraction. 
If the three attractive axes A, B, C are all unequal, the difference 
of density which they produce in the molecules will be related to two 
axes of double refraction, the strongest of which will be negative or 
Positive according as the compression along C is less or greater than 
the dilatation produced along C by the united compressions of A and 
B. Hence all crystals belonging to the prismatic system, in which 
We are informed by cleavage that the particles cohere with unequal 
forces in three diréctions, have invariably two, or, as we have already 
explained, three unequal axes of double refraction, of which the 
strongest is sometimes positive and sometimes negative. 
We have supposed the elementary molecules of bodies to be 
spherical when existing singly, or beyond the sphere of their mutual 
action; but although their form must, in the case of doubly refract- 
ing crystals, be changed into oblate, prolate or compound spheroids, 
yet the deviation of these spheroids from the sphere may be’so small, 
that the forms of the bodies which they compose may be regarded 
48 arising from the union of spherical molecules. It is more proba- 
ble, however, that the form of the molecules suffers a considerable 
change, and we may consider that change as determining the exact 
Primitive form of the crystal and the inclination of its planes. 
The circumstance of almost all rhombohedral crystals having neg- 
ative double refraction, which can only be produced by axes of com- 
Pression in the equator of a prolate spheroid, excludes the supposi- 
ton, that the ultimate molecules are sperical particles converted by 
the forces which unite them into those oblate and prolate spheroids, 
Y means of which, according to the views of Huyexns, all the va- 
Neties of rhombohedrons may be formed ;* for if this were the case, 
obtuse rhombohedrons should possess one positive axis, and the 
Cute ones, one negative axis of double refraction. We are con- 
Stained therefore to suppose that in rhombohedral crystals the mole- 
Berea eth als ee ea 
: See Hvycens’s Traité de la Lumiére, chap. v. and the Edinburgh Journal of 
Stience, No. xvili, pp. $11, 814 
