BY PRESSURE IN THE MOLECULES OF BODIES. 
91 
are most powerfully attracted, and in the direction of which they cohere 
with different degrees of force. Guided by the indications of hemitrope 
forms, and supposing the molecules to be spherical or spheroidal, we infer that 
their axes are three in number and at right angles to each other, and are re- 
lated in position to the geometrical axis of the primitive form. In like manner 
the phenomena of double refraction are related to the same axis of the primi- 
tive form, and may be all rigorously calculated by a reference to three rectan- 
gular axes. In uniaxal crystals, the three axes A, B, C must be such that two 
of them are equal and of the same name ; while the third, corresponding with 
the apparent axis, may be of the same or of a different name. In biaxal 
crystals, the three axes A, B, C are unequal, and in crystals with no double 
refraction the axes are equal and destroy each other*. 
This approximation of these two classes of facts is too remarkable to be ac- 
cidental, and would go far to establish their dependence, even if it were not 
indicated by other arguments which I shall proceed to illustrate. 
Among those crystals which have the obtuse rhomboid for their primitive 
form, there are many with one axis of negative double refraction, and only one 
or two with one axis of positive double refraction. In the former, the nega- 
tive doubly refracting structure will be produced round the axis of the rhom- 
bohedron by the compression arising from attractions in the direction of two 
equal rectangular axes A, B, which will dilate the molecules in the direction 
of the third axis C, and make it a negative axis of double refraction, equal in 
intensity to either of the other two. Here we require the combination only of 
two axes ; but if we suppose that there is in the direction of C a third axis of 
attraction either more or less powerful than the other two, then if it is less 
powerful, the compression of the molecules produced by it will diminish the 
dilatation arising from the united action of A and B, but will still leave an 
unbalanced dilatation, or a single negative axis of double refraction in the axis 
of the rhomb. 
If C, on the contrary, is an axis in which the attractive force of the mole- 
* In uniaxal crystals, the resultant of the two equal axes A, B may have any relation to C but that 
of equality ; excepting when C is of a different name from A and B. 
In biaxal crystals, any two axes A, B, may be converted into three A + C, B + C, + C. See 
Phil. Trans. 1818. 
