On Double Refraction. 299 
arising from the molecules being brought into closer contact, and the 
regular double refraction from the variable density impressed upon 
each elastic molecule, and symmetrically related to the axis of press- 
ure. The effect thus produced on the resinous mass is precisely the 
same as what would take place by subjecting elastic spheres to a regu- 
lar compressing force. The axis of pressure becomes an axis of posi- 
tive double refraction, and the double refraction increases with the in- 
clination of the ray to the axis, and becomes a maximum in the equa- 
tor of the molecules. 
By this view of the preceding facts, we are led to a very simple 
explanation of the origin and general phenomena of double refraction 
inregular crystals. That this property is not inherent in the mole- 
cules themselves may be easily proved. The particles of silex, for 
example, do not possess it in their separate state. In tabasheer, in 
many opals, and in melted quartz, there is not the slightest trace of 
the doubly refracting structure: but when the particles of silex in 
solution are allowed to combine, in virtue of their polarities or mutual 
affinities, they then instantly acquire, at the moment of their combi- 
mation, the property of double refraction, and they retain it while 
they continue in this state of aggregation. The manner in which 
this takes place may be easily conceived: a number of elastic mole- 
cules existing in a state of solution, or in a state of fusion, are kept at 
‘uch a distance by the fluid in the one case, and by the heat in the 
other, as to preclude the operation of their mutual affinities ; but 
When, in the process of evaporation or cooling, any two molecules 
are brought together by the forces or polarities which produce a crys- 
tlline arrangement, and strongly adhere, they will mutually compress 
oe another, and each will have an axis of double refraction in the 
directions of the line joining their centres, in the same manner as if 
had been compressed by an external force. ze : 
From the phenomena of crystallization and cleavage, it is obvious 
that the molecules of crystals have several axes of attraction, or lines 
‘long which they are most powerfully attracted, and in the direction 
Which they cohere with different degrees of force. Guided by 
the indications of hemitrope forms, and supposing the molecules to 
spherical or spheroidal, we infer that their axes are three in num- 
‘and at right angles to each other, and are related in position to 
the geometrical axis of the primitive form. In like manner the phe- 
tomena of double refraction are related to the same axis of the primi- 
We form, and may be all rigorously calculated by a reference to three 
