90 
DR. BREWSTER ON THE DOUBLE REFRACTION PRODUCED 
the crystalline lenses of animals, we can take it away entirely, and substitute 
a new and more powerful doubly refracting structure by induration. 
We may therefore consider it as clearly established that the uniaxal double 
refraction of the resinous mass has been communicated to the individual 
molecules by simple pressure ; the increased transparency arising from the 
molecules being brought into closer contact, and the regular double refraction 
from the variable density impressed upon each elastic molecule, and symmetri- 
cally related to the axis of pressure. The effect thus produced on the resinous 
mass is precisely the same as what would take place by subjecting elastic 
spheres to a regular compressing force. The axis of pressure becomes an axis 
of positive double refraction, and the double refraction increases with the in- 
clination of the ray to the axis, and becomes a maximum in the equator 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 in regular crystals. 
That this property is not inherent in the molecules 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 combination, 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 molecules existing in a state of solution, or in 
a state of fusion, are kept at such 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 crystalline ar- 
rangement, and strongly adhere, they will mutually compress one 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 they had been compressed by an exter- 
nal force. 
From the phenomena of crystallization and cleavage, it is obvious that the 
molecules of crystals have several axes of attraction, or lines along which they 
