BY PRESSURE IN THE MOLECULES OF BODIES. 
89 
glass, I squeezed out the drop into a thin plate. This plate was now almost 
perfectly transparent, as if the pressure had brought the particles of the sub- 
stance into optical contact. 
If we expose this plate to polarized light, we shall find that it possesses one 
axis of positive double refraction, and exhibits the polarized tints as perfectly 
as many crystals of the mineral kingdom. The structure thus communicated 
to the soft film by pressure does not belong to it as a whole, nor has it only 
one axis passing through its centre like a circular piece of unannealed glass. 
In every point of it there is an axis of double refraction perpendicular to the 
film, and the doubly refracting force varies with the inclination of the incident 
ray to this axis, as in all regular uniaxal crystals. When the two plates of 
glass are drawn asunder, we can remove one or more portions of the com- 
pressed film, and these portions act upon light exactly like films of uniaxal 
mica or hydrate of magnesia, and develope a doubly refracting force of equal 
intensity. 
This remarkable experiment presents an interesting subject of inquiry. 
That the regular double refraction of the film is developed by the agency of 
pressure cannot be doubted ; but it does not at first sight appear whether it is 
the immediate effect of the pressure, or is the same doubly refracting force 
which produces the quaquaversus polarization that takes place when the resi- 
nous film indurates without constraint. In this state of the film the axes of 
double refraction are clearly turned in every conceivable direction ; and it is 
impossible to suppose that a pressure in one direction could suddenly arrange 
all these axes in parallel positions. The double refraction of each particle 
of the film has therefore been developed by the compressing force similarly 
applied to them ; and in producing this effect, it must have deprived each par- 
ticle of the doubly refracting structure which it previously possessed. The 
substitution of one doubly refracting structure for another may be easily 
effected in many bodies. Even in regular crystals we can by heat or pressure 
modify or remove their double refraction. Nay, we can take away one axis 
from a biaxal crystal, and communicate a second axis to an uniaxal one. 
When the doubly refracting structure is produced by induration, we can re- 
move it wholly by pressure, and replace it with another even of an opposite 
character ; and when it is generated by the living principle, as in the case of 
MDCCCXXX. 
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