as exhibited in its propagation along plates of glass. 105 
from the central line to each of the black fringes.* This 
inference is not founded on any direct experiment, but it 
derives a support almost amounting to demonstration, from a 
series of new experiments, which I shall soon have the honour 
of submitting to the Royal Society. These experiments were 
made by altering the mechanical state of parallelopipeds of 
animal jellies, both by gradual induration, and by the applica- 
tion of variable pressures ; and I have in this way obtained 
results analogous to those which are described in the preceding 
paper. In every case the compression of the jelly produced a 
set of fringes of an opposite structure to those which are oc- 
casioned by expansion, and every compression was accom- 
panied with a corresponding dilatation. In like manner it will . 
be found, that there is in all crystallized bodies a variation of 
density related to their axes, and connected with their polarity, 
which affords an easy explanation of the fringes of different 
forms which are exhibited by the various crystals of the mi- 
neral kingdom. 'f 
* The appearance of the fracture of glass across the fringes, whether it is tran- 
siently or permanently crystallized, is very instructive. It has always the same aspect, 
and plainly indicates the different mechanical states of the different parts of the glass. 
From this cause crystallized glass is incapable of being cut with a hot iron, like glass 
of uniform density, and there is only one way in which the division of the plate can 
be effected. 
f Since this Paper was written, I have discovered that glass, and all other substances 
that have not the property of double refraction, are capable of receiving it from 
mechanical pressure, and that a compressing force always produces the structure 
which gives the exterior fringes in crystallized glass, while a dilating force produces 
the structure which developes the interior fringes. We are, therefore, entitled to con- 
clude that the middle parts are in a state of dilatation, and the external parts in a state 
of compression. P>y a peculiar application of the compressing forces, I have even 
succeeded in obtaining uniform tints like those produced by plates of sulphate of 
.lime of equal thickness. 
MDCCCXVf. 
P 
