94 DR. BREWSTER ON THE DOUBLE REFRACTION PRODUCED 
on all the sides of the central nucleus, but each successive stratum has an in- 
ferior doubly refracting force till it wholly disappears. Beyond this limit it 
reappears with an opposite character, and gradually increases till the crystal is 
complete. In this case the relative intensities of the axes or poles from which 
the forces of aggregation emanate, have been gradually changed, probably by 
the introduction of some minute matter, which chemical analysis may be un- 
able to detect. If we suppose these axes to be three, and the foreign particles 
to he introduced, so as to weaken the force of aggregation of the greater axis, 
then the doubly refracting force will gradually diminish with the intensity of 
this axis, till it disappears, when the three axes are reduced to equality. By 
continuing to diminish the force of the third axis, the doubly refracting force 
will reappear with an opposite character, exactly as it does in the chabasie un- 
der consideration. 
From the mutual dependence of the forces of aggregation and double refrac- 
tion, it is easy to understand the influence which heat produces on the doubly 
refracting structure, as exhibited in the phenomena discovered by M. Mits- 
cherlich in sulphate of lime and calcareous spar, and in those which I detected 
in glauberite*. This eminent philosopher has found, by direct experiment, that 
heat expands a rhomb of calcareous spar in the direction of its axis, and con- 
tracts it in directions at right angles to that axis-j~ ; that the rhomb thus be- 
comes less obtuse, approaching to the cubical forms which have three equal axes, 
and that its double refraction diminishes. All these effects are the necessary con- 
sequences of the preceding views. The expansion in the direction of the axis, 
and the contraction of all the equatorial diameters diminish the compression 
of the axes of the oblate spheroidal molecules, and must therefore diminish its 
double refraction, as well as the inclination of the faces of the rhomb. In 
* See Edinburgh Transactions, vol. xi. 
-pit follows from this fact, that massive carbonate of lime, in which the axes of the molecules have 
every possible direction, should neither expand nor contract by heat, and would therefore form an 
invariable pendulum. As there must be, in any given length of massive carbonate of lime, as many 
expanding as there are contracting axes, then, if the contractions and expansions in each individual 
crystal are equal, they will destroy one another ; but if they are proportional to their lengths, the con- 
tractions will exceed the dilatations. In this case, we have only to combine the marble with an ordi- 
nary expanding substance, to have an invariable pendulum. The balances of chronometers might 
be thus made of mineral bodies. 
