On Double Refraction. 303 
‘From the mutual dependence of the forces of aggregation and 
double refraction, it is easy to understand the influence which heat 
produces on the doubly refracting structure, as exhibited in the phe- 
nomena discovered by M. Mirscuer.icu 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 ex- 
pands a rhomb of calcareous spar in the direction of its axis, and 
contracts it in directions at right angles to that axis ;+ that the rhomb 
thus becomes less obtuse, approaching to the cubical forms which 
have three equal axes, and that its double refraction diminishes. All 
these effects are the necessary consequences 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 like manner it will be found that in sulphate of lime and glaube- 
Tite the expansions and contractions will be so related to the three 
axes, as to explain the conversion of the biaxal into the uniaxal 
structure, and the subsequent reappearance of the biaxal structure 
ina plane at right angles to that in which the axes are found at ordi- 
nary temperatures. 
The phenomena exhibited by fluids under the influence of heat 
and pressure, and those of doubly refracting crystals, exposed to 
compressing or dilating forces, are in perfect conformity with the 
ve views; so that even without the fundamental experiment de- 
scribed in this paper, we might have been entitled to conclude that 
the forces of double refraction are not resident in the molecules them- 
selves, but are the immediate result of those mechanical forces by 
Which these molecules constitute solid bodies. 
Allerly, October 5, 1829. 
ee Ee ees 
* See Edinburgh Transactions, Vol. XI. 
It 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 
Sven length of massive carbonate of lime, as many expanding as there are contract- 
axes, then, if the contractions and expansions in each individual crystal are 
“qual, they will destroy one another ; but if they are proportional to their lengths, 
Contractions will exceed the dilatations. In this case, we have only to combine 
the marble with an ordinary expanding substance, to have an invariable pendulum. 
balances of chronometers might be thus made of mineral bodies. 
