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. Mitscherlich 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 thataxis ;f 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- 

 rite 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 

 in a 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 

 above 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. 



* See Edinburgh Transactions, Vol. XI. 



\ It folJows 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 contract- 

 ing 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 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. 

 The balances of chronometers might be thus made of mineral bodies. 



