412 FOURTH REPORT — 1834. 



general, heat dilates crystals differently in different directions, 

 and so alters their form ; and their double-refracting properties 

 have been found to undergo a corresponding change. Thus Ice- 

 land spar is dilated by heat in the direction of its axis ; while it 

 actually contracts by a small amount in directions perpendicular 

 to it. The angles of the primitive form thus vary, the rhomboid 

 becoming less obtuse*, and approaching the form of the cube. 

 M.Mitscherlich, accordinglj;^, conjectured that its double-i-efract- 

 ing energy must in these circumstances be diminished ; and the 

 conjectui-e was fully verified by experiment. This inquiry has 

 been followed up by M. Rudberg ; and the effects of heat on the 

 refractive indices of double-refracting crystals examined by the 

 direct method of prismatic refraction. In conformity with the 

 observations of M. Mitscherlich, it was found that the extra- 

 ordinary index in Iceland spar increased considerably with the 

 temperature, while the ordinary index underwent little or no 

 change. In rock crystal, on the other hand, both indices diini- 

 nislied as the temperature augmented, and nearly by the same 

 amount. In arragonite a similar effect was produced on the 

 three principal refractive indices, — the least index, however, 

 undergoing the smallest proportionate diminution f. 



The inclination of the optic axes, in biaxal crystals, is a simple 

 function of the elasticities of the vibrating medium in the direc- 

 tion of three rectangular axes ; and the plane of the optic axes is 

 that of the greatest and least elasticities. If, then, these three 

 principal elasticities be altered by heat in different proportions, 

 the inclination of the axes will likewise vary ; and if, in the 

 course of this change, the difference between the greatest elasti- 

 city and the mean, or between the mean and the least, should 

 vanish and afterwards change sign, the two axes will collapse 

 into one, and finally open out in a plane perpendicular to their 

 former plane. All these variations have been actually observed. 

 Professor Mitscherlich found that, in sulphate of lime, the angle 

 between the axes (which is about 60° at the ordinary tempera- 

 ture) diminishes on the application of heat ; that, as the tem- 

 perature increases, these axes approach until they unite ; and 

 that, on a still further augmentation of heat, they again separate 

 and open out in a perpendicular plane. The primitive form of 

 the crystal undergoes a corresponding change, the dilatation 

 being greater in one direction than in another at right angles to 



• A change of temperature, from the freezing-to the boiling-point, produced 

 a change of 85' in tlie dihedral angles at the extremity of the axis. — Bull. Soc. 

 Phil., March 1824. 



t Pk'iL Mag., Third Series, vol. i. 400. 



