302 On Double Refraction. 



cules have the form of an oblate spheroid, with its axes so related, that 

 the change superinduced upon it by the forces of aggregation deter- 

 mines the exact form of the combination. In carbonate of lime for 

 example, where the precise inclination of the faces of the rhombo- 

 hedron can be produced only by oblate spheroids whose polar is to 

 their equatorial axis as 1 to 2.8204, we may suppose that the sphe- 

 roids were originally more oblate, and that the forces by which they 

 receive the doubly refracting structure dilated them in the direction 

 of the smaller axis, so as to produce a spheroid having its axis as 1 

 to 2.8204. Hence if we could suppose the molecules placed togeth- 

 er without any forces which would alter their form, they would com- 

 pose a rhombohedron with a greater angle and having no double re- 

 fraction. But when they are combined by the attractive forces of 

 crystallization, they compose a rhombohedron of 105°, possessing 

 negative double refraction. 



In this view of the subject, the form of the ultimate molecules of 

 crystals existing separately, may be regarded as determining within 

 certain limits the primitive form to which they belong ; while the 

 doubly refracting structure and the precise form of the crystal are 

 simultaneously produced by the action of the forces of aggregation. 



These views receive a remarkable illustration from a new doubly 

 refracting structure, which I discovered many years ago in chabasie, 

 and which will form the subject of a separate communication. In 

 certain specimens of this mineral, the molecules compose a regular 

 central crystal, developing the phenomena of regular double refrac- 

 tion ; but in consequence of some change in the state of the solution, 

 the molecules not only begin to form a hemitrope crystal 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 unable to detect. If we 

 suppose these axes to be three, and the foreign particles to be intro- 

 duced, so as to weaken the force of aggregation of the greater axis, 

 then the doubly refracting force will gradually diminish with the in- 

 tensity 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 under consideration. 



