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 ulumate 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 
simultanéously 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 ag° in chabasie, 
and which will form the subject of a separate communication: In 
certain specimens of this mineral, the molecules compose @ 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 ap !0- 
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 © the 
axes or poles from which the forces of aggregation emanate, opt 
been gradually changed, probably by the ntroduction of some minute 
-matter, which chemical analysis may be unable to detect. iat 
suppose these axes to be three, and the foreign particles to be nt 
duced, so as to weaken the force of aggregation of the greater aX! 
‘then the doubly refracting force will gradually diminish with the ™ 
tensity of this axis, till it disappears, when the three axes are reduce 
to equality. By continuing to diminish the force of the third ax! 
the doubly refracting force will reappear with an opposite characters 
exactly as it does in the chabasie under consideration. 
