OP ARTS AND SCIENCES. 



55 



the vermiculites and micas, I do not feel confident. It is seldoui 

 that more than three individuals 

 can be distinguished on a given 

 plate ; and the very unequal de- 

 velopment of the several individ- 

 uals, and the indefiniteness of tlie 

 lines of demarcation, resulting from 

 the phenomena Avhich have been 

 described, render what would seem 

 to be a characteristic feature of the 

 more complex group not neces- 

 sarily a certain indication of the 

 structure. I refer to the fact, very 

 constantly noticed, that the plane 

 of the optical axes is parallel to 

 the nearest hexagonal edo'e, as 

 shown in Figs. 2 and 3. In Figs. 5 and 6 this same plane is parallel 

 to the shorter axis of the ellipse ; and it can easily be seen that if 

 either of the individual of Fig. 5 were developed over any large 

 portion of the space of its neighbors, the optical plane might appear 

 parallel to the adjacent edge. 



Plaving made the two suppositions, as above, to explain the phe- 

 nomena of twinning, whicli have been long familiar and externally 

 visible, it will not, we trust, appear unreasonable if we make a third 

 supposition to explain the phenomena first described in this paper. 

 We may conceive that the ellipsoidal molecules, instead of grouping 

 together on the same plane, become associated by their alternate poles, 

 one over the other, as represented in Fig. 7. 

 Molecules so associated, developing later- 

 ally, would produce the laminae of a mica 

 plate in the relative position in which we 

 have placed them in our artificial crystals, 

 with only this difference, that the laminse 

 would be indefinitely thin, and in exact 

 position ; and the effect of such compound 

 molecules in modifying tlie elasticity of 

 the crystalline structure must be, in most 

 respects at least, like that of single mole- 

 cules, symmetrical on all sides of one line 



or axis, — in other words, they must produce a structure similar to that 

 of uniaxial crystals. Under what further conditions the grouping of 



Fig. 7. 



