the Disintegrated Surfaces of Crystals, 17 



tain fixed points of their primitive forms ; and the phsenomena of 

 circular polarization in quartz and amethyst, connected with the 

 plagiedral faces of the crystal, indicate remarkable peculiarities 

 of structure ; and I have had occasion to show that all the pro- 

 perties comprehended under double refraction and polarization 

 do not exist in the ultimate molecules of the body, but are wholly 

 the result of those forces by which these molecules are combined. 

 Structures still more complicated have been discovered by the 

 analysis of polarized light; and in the complex formations of 

 Apophyllite and Analcime, we witness the operation of laws 

 resembling more those which regulate the structures of animal 

 life than those which had previously been observed in crystalline 

 formations. 



The doubly refracting structure of crystals, or to use the lan- 

 guage of the undulatory theory, the law according to which this 

 structure permits the aether to be distributed in their interior, 

 relative to one or more axes, becomes the index as well as the 

 measure of certain changes of structure which in some cases 

 arise during the process of crystallization. When the atoms 

 approach each other in a pure and undisturbed solution, the 

 crystal which they form will be a correct type of the species ; 

 but if the solution has been exposed to agitation, — if its electrical 

 condition has been changed, — if foreign matter, crystallized or 

 uncrystallized, opake or transparent, coloured or uncoloured, 

 amorphous or isomorphous with the crystal ; — if any such matter 

 has been introduced into the solution, we may expect a crystal 

 deviating from the type of perfect crystallization, in transparency, 

 or colour, or density, or hardness, or refractive power, or in 

 doubly refracting and polarizing structure. A very remarkable 

 example of such changes I discovered long ago in Chabasie. 

 When the crystal had begun to form, it possessed the structure 

 of the perfect mineral, but the force oi positive double refraction 

 of each successive layer began to diminish till it wholly disap- 

 peared. The changes, however, did not stop here ; a negative 

 doubly refracting structure commenced at the neutral line, and 

 gradually increased till the crystal was completed. This singular 

 effect I ascribed to the introduction of foreign matter between 

 the integrant molecules of chabasie, which weakened their force 

 of aggregation, and consequently the double refraction produced 

 by the mutual compression which arises from that force. By 

 pursuing the same idea, I have been recently led to discover the 

 cause of the beautiful but perplexing phsenomena of dichroism, 

 and I hope to be able to lay before the Society an artificial com- 

 bination in which the actual phsenomena are reproduced. 



Having thus briefly adverted to the present state of our know- 

 ledge of the interior constitution of crystals, I shall now proceed 



Phil, Mag. S. 4. Vol. 5. No. 29. Jan. 1853. C 



