SIR DAVID BREWSTER ON THE OPTICAL FIGURES, &c. 1(55 



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 du- 

 ring the process of crystallisation. 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 hasbeen changed, — if foreign matter, crystallised or uncrystallised, opaque 

 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 crystallisation, in transparency, or 

 colour, or density, or hardness, or refractive power, or in doubly refracting and 

 polarising structure. A very remarkable example of such changes I discovered 

 long ago in chabasie. When the crystal had begun to form, it possessed the struc- 

 ture of the perfect mineral, but the force of positive double refraction of each 

 successive layer began to diminish till it wholly disappeared. The changes, how- 

 ever, 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 mole- 

 cules 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 phenomena of dichroism, and I hope to be able to 

 lay before the Society an artificial combination in which the actual phenomena 

 are reproduced. 



Having thus briefly adverted to the present state of our knowledge of the 

 interior constitution of crystals, I shall now proceed to the proper subject of this 

 paper, which is to describe the optical figures produced by the disintegrated sur- 

 faces of minerals and artificial crystals. The disintegration by which these figures 

 are developed, is produced by three causes : — 



I. By the natural action of solvents on the mineral, either at the time of its 

 formation or at some subsequent period in the bowels of the earth. 



II. By the action of acids and other solvents upon the surfaces of perfect 

 crystals; and, 



III. By mechanical abrasion. 



I. The first examples of Natural disintegration which I met with, were in 

 Brazil Topaz. In a great number of these topazes, I observed cavities filled with 

 a white pulverulent substance, which Berzelius, who analyzed it at my request, 

 found to be a sort of marl, consisting of silex, alumina, lime, and mater, and which, 

 as he remarks, would have formed a zeolite had it been crystallised. Upon ex- 

 amining the sides of the cavities which contained this substance, I found that 



