302 Dr Brewster on the Structure and 



nion, that there was in this case a real difficulty worthy of be- 

 ing solved, and I had sufficient confidence in the powers of 

 optical analysis, to believe that it was peculiarly fitted for con- 

 ducting us to the required solution. As Mr Phillips had de- 

 scribed some of the crystals as almost perfectly transparent, a 

 character which does not belong to chalcedony, I confidently 

 expected that the determination of the axes of double refrac- 

 tion, and of their relation to the primary or secondary planes, 

 would settle at once the question, whether the crystals of Hay- 

 torite were modelled upon those of any other substance, as Mr 

 Levy expresses it, or derived their form from their own pro- 

 per laws of crystallization. 



As the crystals, however, are not transparent, in the proper 

 sense of the word, but disperse all the light which passes 

 through them like chalcedony, or ground glass, I was disap- 

 pointed in this expectation ; but other modes of observation 

 presented themselves, which I have no doubt will be regarded 

 as sufficient to remove all ambiguity from the results to which 

 they lead. 



Having long ago had occasion to examine the structure of 

 chalcedony, and to study the way in which it passes through 

 agate into perfectly crystallized quartz, I was somewhat pre- 

 pared for this examination. If we take a thin splinter of chal- 

 cedony, and examine it with a powerful microscope and polar- 

 ized light, we shall observe that it is composed of minute par- 

 ticles possessing double refraction, but having their axes lying 

 in every possible direction. When the splinter is very thin, 

 we perceive even the polarized colours of some of the indivi- 

 dual particles of quartz. Hence it is manifest that chalcedony 

 is an aggregation of particles of quartz, having the axes of the 

 primitive rhombohedron lying in every possible direction ; and 

 as the specific gravity of chalcedony is exactly the same as that 

 of quartz, it is obvious that the particles must be cemented by 

 a substance of the same density, or agglutinated by the fusion 

 of their surfaces, or held together by molecular attraction. 



If the particles thus combined had been particles of glass, 

 or of any singly refracting body, the mass would possess that 

 homogeneous transparency, through which objects can be dis- 



