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XXXVIII. On the Production of Crystalline Structure in Cry- 

 stallized Powders, by Compression and Traction. By Sir 

 David Brewster, K.H., D.C.L., F.R.S., V.P.R.S. Edin., 

 and Associate of the Institute of France*. 



THE influence of compression and dilatation in producing 

 the doubly refracting structure in solids of all kinds, 

 whether crystallized or uncry stallized, which do not possess it, 

 and in modifying that structure in all crystals which do possess it, 

 has been long known ; but with this class of phamouiena, those 

 which I am about to describe have no connexion whatever. 



In the course of experiments on the double reflexion and polar- 

 ization of light which 1 discovered in the chrysammates of potash 

 and magnesia, murexide, and other crystals, I was surprised to 

 find that these substances could be spread out upon glass by 

 hard pressure, like grease or soft wax ; and that in the case of 

 chrysammate of potash and other bodies, when the powder could 

 scarcely be distinguished from snuff, I obtained a transparent 

 film, exhibiting the phenomena of double reflexion and polariza- 

 tion from its surface as perfectly as if I had been using a large 

 crystal. 



In subsequently repeating these experiments, and examining 

 under polarized light the film thus produced by compression and 

 traction, I was surprised to observe that the streaks and separate 

 lines of the film, as well as the film itself, had regular axes of 

 double refraction, as if they were regularly crystallized portions 

 of the substance under examination. These streaks and capil- 

 lary lines, which were often of extreme minuteness, did not 

 appear to consist of insulated particles merely dragged into a line ; 

 but when the substance possessed the new property in perfection, 

 the lines of polarized light were continuous, and the crystallo- 

 graphic as well as the optical axes of the particles were placed 

 in that line. In other cases, where the experiment was less 

 successful, the insulation of the particles was easily recognised, 

 though the general mass of them was crystallographically ar- 

 ranged. 



In making these experiments, the natural crystalline powder, 

 or the particles of the crushed crystal, may be placed either 

 upon a polished glass surface or upon a piece of glass ground on 

 one side. In those cases where the substance is soft, the polished 

 surface is preferable ; but when the powder is hard, and con- 

 siderable pressure necessary, it is better to place it upon the 

 ground surface of a piece of glass, as the particles are detained 

 between its minute elevations, and submit more readily to the 



* From the Transactions of the Royal Society of Edinburgh, vol. xx. 

 part 4. 



