Sir David Brewster on the Production of 



Strychnine, sulphate of. 



acetate of. 

 Soda, native nitrate of. 

 Berberine. 

 Mucic acid. 

 Solanine. 

 Asparagine. 



Mercury, oxyrauriate of. 



Isatine. 



Alizarine. 



Manganese, sesquioxide of. 



Lead, protoxide of. 



Tungstic acid. 



Chromo-oxalate of potash. 



In submitting other crystals to the influence of compression 

 and traction, I have found great numbers which do not exhibit 

 the least trace of transparent streaks and lines, the separate 

 particles being merely dragged into lines, and exhibiting only a 

 quaquaversus polarization. On the other hand, there is another 

 class of crystals whose powders or particles are forced into 

 distinct and transparent streaks and lines, in which the indivi- 

 dual particles have a quaquaversus polarization and no trace of 

 a prismatic arrangement. As these crystals have a peculiar rela- 

 tion to those in the preceding list, I shall enumerate the most 

 important of them in the following table ; that is, those in which 

 the powder has been dragged into transparent and continuous 

 streaks and lines, resembling externally portions of a solid body ; 

 for it is only by a comparison of the physical, or perhaps the 

 chemical qualities of the two classes of bodies, that we can expect 

 to explain the new property which is possessed only by one of 

 them. 



Soda, acetate of. 

 Mercury, prussiate of. 



muriate of. 



sulphuret of. 

 Barytes, acetate of. 

 Zinc, chromate of. 

 ... sulphate of. 

 Cobalt, sulphate of. 

 Magn esia and soda, sulphate of . 

 Borax. 



Hydrate of potash, pure. 



Indigotic acid. 



Urea. 



Citric acid. 



Silver, nitrate of. 



Meconine. 



Naphthaline. 



Soda, nitrate of. 



Potash an d copper,sulphate of. 



Soda, phosphate of. 



As both compression and traction are necessary in producing 

 the transparent streaks and lines in both classes of the substances 

 I have enumerated, it became interesting to ascertain what effect 

 was produced by each of these forces acting separately, and 

 which of them was chiefly influential in developing the doubly 

 refracting arrangement exhibited by the substances that pos- 

 sessed it. 



The force of compression was undoubtedly the agent in forcing 

 the separate particles into optical contact, while that of traction 

 drew them into a line, and tended to dilate the film in the direc- 

 tion of that line, and to draw its particles from each other ; or 

 overcome their attraction of aggregation in that direction. It 



