On the Production of Crystalline Structure. 



261 



combined force of pressure and traction. When the powder is 

 thus placed, I take a polished and elastic knife, and with its 

 broad point I compress and drag the powder in a given direction 

 till there is the appearance of a polished surface on the com- 

 pressed substance. In general, I have used both the smooth 

 and the rough glass, and have frequently obtained results with 

 the one which were not given by the other. 



If we now place the plate of glass in a polarizing microscope 

 with the field dark, we shall find that the streaks and lines pro- 

 duced by traction have, in certain substances, regular neutral 

 and depolarizing axes, as if they were prismatic crystals of the 

 substance under examination. With the chrysammate of magnesia, 

 a red powder with specks of yellow reflected light, the pheno- 

 mena are peculiarly splendid; the natural colours of the sub- 

 stance, which vary greatly with the thickness of the streaks and 

 films, being combined with the different tints which they polarize. 

 As the crystals of this substance possess unusual reflexion, this 

 property is displayed in the crystallized streaks produced by 

 traction ; and the superficial colours which they reflect vary with 

 the azimuth which the plane of incidence forms with the plane 

 passing through the axis of the prism. 



The remarkable property which I have now described I have 

 found, in a greater or a less degree, in the following crystals : — 



Chrysammate of magnesia. 



of potash. 

 Hydro-chrysammid. 

 Murexide. 



Aloetinate of potash. 

 Aloetinic acid. 

 Oxamide. 

 Palmine. 

 Palmic acid. 

 Amygdaline. 

 Tannin, pure. 

 Quinine, pure. 



acetate of. 



sulphate of. 



muriate of. 



phosphate of. 



citrate of. 

 Cacao butter. 

 Veratric acid. 

 Esculine. 

 Theine. 



Silver, cyanide of. 

 ... acetate of. 



Platina and magnesia, cyanuret of. 

 and bary tes, cyanuret of. 



... potassium, cyanuret of. 



... ammonia, chloride of. 

 Potash, oxymuriate of, 



chromate of. 

 Urea, nitrate of. 

 Sulphur. 

 Camphor. 

 Cinchonine. 



sulphate of. 

 Meconic acid. 

 Brucine, sulphate of. 

 Morphia, acetate of. 

 Tin, iodide of. 

 Cerium, oxide of. 

 Parmeline. 

 Lecanorine. 

 Indigo red. 

 Ammonia, oxalate of. 



sulphate of. 

 Soda, chromate of. 

 Lead, iodide of. 



