178 



2. On the Production of Crystalline Structure in Crystallised 

 Powders, by Compression and Traction. By Sir David 

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



The author, after alluding to the influence of compression and di- 

 latation in producing the doubly refracting structure in solids of all 

 kinds devoid of it, and in modifying it where it exists, mentioned that 

 the phenomena to be described have no relation to those alluded to. 



In experimenting on the double reflexion and polarization of light 

 discovered by him in the chrysammates of potash, and magnesia, 

 murexide, and other crystals, he found that they could be spread out on 

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

 of dark powders, he could thus obtain a transparent film, exhibiting 

 double reflexion and polarization from its surface, as well as if it had 

 been a large crystal. 



In studying these phenomena under polarized light, he found that 

 the streaks and lines had axes of double refraction, as well as the film 

 composed of them, just as if they were regular crystals. When the 

 substance possessed the new property in perfection, these lines, though 

 very minute, were not formed of insulated particles dragged into a 

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

 graphic as well as the optical axis of the particles, were .placed in that 

 line. In other cases, the insulation of the particles was easily seen. 



The substance may be subjected to pressure and traction, either on 

 smooth or on ground glass, the latter being preferable for hard sub- 

 stances. A polished and elastic knife is used to give the pressure. 

 The lines thus formed, examined in the polariscope, exhibit regular 

 neutral and depolarizing axes. With the chrysammate of magnesia, 

 the appearances are peculiarly splendid; its natural colours, which 

 vary with the thickness, being combined with the tints depolarized by 

 the streaks. As these crystals are dichroitic, and possess unusual 

 reflexion, so also the streaks exhibit the same ; the two pencils be- 

 ing carmine red and pale yellow. 



This pi-operty the author has found more or less in the follow- 

 ing crystals : — 



Chrysammate of magnesia. 



potash. 

 Hydro-chry sam m ide . 



Murexide. 



Aloetinate of potash. 

 Aloetinic acid. 



