556 SIR DAVID BREWSTER ON THE PRODUCTION OF 
as the particles are detained between its minute elevations, and submit more 
readily to the 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 compressed substance. In general, [ 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 polarising microscope, with the field 
dark, we shall find that the streaks and lines produced by traction have, in cer- 
tain substances, regular neutral and depolarising 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 phenomena are peculiarly 
splendid ; the natural colours of the substance, which vary greatly with the thick- 
ness of the streaks and films, being combined with the different tints which they 
polarise. As the crystals of this substance possess unusual reflexion, this pro- 
perty is displayed in the crystallised 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. Platina and magnesia, cyanuret of. 
of potash. ... and barytes, cyanuret of. 
potassium, cyanuret of. 
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. 
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. 
