
CRYSTALLINE STRUCTURE BY COMPRESSION AND TRACTION. 557 
Strychnine, sulphate of. Mercury, oxymuriate of. 
acetate of. | Isatine. 
Soda, native nitrate of. Alizarine. 
Berberine. Manganese, sesquioxide of. 
Mucic acid. Lead, protoxide of. 
Solanine. Tungstic acid. 
Asparagine. 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 ex- 
hibiting only a quaquaversus polarisation. On the other hand, there is another 
class of crystals, whose powders or particles are forced into distinct and transpa- 
rent streaks and lines in which the individual particles have a quaquaversus 
polarisation, and no trace of a prismatic arrangement. As these crystals have a 
peculiar relation to those in the preceding list, I shall enumerate the most im- 
portant of them in the following table; that is, those in which the powder has 
been dragged into transparent and continuous streaks and lines, resembling exter- 
nally 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. 

Hydrate of potash, pure. Soda, acetate of, 
Indigotie acid. Mercury, prussiate of. 
Urea. As muriate of. 
Citric acid. 3 sulphuret of. 
Silver, nitrate of. Barytes, acetate of. 
Meconine. Zinc, chromate of. 
Napthaline. ... sulphate of, 
Soda, nitrate of. Cobalt, sulphate of. 
Potash and copper, sulphate of. Magnesia and soda, sulphate of. 
Soda, phosphate of. Borax. 
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 possessed 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 direction of that line, and to draw its particles 
from each other; or overcome their attraction of aggregation in that direction. It 
is quite possible, too, that these forces may have exercised some influence in 
modifying the doubly refracting structure of the substance under examination ; 
but as such a question has no bearing upon our present subject, I have not at- 
tempted its solution. ” 
