558 SIR DAVID BREWSTER ON THE PRODUCTION OF 
Without expecting any very interesting result, I submitted to examination 
several of the soft solids which possess double refraction, such as bees’ waz, oil of 
mace, tallow, and almond soap. The last of these substances, though in common 
use, is a very remarkable one. Owing to its particles not being in optical contact, 
it has a fine pearly lustre, and may be drawn out into long and slender strings. 
Upon laying a portion of it on glass, it has a quaquaversus polarising structure, 
with a tendency to form circular crystals, but when it is drawn out into strings, 
and laid upon glass, these strings have neutral and depolarising axes, like the 
streaks formed by compression and traction. In the present case, it is by traction 
alone, that this crystalline arrangement of the particles is produced. 
In oil of mace and tallow, a similar effect is produced by compression and 
traction. With bees’ wax, the depolarising lines are still better displayed, and the 
effect is considerably increased by mixing the bees’ wax with a small quantity 
of rosin. 
As the preceding experiments place it beyond a doubt, that the optical or 
crystallographic axes of a number of minute particles are dragged by pressure 
and traction into the same direction, so as to act upon light like regular crystals, 
it became interesting to discover the cause of phenomena which certainly could 
not have been anticipated from any theoretical principle with which we are 
acquainted. The primary force, and indeed the only apparent one exerted in 
these experiments, is a mechanical force; but it is not improbable that a secondary 
force, namely, that of electricity, may be generated by the friction which accom- 
panies the forces of pressure and traction. That such a force is excited with 
certain crystals will not admit of a doubt; but even if it were developed in every 
case, this would not prove that electricity was the agent in producing the pheno- 
mena under consideration. In subjecting asparagine to compression and traction, 
I observed, upon placing it in the polarising microscope, that its particles were 
moving about under an electrical influence, but in no other case did the same 
phenomenon present itself to me. 
The experiments with soft solids, but especially those made with the almond 
soap, exclude the supposition that the electricity of friction is the cause of the 
crystalline arrangement of its particles; though it is not improbable that the 
sliding of the particles upon one another, as produced by traction, and their 
mutual separation, as in the case of tearing asunder mica or paper, may produce 
enough of electricity to have some share in giving the same direction to the axes 
of the particles. 
When a portion of almond soap is placed upon glass, the axes of its particles 
lie in every direction, and have no tendency to assume the crystalline arrange- 
ment. The forces of aggregation emanating from three rectangular axes, are not 
strong enough to overcome the inertia, as we may call it, arising from the natural 
quaquacersus adhesiveness of the substance, and from the water interposed be- 
