84 Dr * Brewster on new properties of heat, 
structure to the opposite structure, and from one degree of 
crystallization to another, according to its position with regard 
to the edge of the plate ; and there cannot be an equilibrium 
among the forces, by which this change is produced, unless 
the plate exhibits the different sets of fringes which have al- 
ready been described. 
This optical polarity is produced by heat, just as electrical 
polarity is developed in the tourmaline, and other minerals 
by the same agent ; and there is as much reason to ascribe 
the production of the optical phenomena to the action of a 
peculiar fluid, as there is to explain the phenomena of elec- 
tricity and magnetism by the operation of magnetical and elec- 
trical fluids. The optical fluid, as we may call it, may be 
supposed to reside in all bodies whatever in its natural state, 
consisting of two fluids in a state of combination, and capable 
of being decomposed, and fixed in particular parts of a body 
by the agency of various causes. It would be a waste of time 
to point out the numerous and striking analogies, which exist 
between many of the results contained in this Proposition and 
some of the most interesting phenomena of electricity and 
magnetism. Some of them will be noticed in the demonstra- 
tion of a subsequent Proposition. 
