4$ Dr, Brewster on new properties of heat, 
■9 
inequality of temperature is necessary to the developement of 
this structure, I held a small plate of glass in a pair of hot 
pincers with globular ends. It instantly acquired the depola- 
rising structure, and lost it when the diffusion o the heat be- 
came uniform. I then cooled the glass, and held it a second 
time in the same pincers, which were now much colder than 
before : the depolarising structure was again communicated to 
it as formerly. 
The same result was obtained wdten 12 plates of glass 
were placed upon a bar of red hot iron. 
Proposition II. 
JVhen a plate of glass is brought to an uniform temperature con- 
siderably above that of the atmosphere, the communication of its 
heat to the surrounding air, or to other contiguous bodies colder 
than itself, is marked by the production of a crystalline structure, 
similar to that which is described under the preceding propo- 
sition. 
I took three plates of thick mirror glass, and brought them 
to an uniform temperature by immersion in boiling water. 
In this state they exercised no action upon polarised light ; but 
when their edges were placed upon a mass of cold iron, the 
inequality of temperature, occasioned by the abstraction of 
their heat, produced a crystalline structure at the very edge 
of the plates, which polarised a bluish white tint of the first 
order. At a greater distance from the edges, the plates de- 
polarised a lower" tint in Newton's scale. When the plates 
* One tint is said to be higher than another, when it belongs to a higher order, or 
is at a greater distance from the black, or the commencement of the scale. This ex- 
planation is rendered necessary, in consequence of M. Biot’s having used this term 
in the opposite sense. 
