94 2 >. Brewster on nezv properties of heat, 
lized plate. The state of the crystallized plate is analogous 
to that of a bar of steel not saturated with magnetism. It is 
capable of receiving from heat a much higher degree of crys- 
tallization. See Prop. XXL 
Proposition XXXiV. 
When a plate of permanently crystallized glass is brought to an 
uniform temperature in boiling water , or boiling oil , and is then 
cooled in the open air , the tints descend in the scale , in propo?'- 
tion to the temperature employed , but , they again resume their 
former intensity when the plate acquires the temperature of the 
surrounding air. 
This diminution of the tints, arises from the production of 
the transient and unusual series of fringes described in Prop. 
XIV., which, being of an opposite character from the perma- 
nent fringes, necessarily causes them to descend in the scale. 
The effect is here precisely the same, as if the permanently 
crystallized plate had been combined, when cold, with a hot 
plate of the same thickness, oppositely and transiently crystal- 
lized by cooling. 
Proposition XXXV. 
When the centre of a plate of glass brought to a red heat is laid 
upon the summit of a small cylinder of iron standing vertically , 
it acquires in cooling a permanent structure . which exhibits black 
spaces , and fringes of a circular form, and the black cross exhi- 
bited in Fig. 19. (FI. III . ) 
In a specimen of plate glass crystallized in this manner, the 
dark spaces and the black cross are very distinctly developed, 
a yellow tint of the first order appearing between the dark 
