78 Dr. Brewster on new properties of heat , 
stances, both of animal and vegetable origin, receive a new • 
structure during the propagation of heat. 
Sect. II. On the permanent effects produced upon glass by the com- 
munication of its heat to surrounding bodies. 
The phenomena described in the preceding Section are of 
the most transitory nature. Every fringe is in a state of per- 
petual change : one colour quickly succeeds another, and 
after heat has rapidly developed all the various tints due to 
its intensity, they repass through the same hues which they 
exhibited in their formation, and they finally disappear after a 
slow and gradual decline. In this respect, only, do the phe- 
nomena of crystallized glass differ from those of the regularly 
organised bodies that compose the three kingdoms of nature. 
The fine display of colours which characterises the action of 
crystalline laminae upon polarised light, are in every respect 
permanent. The same mineral possesses an invariable struc- 
ture, and patience only is necessary to detect the phenomena 
which it presents, and to obtain an accurate knowledge of the 
character and intensity of its action. The coloured fringes of 
heated glass, on the contrary, are not susceptible of correct 
mensuration. Where every thing is in a state of change, no 
fixed character can be seized, and, instead of measuring, it is 
often difficult to observe their variations. From this perplexity, 
however, I have been fortunately relieved by the discovery of 
a method of fixing glass in a crystalline state, and giving it a 
character as permanent as that of the most perfect minerals. 
An account of this method, and of the results which it has 
enabled me to obtain, will form the subject of the present 
Section. 
