98 
Dr. Brewster on new properties of heat , 
Proposition XXXVII. 
When a piece of glass is regularly crystallized, every set of lateral 
fringes which it exhibits is accompanied with another set of an 
opposite kind , and the forces by which these fringes are produced, 
are not in equilibrio, unless when two sets of fringes of one 
character are opposed to two sets of fringes of the opposite 
character. 
The truth of this Proposition is demonstrated by all the 
preceding experiments. Some apparent exceptions to it will 
be stated in the Scholium. 
Scholium. 
The result announced in the Proposition, naturally leads us 
to point out the striking analogy which subsists between the 
phenomena of crystallized glass and those of magnetism. 
' In order to avoid circuitous expressions, I shall consider the 
part of the glass which polarises the highest tint in one set of 
fringes as a north pole , and the part which polarises the highest 
tint in the opposite set as a south pole. 
1. When heat is propagated along a plate of glass, or when 
glass is permanently crystallized by cooling, and exhibits the 
fringes shown in Fig. 2. (PI. II.), its poles will be arranged 
as in Fig. 43. (PI. IV.) which represents a section of the 
glass across the fringes. The north poles are situated at 
N, N', and there is a south pole in the middle at S', A, and 
B being the neutral points corresponding to the black spaces, 
where the one kind of polarity passes into the other. This 
arrangement of the poles is precisely the same as that of a 
