74 Dr. Brewster on new properties of heat , 
Now it is manifest, that the two pieces AB, BD, though 
they touch one another optically, are not in physical contact, 
or in the same state in which they were before the fissure was 
formed. If we were to make several other notches in the 
glass with a file, it would always break at the place of the 
fissure ; which proves that the force of cohesion has there been 
weakened, and that the surfaces, though optically in contact, 
are physically at a distance. The crystallization of the solid 
AD, as if it were continuous, forms a fine analogy with the 
curious fact in magnetism, that two bars of steel pressed 
together at their extremities, may be magnetised as if they 
had formed only a single bar, and will exhibit a neutral point 
at the place of junction. 
Proposition XIX. 
When heat is propagated from the centre of a plate of glass in 
radial lines , all the fringes and the black spaces form concen- 
tric circles , and four black radial spaces , at right angles to each 
other , diverge from the centre in directions parallel and perpen- 
dicular to the plane of primitive polarisation . 
I took a large plate of glass, and applied to its centre a ball 
of red hot iron. The four black radial lines were distinctly 
seen diverging from each other at right angles, but the two 
concentric dark spaces were indistinctly developed. I next in- 
tended to grind a hole in the centre of the plate, and to place 
in it a red hot ball, but having discovered a much better 
method of generating the circular fringes, which will be ex- 
plained in the next section, I proceeded no farther in the 
experimental illustration of the Proposition. 
If we suppose ABCDEFGH, Fig. 18, (PI. Ill,) to be eight 
