refraction to glass, &c. by pressure. 1 6 g 
Proposition XV. 
If a parallelopiped of glass is enclosed on all sides , except two , 
in a mass of fluid metal , the contractions and dilatations which 
the metal experiences in passing to a state of permanent solidity , 
zvill be rendered visible by the communication of the doubly 
refracting structure to the glass. 
I took a cylinder of tin plate AB, Fig. 15, (PL X.) open at both 
ends, and having placed a piece of glass CDEF on its lower 
edge EF, I surrounded it with melted lead. As soon as the 
lead lost its fluidity I exposed it to a polarised ray, and found 
that the glass exhibited no colour. As the metal contracted 
in its dimensions, there appeared a bluish white tint, which 
gradually rose through all the tints of the first order, and 
reached the red of the second order, when plunged in a 
freezing mixture. 
The same result was obtained when the glass was sur- 
rounded by tin ; but when it was incased in the fusible 
metal, consisting of eleven parts of bismuth, three of lead, 
and five of tin, it exhibited after cooling the same tints as if it 
had been dilated. In order to examine this point with greater 
care, I exposed the glass to a polarised ray as soon as the 
fusible metal was fixed. It then displayed no tints whatever, 
but as the cooling advanced, a tint appeared which rose to a 
yellow of the first order, as if the glass were highly com- 
pressed. At a certain temperature, however, the tints gra- 
dually diminished, and passed into the opposite tints produced 
by dilatation. Hence it follows, that after the fusible metal 
has assumed the solid state, it contracts its dimensions, and at 
a certain temperature is again expanded. 
MDCCCXVI, Z 
