optical properties and structure of heated glass , &c. 3 
fig. l, so that I was able to obtain only approximate results 
by estimating the magnitude of these cavities. 
The following specific gravities were measured by my 
friend Mr. Jardine, with his usual correctness. 
Unannealed flint glass drop, fig. 1, - 3.20405 
Annealed flint glass from the same pot 3-2763 
In order to correct the first of these measures, I moulded a 
piece of bees’ wax into the size and form of the cavities E, F, 
fig. 1, by examining them under a fluid of the same refrac- 
tive power as the glass. I then formed the two pieces of wax 
into a sphere, and thus ascertained, with tolerable accuracy, 
the weight of a quantity of water of the same magnitude as the 
cavities. By this means I obtained the following measure, 
Corrected specific gravity of the unannealed drop 3.264, 
a result differing so little from that of the annealed glass, that 
we may consider them as having nearly the same density. 
With the view of obtaining some farther insight into the 
structure of the crystallized drop, I brought the one, repre- 
sented in fig. 1, nearly to a red heat. Its shape suffered no 
change at this temperature, and the vacuities E, F, still re- 
mained; but it had now lost the faculty of depolarisation, and 
the particles had therefore assumed a new arrangement. By 
increasing the temperature, the cavities E, F, disappeared : 
the lower side of the drop, upon which it rested, was indented 
by the bottom of the crucible ; but it had in no other respect 
lost its external shape, the appearance of the cleavage in fig. 2 
remaining unaltered. In this state Mr. Jardine measured 
the specific gravity of the drop, and found it to be 3.278, 
which is almost exactly the same as that of the annealed 
drop. 
