6 Dr. Brewster’s additional observations on the 
The contraction of the flint glass drops, as computed from 
the magnitude of the cavities, must always err in defect ; but 
the maximum result obtained from a considerable number 
of drops may be regarded as a tolerably correct measure of 
the diminution of density. * For this purpose, those drops 
should be employed in which the cavities are numerous, 
and scattered over every part of their length. The largest 
drops have generally this character, and I have one of these 
in my possession, in which there are no fewer than seven 
cavities. 
A considerable degree of difficulty is experienced in pro- 
curing unannealed drops of flint glass. Owing, I presume, 
to the softness of this kind of glass, the greater number of 
the drops burst as soon as they are cooled, and from some 
pots of glass I have been able to procure only four drops out 
of twenty-four that were plunged into the water. One of these 
burst in my hand some hours after it was made, without any 
part of the tail having been previously broken, and another 
burst on the following day when lying on the table exposed 
to no change of temperature. The best method of obtaining 
the drops entire, is to watch the moment when the red heat 
disappears in the centre of the bulb, and to remove it instantly 
from the water. 
As the minutest fragments of all crystallized bodies have 
the same action upon light, as the crystals of which they 
formed a part, I expected a similar property in the fragments 
* The specific gravity of the unannealed drop, as corrected in page 3, from an 
estimate of the size of the cavities, approaches very near to that of the annealed drop. 
The difference is only 0.012, and would have been considerably less had the cavities 
been more numerous. 
