MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
33 
which;, at the same time that it allowed sufficient air to pass, seemed, from the 
appearance of the tube afterwards, to have excluded every impurity. 
7 1. There are two conditions of the finished glass, each of great importance, 
which it is the object of the process to obtain in this state of the sub- 
stance. One, and the most essential, is the absence of all striae and irregu- 
larities of composition; the other, the absence of even the most minute bubbles. 
The first is obtained by agitation and perfect mixture of the whole ; the latter, 
principally by a state of repose: so that the means required to be successful on 
both points are directly opposed to each other. Were the glass absolutely 
incapable of change by the long-continued action of heat, it would be easy first 
to render it uniform by stirring, and then to leave it in a quiescent state, until 
the bubbles had disappeared; but I am not yet fully assured of the fact which is 
necessary to this order of proceedings. That the glass as far as proportions are 
concerned, if changed at all, is altered only in an extremely minute and inap- 
preciable degree, is shown by some experiments, in which, after a portion 
had been prepared and heated for many hours, and also stirred well, the resulting 
piece was divided into smaller portions, and these heated at different tem- 
peratures, in platina trays, for sixteen hours. Three portions were heated as 
powerfully as the furnace would admit of; three only to redness, which may be 
considered as a very low heat ; and three to an intermediate degree: all were 
cooled slowly and annealed for an equal time. The specific gravities of each 
after the experiments were as follows : 
Highest heat . . 5.4206 . 5.4211 . 5.4203 . . Mean sp. gr. 5.42066 
Intermediate heat . 5.4253 . 5.4242 . 5.4255 . . — • 5.42500 
Least heat . . . 5.4258 . 5.4262 . 5.4235 . . 5.42516 
Original glass . . 5.4247 • 5.4261 * — • 5.42540 
72. Here, notwithstanding the irregularities between the similar experi- 
ments, there seems, from the comparison of the mean specific gravities, to be 
a gradual though minute diminution of density, as the glasses have been more 
powerfully heated ; and I found also, that when glass was so well stirred as to 
leave no doubt that it was thoroughly well mixed, yet being left in the furnace 
at a high temperature for eight or nine hours, it contained striae. 
73. On the other hand, first to render the glass perfectly free from bubbles 
and clear, and then to stir out the irregularities of composition, I have not 
MDCCCXXX. 
F 
