MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
39 
been finally determined. Stirring introduces bubbles., and therefore should, if 
possible, be avoided towards the conclusion of the experiment. Rest, or at 
least that condition in which there is no other motion than what is due to the 
currents produced by slight differences of temperature, causes striae even 
after very careful mixture (71. 72), and is therefore equally to be feared ; and 
whatever other variations may have been adopted, I have always found it im- 
portant to apply a careful concluding stirring. The following may be consi- 
dered as the order of an experiment. If the spongy platina has not been in- 
troduced into the tray with the rough glass, then about the sixth hour after 
lighting the fire it is added in the manner already directed (85), and the 
glass well stirred (76). At about the twelfth hour the stirrings are recom- 
menced, for the purpose of making the mixture perfect, and are repeated 
every 20 or 30 minutes, according to the fusibility of the glass and the 
state of the heat (60), for 8 or 9 times. The glass is then allowed to 
remain at rest for 6 or 8 hours, that bubbles may ascend and be dissipated, 
after which it is well stirred twice or thrice more with particular attention, 
that if possible no air may be introduced, and is finally mixed for the last 
time. 
89. This concluding stirring is peculiar, in that it has to be continued until 
the glass is so cold and thick that no ascending and descending currents can 
be formed in it ; after which its temperature is not again to be allowed to rise : 
hence the operation requires certain preliminary arrangements. The first point 
necessary is to clear out a considerable quantity of slag from the flue furnace, 
or that part beneath the chamber (47). This slag results from the fused ashes 
of all the coke which has been consumed there, with other portions that have 
passed on from the coal fire. It is to be drawn on to the bars of the furnace 
by a fire-rake which will pass into the passages beneath the chamber. If not 
taken out in its fused state, it would be impossible afterwards to remove it 
without risk of great injury to the furnace. At the same time that the slag is 
removed, all the coke is likewise to be withdrawn. All the fuel in the fire 
bars is also to be brought out of the furnace ; and if the bars are embarrassed 
with clinkers, they are to be loosened. These things being done quickly and 
quietly, and the furnace apertures closed, a few moments are to be allowed for 
the little dust that may have been agitated to settle, and then the chamber 
