MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 35 
which may cause bubbles in the heated matter and do much harm. A little 
gold, therefore, should be applied to the part where the stem is attached, and 
fused, so that all hollows may be filled up. Stirrers of different dimensions are 
to be provided for different sized plates of glass. Before being used, they 
should be steeped in dilute nitric acid, and also heated to redness in the spirit 
lamp, just previous to their immersion in the glass for the first time in each 
experiment. 
76. When a stirring is to be performed, the tiles and iron covers are removed 
from the first part of the chamber (44. 49. 65), the glass covers also taken off 
and put into the back part of the chamber (61. 65), the glass quickly examined, 
to give assurance that all is in good condition, and then the stirring com- 
menced. The stirrer should be put in gently, that no air may be carried down 
with it, and then drawn through the glass quickly but steadily, so as to mingle 
effectually, but not to endanger forcing the substance over the edges of the 
tray or to run the risk of involving air bubbles. The chamber and its con- 
tents are cooled by the necessary exposure to the atmosphere, and therefore, 
when the agitation has been continued until the glass is so much lowered in 
temperature as to become thick, it should be discontinued, the stirrer carefully 
removed, the glass covers replaced, the chamber covers restored to their situa- 
tion, and the temperature allowed to rise for fifteen or twenty minutes, when the 
operation may be renewed. 
77 - All the precautions against loose particles, dust, and soot, that were be- 
fore spoken of (66), should be adopted in this operation. In the act of stirring, 
the instrument should not be struck carelessly against the bottom or sides of 
the tray ; for the platina in this highly heated state is very soft, and a hole 
would readily be forced through it ; nor should it be brought forcibly against 
the corners, for the metal is in such a favourable condition for welding, that 
the least blow upon a doubled part causes adhesion. By merely allowing the 
stirrer, when ignited, to sink upon the bottom of the tray rather more hastily 
than usual, it has adhered to the place ; and when, for safety, an underlying 
plate of platina was used (50), it was always found welded to the tray at 
the places which the stirrer had touched a little more forcibly than the ad- 
jacent parts, and could not afterwards be separated without leaving holes in 
f 2 
