40 
MR. FARADAY ON THE 
is to be opened and the glass stirred. The heat will have fallen very little by 
the preceding operations, and the glass may be well mixed, but with this pre- 
caution, that when once the stirrer is beneath the surface, it should not again 
be taken out until the conclusion. By opening the feed-hole, or the ash-pit, 
air may now be allowed freely to enter the furnace, and will rapidly lower its 
temperature, especially at such parts as the bottom of the pan, which are thin 
and at this moment exposed to the atmosphere on both surfaces. The tempera- 
ture of the glass will fall in a corresponding degree, and the stirring being all 
this while continued, though more slowly if convenient, the substance will 
gradually thicken, until at last motion will endanger its being pushed out of 
the tray, and then the stirrer is to be carefully withdrawn. No currents in the 
glass need be feared, for the temperature cannot now rise higher. But a 
single cover being put over the tray, and the outer orifice of the air-tube 
closed by a good cork, the whole may be left a few minutes to cool still further 
for perfect security, until, the glass being supposed to have arrived at the 
state of a thick paste, the annealing should commence. Then the ash-pit, the 
fire-place, and all the other apertures to the furnace are to be closed ; the se- 
cond glass-cover put into its place ; the chamber shut up by its iron and tile 
covers ; a layer of bricks arranged close together over the whole upper sur- 
face of the chamber and furnace; the damper of the flue closed to prevent air 
passing through the fire-place, and the whole left to cool gradually for several 
days. 
90. The interval between the common temperature and that at which the 
glass begins to lose solidity and acquire softness, is so much less Avith this va- 
riety than with flint glass, that it is probable a much shorter period of time is 
required for its perfect annealing than for the latter. That no failure might 
occur in this point, however, four days and nights have been allowed for the 
annealing of the large plates. If every thing were left as just described, the 
contents of the chamber would be warm on the sixth or even the seventh day, 
so gradually do the arrangements allow it to cool; but on the morning or the 
evening of the third day, according to circumstances, the damper in the flue is 
withdrawn a very little to allow the passage of a small quantity of air, and by 
this means the cooling facilitated and regulated. 
91. When the furnace and its contents arc cold, the chamber is opened; if 
