272 On the Manufacture of Optical Glass. [1829. 



then allowed to remain at rest for six or eight 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, being thus finally mixed for the last 

 time. 



89. The concluding mixture 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 the 

 temperature is not again to be allowed to rise ; hence the opera- 

 tion 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 im- 

 possible 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 coke is likewise to be withdrawn.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 is to 

 be opened and the glass stirred. The heat will have fallen 

 very little during the preceding operations, and the glass may be 

 well mixed ; but with this precaution, 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 temperature 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 



