1829.] On the Manufacture of Optical Glass. 263 



quantity of lateral brick-work to be beated, it is usually many 

 bours before tbe sides of tbe chamber are so hot, that the tray 

 and its contents have attained their highest temperature. At 

 the same time it must be understood that the heat of the glass is 

 very much governed, especially at the early part of an experi- 

 ment, by the number of tray-covers over it, and rises far more 

 rapidly, and much higher, with two or three covers than with 

 one. 



65. Perhaps the glass may with propriety be examined once, 

 early in the experiment, for the purpose of ascertaining that 

 the tray and its contents are safe ; but usually it is left for six 

 or eight, or a greater number of hours, that the whole may 

 fuse, the temperature rise, and the bubbles escape. When the 

 glass is to be examined, the tile and iron covers are to be re- 

 moved from over that half of the chamber containing it, by 

 which, consequently, the tray-covers are exposed ; these are 

 next to be carefully raised, one by one, using the iron instru- 

 ment before described, for the purpose (61), and, as they are 

 removed, are to be carefully put into the further part of the 

 chamber, which still remains covered, where they will be re- 

 tained in a heated state. This prevents their cracking arid 

 falling to pieces, as they would do if brought into the open air. 

 If the experiment, and consequently the covers, are upon so 

 large a scale that the latter cannot all be placed in this situa- 

 tion, then the exterior ones may be placed upon the top of the 

 heated covers and tiles ; but the particular cover, which imme- 

 diately encloses the glass, being of great importance, must be 

 put into the further safe part of the furnace, that it may be 

 carefully preserved from injury, and ready to be replaced over 

 the glass with the least possible disturbance. 



66. The moment the last cover is removed, the glass is ex- 

 posed to any falling substance from the iron plates, or tiles, or 

 other sources, so that extreme attention is required at such 

 times to keep the place free from dust, and to perform every 

 requisite operation as quietly as possible. The current of hot 

 air which rises from the chamber, ascending and striking against 

 the ceiling, frequently causes, by change of temperature and 

 mechanical agitation, the separation of small particles of mat- 

 ter, which, descending, cause risk of injury to the glass; for 

 which reason it may sometimes be needful to have a temporary 



