MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
31 
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 removed from over 
that half of the chamber containing it, by which, consequently, the glass covers 
are exposed , these are next to be carefully raised, one by one, using the iron 
instrument 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 re- 
mains covered, where they will be retained in a heated state. This prevents 
their cracking and 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 situation, then the exterior 
ones may be placed upon the top of the heated covers and tiles ; but the par- 
ticular cover, which immediately incloses 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 exposed to any fall- 
ing 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 se- 
paration of small particles of matter, which, descending, cause risk of injury to 
the glass ; for which reason, it may sometimes be needful to have a temporary 
shelter fixed over the furnace, either of tin plate, clean boards, or some other 
material which shall not throw off scales or impurities of any kind. 
67. If, by any unfortunate accident, a fragment of matter does fall into the 
glass, it should be instantly removed. It certainly will not sink, because of 
the great density of the glass, and may be taken out, usually with facility, by 
touching it, and the glass in its neighbourhood, with the platina stirrer (28), 
or the bottom of the platina ladle (28). In carrying it and the adhering 
glass away, great attention should be given, that none of the latter fall over 
the sides of the tray ; since such portion might be a means of introducing im- 
