1829.] On the Manufacture of Optical Glass. 255 



ported on ledges at the sides, and upon the ends of fire-bricks 

 in the middle, firmly placed at intervals so as neither to stop 

 the passage for smoke and flame, nor the cross passages for 

 the introduction of coke. 



48. The value of the coke arrangement in this as in the other 

 furnace is very great. The heat obtained by the united action 

 of the coke and the flame from the fire-place, is abundantly 

 sufficient, and, whilst obtained at the necessary point, does 

 not involve that degree of mechanical action required for 

 stoking and stirring, which is necessary with coals, and would 

 risk the destruction of the soft thin bottom of the glass 

 chamber. It further occasioned the perfect combustion of 

 the smoke produced in the coal fire, which at first was so 

 considerable in quantity that, had it continued unaltered, the 

 experiments must have been removed from the Royal Institu- 

 tion ; in which case they would probably have been discon- 

 tinued altogether. 



The flue is the same as that connected with the former 

 furnace, and has a damper for regulating the heat, especially 

 useful during the annealing operation. 



49. The chamber was at first of cast iron, that material 

 being selected as one which would bear a sufficient tempe- 

 rature without melting, would conduct and transmit the heat 

 freely to the substances within, and could be easily obtained 

 of the requisite form. The upper aperture was closed by 

 plate-iron covers, and in the first trials all appeared to answer 

 well; but when large experiments were made, and the heat 

 was continued for a long time, the bottom gave way and 

 became irregular : and upon endeavouring to rectify this, and 

 place the tray of glass level by means of sand, the transmission 

 of heat to the glass was prevented, the temperature of the iron 

 rose, and the bottom melted. Besides these injurious liabilities, 

 if the smallest portion of glass passed out of the tray, the 

 moment it touched the iron it was reduced, the lead im- 

 mediately caused fusion of the platinum, and in an instant the 

 tray was destroyed, the experiment stopped, the glass rendered 

 black and useless, and the bottom of the chamber covered with 

 lead and rendered unfit for another operation. 



50. Finally, one very curious action of the iron was dis- 

 covered, which immediately caused its rejection, Plates of 



