254 On the Manufacture of Optical Glass. [1829. 



be helped, but always to make the largest plate upon it for 

 which it is competent. Then, when operated with a second or 

 third time, smaller gauges may be used, and the folds will not 

 be repeated in the same place; and if injury occurs to the 

 metal, being generally at the sides of the tray, the middle part 

 will still be left for the preparation of smaller plates of glass. 



If such large plates of platinum are required for trays as can 

 hardly be rolled at once, there is no difficulty in making a 

 folded joint and rendering it tight by soldering with gold. 



44. A kind of furnace, unlike the former, is now required 

 for the completion of the glass, and its delivery in the state of 

 an annealed plate. This furnace shall be described accurately 

 in the Appendix. It may here be sufficient to state that it 

 consists of a fire-place in which coals are burnt; of a part 

 beyond, acting both as furnace and flue, in which coke is 

 used; and of a chamber above, to be heated by the fire, 

 though out of the course of both flame and smoke. It is in 

 this chamber that the glass is made ; so that, by the arrange- 

 ment adopted, at the same time the substances are fused and 

 access for stirring allowed, the essential condition of excluding 

 impurity or reducing matter is also fulfilled. 



45. The fire-place itself is of the ordinary construction, and 

 fed with fuel by an aperture in front in the usual way. I have 

 found abundant reason to be satisfied that the passage of steam 

 beneath the bars of the grate is of considerable use ; for which 

 reason an iron trough charged with water occupies the lower 

 part of the ash-pit. The bars are by this arrangement pre- 

 served very cool and do not burn away ; they are easily kept 

 open and clear of clinkers ; the free passage of air to the fire 

 is permitted ; and the action of the furnace retained at a high 

 point for any number of hours together. 



46. That part of the furnace beneath the chamber requires 

 peculiar and careful arrangement ; for at the same time that 

 such a heat as will soften the neighbouring materials is pro- 

 duced there, the bottom of the chamber in its softened state 

 and charged with several pounds of materials, has to be firmly 

 supported for many hours together without change of position. 



47. The coke necessary in this part is introduced by two or 

 more holes in the side of the furnace, which, when necessary, 

 are stopped by bricks. The bottom of the chamber is sup- 



