262 On the Manufacture of Optical Glass. [1829. 



strength, and no injurious degree of elasticity, are made taper 

 at one extremity, and slightly curved there. This end is 

 easily introduced beneath the platinum suspension wire, and as 

 easily withdrawn when the cover is removed. 



62. All these matters being preliminarily arranged, the final 

 disposition of the tray and its charge is made. The air-tube 

 is carefully wiped, and its external aperture closed by a clean 

 loose plug of dry sponge. The tray is for the last time freed 

 from dust by inversion and blowing upon it, and is put into its 

 place. The quantity of rough glass necessary for the required 

 plate, about 8lbs in the present instance (30), is carefully 

 weighed out, and then introduced by an evaporating basin, or 

 some other means which shall not allow of the admission of 

 any reducing or colouring matter, or permit any portion of 

 glass to pass beyond the edges of the tray. The tray-covers 

 are then to be arranged in their places ; the iron covers of the 

 chamber likewise adjusted, and over all are to be placed a set 

 of thick earthenware tiles, which have been fitted together so 

 as to constitute a general covering to the whole, well calculated 

 to retain heat. 



63. The ensuing part of the process is one in which the 

 precise order and most advantageous proceedings have not 

 yet been ascertained. Variations have been made up to the 

 very last experiment, and it is only by still more extensive 

 experience that the arrangement will ultimately be settled. 



64. A fire being lighted in the furnace, and some coke put 

 beneath the glass chamber, the temperature is gradually raised. 

 In about an hour the bottom of the chamber begins to appear 

 ignited, and in four hours the top iron covers are usually dull 

 red-hot. These appearances are useful as indications of the 

 progress of the operation. When the furnace has been heated 

 for the first half hour, then every care is taken that the tem- 

 perature may be fully sustained to the end of the experiment ; 

 and besides the ordinary kind of attention to the fire, par- 

 ticular care is taken that coke be supplied, by the lateral 

 holes, to the part beneath the chamber ; for, if the fuel there 

 be allowed to burn out, the heat soon falls, notwithstanding 

 the flame from the coals. Although the fire may seem quickly 

 to have attained its best condition, yet the temperature con- 

 tinues to rise in the chamber long afterwards ; for, from the 



