1839.] On the Manufacture of Optical Glass. 273 



single cover being put over the tray, and the outer orifice of 

 the air-tube closed by a good cork, the whole may be left a 

 few minutes to cool still further for perfect security, until, the 

 glass being supposed to have arrived at the state of a thick 

 paste, the annealing should commence. Then the ash-pit, the 

 fire-place, and all the other apertures to the furnace are to be 

 closed ; the second glass cover put into its place ; the chamber 

 shut up by its iron and tile covers ; a layer of bricks arranged 

 close together over the whole upper surface of the chamber 

 and furnace ; the clamper of the flue closed to prevent air 

 passing through the fire-place, and the whole left to cool gra- 

 dually for several days. 



90. The interval between the common temperature and that 

 at which the glass begins to lose solidity and acquire softness, 

 is so much less, with this variety than with flint glass, that it is 

 probable a much shorter period of time is required for its perfect 

 annealing than for the latter. That no failure might occur in 

 this point, however, four days and nights have been allowed for 

 the annealing of the large plates. If everything were left as 

 just described, the contents of the chamber would be warm on 

 the sixth or even the seventh day, so gradually do the arrange- 

 ments allow it to cool ; but on the morning or the evening of the 

 third day, according to circumstances, the damper in the flue is 

 withdrawn a very little to allow the passage of a small quantity 

 of air, and by this means the cooling facilitated and regulated. 



91. When the furnace and its contents are cold, the chamber 

 is opened : if the experiment has been well conducted, every- 

 thing will be found loose, and unaltered in disposition from 

 what they were when first arranged. The earthenware supports 

 are to be removed, and the tray taken out. After examining 

 the glass itself, the exterior of the tray should be carefully 

 observed, whether there be any appearance of leakages either 

 through imperceptible holes or at the corners ; and such places 

 as can be rectified by a patch should be noted in reference to 

 the future use of the platinum. 



92. An operation which, to be successful, requires much care, 

 is then to be performed ; namely, the separation of the platinum 

 from the glass. The tray should be placed on clean smooth 

 paper upon a cloth. The corners are one by one to be opened 

 by a blunt smooth knife, or some softer instrument, from the 

 side towards which they were folded ; and being then carefully 



