264t On the Manufacture of Optical Glass. [1829. 



shelter fixed over the furnace, either of tin plate, clean boards, 

 or some other material which shall not throw oft* scales or im- 

 purities 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 platinum stirrer (28), 

 or the bottom of the platinum 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 impurity hereafter, or of 

 cementing the tray and the earthenware together in a very in- 

 convenient and injurious manner. 



68. If, also, it should be observed at this time, that there is 

 a superabundance of glass in the tray, and not sufficient distance 

 between its surface and the edges of the platinum, the excess 

 should be ladled out (28), an operation easily performed, but 

 which must be done with care. 



69. When the glass is ascertained to be in a proper condition, 

 and that there is no appearance of any portion of it outside the 

 tray, the covers are to be replaced, the chamber closed, and the 

 heat continued. If the tray-covers be glazed, some precaution is 

 required in their arrangement ; for on putting the second cover 

 over the first, if they are left in contact by a portion of glazed 

 surface, they will be found, upon their next removal, to adhere 

 at that place. They should never be put in contact therefore 

 with each other, or, if that cannot be avoided, a piece of old 

 platina foil should be laid upon the place where the contact is 

 necessary (58). 



70. Whilst the glass is covered and subjected to a high tem- 

 perature, there is, as before stated, an inward current of fresh 

 air passing continually to and about it through the air-tube, 

 during the whole time of the experiment (55). 



It was necessary to apply a valve to the external orifice of 

 this tube to regulate the supply; for the draught was so con- 

 siderable, that the glass was cooled by it, and much dust car- 

 ried in. Finding reason to believe that even when very much 

 diminished, the quantity of soots and dust in a London atmo- 

 sphere, and especially in that portion of it taken from an expe- 



