1829.] On the Manufacture of Optical Glass. 257 



collected together into little drops like fat upon hot water, and 

 upon examination each little globule was found to be soft 

 brilliant metallic lead. At other times a much larger globule 

 hung from the middle of the surface into the glass, barely 

 sustained there, and ready to sink by the least agitation when 

 in a heated state, and in some instances the bottom of the tray 

 was alloyed and perforated by globules of lead which had thus 

 been formed and deposited, and the glass just running out, 

 whilst another globule was in progress of formation at the sur- 

 face exactly over the place of the hole. 



51. When iron was dismissed as the material of the chamber, 

 earthenware was resorted to. The sides were built up of 

 brick, and the bottom formed of tiles, which resting at the 

 sides upon ledges, and at the middle upon the fire-brick sup- 

 ports (47), could be replaced at pleasure. The same iron 

 covers were used for the upper aperture of the chamber as 

 before. 



52. The use of earthenware as the material, made it far 

 more difficult to apply a sufficient heat to the contents of the 

 chamber than before, because of its inferiority to the iron as a 

 conductor of heat ; and a series of investigations were required 

 to discover that substance which, at the same time that it had 

 sufficient strength and exerted no injurious influence, was also 

 a sufficiently good conductor. Reigate fire-stone, recommended 

 by the builders, did not answer the purpose, and moreover in 

 thin plates was liable to fuse and slag. Slate, however care- 

 fully heated, shivered and split not only across but parallel 

 to its structure ; and then, as soon as air intervened, it trans- 

 mitted too little heat. It also softened, became curved, and 

 let in air and smoke, and at last gradually fused, becoming 

 unable to bear the weight of a large experiment. Yorkshire 

 stone, rubbed down into plates fthsof an inch thick, answered 

 moderately well, if the application of heat was carefully made 

 and gradually raised. It cracked in a few places, but did not 

 fall to pieces ; and it was more difficult of fusion than the 

 former substances. Fire-tiles of various kinds were tried ; 

 those made of Stourbridge clay answered the best, and, when 

 about f ths of an inch thick and carefully heated, might be 

 successfully used ; but that which we finally arrived at was 

 the use of plates made of the materials from which Cornish 



