MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
25 
carbonic acid ; but as soon as the carbonic acid so produced came in contact 
with the heated iron, it was again converted, according to the well known 
condition of the chemical affinities at these temperatures, into carbonic oxide, 
and went back to the glass to repeat its evil operation and produce more 
metallic lead. In this way it was that the glass became sullied by smoky 
clouds consisting of metallic lead. It was the lead thus evolved also, that, 
by alloying with the platina, had produced the appearance of chemical action 
always visible in these cases ; and now I knew how to account for the failure 
of many experiments in consequence of the formation of holes in the trays in 
a manner before quite inexplicable : for in the experiments purposely made to 
investigate this point, sometimes the glass was darkened only at the surface, 
the lower part being quite clear and good; and then, though the platina tray 
was frequently cut through as with a knife all round level with the surface of 
the glass, it was quite unaltered below. At other times the superficial stain was 
in a greater quantity, and had collected together into little drops like fat upon 
hot water, and upon examination each little globule was found to be soft me- 
tallic brilliant 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 surface 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 supports (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. 
MDCCCXXX. 
£ 
