MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
9 
to such an extent, occurs in every pot of flint glass made in the ordinary 
way. 
8. Another curious illustration of the predominance of oxide of lead at 
the bottom is shown in many of our specimens, which have been broken 
through vertically : they have been affected by sulphuretted vapours and tar- 
nished ; but the tarnish has occurred only at the bottom, where the lead is 
abundant, and is there very strong, whilst there is no appearance of it towards 
the top. 
9. Whilst the crucible is in the condition described, it is clear that all those 
circumstances, as currents, bubbles, &c., which tend to mix the glass, form 
abundant striae and veins of enormous strength, and do harm unless they are 
continued in activity until the mixture is nearly complete; a state rarely if ever 
acquired in the ordinary flint glass pot. But even if this could be the case, 
there is a constant cause of deterioration, arising from the highly fluxing and 
dissolving quality given to the glass by the oxide of lead. In this respect, flint 
glass far surpasses crown or plate glass, and it is also during one stage of its 
preparation more fluid : it consequently is continually exerting a solvent 
power upon the crucible to a considerable extent, occasioning that very 
irregularity in composition which produces striae, whilst the comparative levity 
of the matter dissolved at the sides and bottom, and the ascending currents 
at the hottest parts of the crucible are constantly mixing this deteriorating 
portion with the general mass. 
10. The difficulties which are thus introduced into the manufacture of flint 
glass fit for optical uses appeared to the committee, who, however, were none 
of them practical glass-makers, to increase, as the scale upon which the inqui- 
ries were carried on diminished : and the enormous expense of large expe- 
riments, — the time required for each, — the number necessary to give that expe- 
rience which should render any one who undertook the charge of this part of 
the inquiry an ordinary practical workman, — and the uselessness of the resulting 
glass for any other purpose than the one directly contemplated, — compelled the 
sub-committee to consider seriously on the possibility of making other glasses 
than those ordinarily in use, which, at the same time that they had the high 
dispersive power enabling them to replace flint glass, might have also such fusi- 
bility as would allow of their being perfectly stirred and mixed, and might be 
MDCCCXXX. 
c 
