For Crown Glass—
|Carbonate of Soda
|Carbonate of Lime
On examining the records of these practical
modes of obtaining the best achromatic Flint Glass, it will be found
that our philosophic countryman, Dr. Faraday, (the plan of Guinand being
then a secret,) suggested the same idea of stirring, which was carried
out successfully in the heavy Glass he manufactured. This was composed
||Protoxide of Lead.
||Silicate of Lead.
||Dry Boracic Acid.
His Glass required but a red heat for fusion,
thereby offering facilities for minute agitating operations. Although it
is not calculated for permanence or general use, its manufacture developed
the fact, that the destruction of striæ is more dependent upon
mechanical than chemical appliances; and that Dr. Faraday's conclusion
and those of M. Bontemps and Guinand, jun., were identical in principle.
The question may be asked, Why should there be difficulty in obtaining
good achromatic flint Glass? There would be none, (is the reply,) if
a manufacturer could anticipate a fair remunerative price and demand,
after having succeeded in obtaining the quality. Probably, he could
only sell 600 cwt. per annum, even were he to supply all the opticians
in Great Britain at ten times the price of ordinary Flint Glass, which
would scarcely be remunerative, whether he manufactured in one of the
pots of a ten-pot furnace, containing each fifteen or eighteen cwt.;
or upon a smaller scale, in