about 50 per cent, of oxide of lead among its constituents.
There existed in tine middle ages, and probably had existed among the ancients also, a substance called at first amasa, then encansta, and lastly smalta, from which last term our modern émail (enamel)
is derived. These were generic expressions for substances formed of
glass and a metallic oxide ; and the basis employed was certainly a
kind of strass—that is to say, glass containing a great quantity of oxide of lead.
The
improvement made in strass since the middle ages is due to our modern
chemistry, which furnishes productions of a perfect purity, otherwise
the ingredients, and probably their proportions, remain the same ; and
the same rule is still observed that the longer the fusion is
prolonged, the finer will be the quality of the strass. According to M.
Dumas, the strass now employed consists of—
