certain effects. But the common transparent glass-ware used in every
house is the kind we call flint, and it is composed of the materials
I have named,-- silica, oxide of lead, and the alkalies, with arsenic
and oxide of manganese to decolorize it."
"What do you use for alkalies?" asked the Doctor.
"Pearlash and saltpetre, or pearlash and soda.
Here is where we purify the saltpetre."
The gaffer showed a tank, the bottom and sides
of which were thickly incrusted with beautiful large crystals. "The
saltpetre," he explained, "is dissolved in hot water. The liquid is
skimmed, and allowed to cool. As the crystals form, they exclude all
impurities, which are drained off with the remaining liquid. The pearlash
is purified in a different way. It is dissolved, like the saltpetre; but
the impurities, except what are skimmed off, settle to the bottom, in what
we call slurry, which we sell to chemical works. The liquid is then
evaporated in these large cauldrons, until only the dry, clear pearlash
remains."
The gaffer then showed where the several materials
were all thrown together into a tank, and mixed. "They are then ready
to be loaded upon this carriage, and taken to the blowing-house,-- which
we will now go and see."
This was delightful news to Lawrence, who was
getting tired of these preliminaries, and eager to witness the wonders of
blowing and working the melted material.
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