loadstone is added. "Because that force is
believed, in our times just as in former times, to attract the liquor of
glass to itself, as it attracts iron to itself, purges it when drawn, and
makes clear glass from green or muddy; but the fire afterwards burns up the
loadstone." It is true indeed that some sort of _magnes_ (as the magnesia
of the glass-makers imbued with no magnetick virtues) is sometimes put in
and mixed with the material of the glass; not, however, because it attracts
glass. But when a loadstone is burnt, it does not lay hold of iron at all,
nor is iron when red-hot allured by any loadstone; and loadstone also is
burnt up by more powerful fires and loses its attractive potency. Nor is
this a function of loadstone alone in the glass furnaces; but also of
certain pyrites and of some easily combustible iron ores, which are the
only ones used by our glass-makers, who make clear, bright glass. They are
mixed with the sand, ashes, and natron (just as they are accustomed to make
additions in the case of metallick ores whilst they are smelted), so that
when the material slows down into glass, the green and muddy colour of the
glass may be purged by the penetrating heat. For no other material becomes
so hot, {112} or bears the fire for such a convenient time, until the
material of the glass is perfectly fluid, and is at the same time burnt up
by that ardent fire. It happens, however, sometimes, that on account of the
magnetick stone, the magnesia, or the ore, or the pyrites, the glass has a
dusky colour, when they resist the fire too much and are not burnt up, or
are put in in too great quantity. Wherefore manufacturers are seeking for a
stone suitable for them, and are observing also more diligently the
proportion of the mixture. Badly therefore did the unskilful philosophy of
Pliny impose upon Georgius Agricola and the more recent writers, so that
they thought the loadstone was wanted by glass-makers on account of its
magnetick strength and attraction. But Scaliger in _De Subtilitate ad
Cardanum_, in making diamond attract iron, when he is discussing
magneticks, wanders far from the truth, unless it be that diamond attracts
iron electrically, as it attracts wood, straws, and all other minute bodies
when it is rubbed. Fallopius reckons that quicksilver draws metals by
reason of an occult property, just as a loadstone iron, amber chaff. But
when quicksilver enters metals, it is wrongly called attraction. For metals