Professor Barlocci found that an armed natural loadstone, which would
carry 1½ Roman pounds, had its power nearly _doubled_ by twenty-four
hours’ exposure to the strong light of the sun. M. Zantedeschi found
that an artificial horse-shoe loadstone, which carried 13½ oz., carried
3½ more by three days’ exposure, and at last arrived to 31 oz. by
continuing it in the sun’s light. He found that while the strength
increased in oxidated magnets, it diminished in those which were not
oxidated, the diminution becoming insensible when the loadstone was
highly polished. He now concentrated the solar rays upon the loadstone
by means of a lens; and he found that, both in oxidated and polished
magnets, they _acquire_ strength when their _north_ pole is exposed
to the sun’s rays, and _lose_ strength when the _south_ pole is
exposed.--_Sir David Brewster._