ichman, of St. Petersburg, while experimenting during
a thunder-storm, with an iron rod which he had erected on his house,
received a shock that killed him instantly.

About 1733, as we have seen, Dufay had demonstrated that there were two
apparently different kinds of electricity; one called VITREOUS because
produced by rubbing glass, and the other RESINOUS because produced
by rubbed resinous bodies. Dufay supposed that these two apparently
different electricities could only be produced by their respective
substances; but twenty years later, John Canton (1715-1772), an
Englishman, demonstrated that under certain conditions both might be
produced by rubbing the same substance. Canton's experiment, made upon
a glass tube with a roughened surface, proved that if the surface of the
tube were rubbed with oiled silk, vitreous or positive electricity was
produced, but if rubbed with flannel, resinous electricity was produced.
He discovered still further that both kinds could be excited on the same
tube simultaneously with a single rubber. To demonstrate this he used a
tube, one-half of which had a roughened the other a glazed surface.
With a single stroke of the rubber he was able to excite both kinds of
electricity on this tube. He found also that certain substances, such as
glass and amber, were electrified positively when taken out of mercury,
and this led to his important discovery that an amalgam of mercury
and tin, when used on the surface of the rubber, was very effective in
exciting glass.

XV. NATURAL HISTORY TO THE TIME OF LINNAEUS

Modern systematic botany and zoology are usually held to have their
beginnings with Linnaeus. But there were certain precursors of the
famous Swedish naturalist, some of them antedating him by more than a
century, whose work must not be altogether ignored--such men as Konrad
Gesner (1516-1565), Andreas Caesalpinus (1579-1603), Francisco Redi
(1618-1676), Giovanni Alfonso Borelli (1608-1679), John Ray (1628-1705),
Robert Hooke (1635-1703), John Swammerdam (1637-1680), Marcello Malpighi
(1628-1694), Nehemiah Grew (1628-1711), Joseph Tournefort (1656-1708),
Rudolf Jacob Camerarius (1665-1721), and Stephen Hales (1677-1761). The
last named of these was, to be sure, a contemporary of Linnaeus himself,
but Gesner and Caesalpinus belong, it will be observed, to so remote an
epoch as that of Copernicus.

Reference has been made in an earlier chapter to the microscopic
investigations of