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.