The next step in electrical discovery, and one which marks an epoch in the history of the science, was made by Stephen Gray, of England, in 1729. To him is due the credit of finding out that electricity from an excited glass cylinder could be conducted away from it to objects at a remote distance. Though he used only a packthread as a conductor, he thus carried electricity to a distance of several hundred feet, and his novel discovery opened up what, for the time, was a brilliant series of experiments in England and throughout France and Germany. Out of these experiments came the knowledge that some substances were natural conductors of electricity, while others were non-conductors; and that the non-conductors were the very substances—glass, resin, sulphur, etc.—which were then in popular use as electrics. Here was laid the foundation of those after-discoveries which led to the selection of copper, iron, and other metals as the natural and therefore best conductors of electricity, and glass, etc., as the best insulators or non-conductors. Up to this time an excited electric, such as a glass cylinder or wheel, had furnished the only source whence electricity had been drawn for purposes of experiment. But now another great step forward was taken by the momentous discovery that electricity, as furnished by the excited but quickly exhausted electric, could be bottled up, as it were, and so accumulated and preserved in large quantities, to be drawn upon when needed for experiment. It is not known who made this important discovery; but by common consent the storage apparatus, which was to play so conspicuous a part in after-investigations, was named the _Leyden Jar_ or _Phial_, from the city of Leyden in Holland. It consisted of a simple glass jar lined inside and out with tinfoil to within an inch or two of the top, the tinfoil of the inside being connected by a conductor passing up through the stopper of the jar to a metallic knob on top. This jar could be charged or filled with electricity from a common electric, and it had the power of retaining the charge till the knob on top was touched by the knuckle, or some unelectrified substance, when a spark ensued, and the jar was said to be discharged. By conductors attached to the knob, guns were fired off at a distance by means of the spark, and it is said that Dr. Benjamin Franklin ignited a glass of brandy at the house of a friend by means of a wire attached to a Leyden jar and stretched the full width of the Schuylkill River at Philadelphia. [Illustration: LEYDEN JAR.]