391. =Heat Produced by Electricity.=--Electricity always produces in its passage some amount of heat, probably by its mechanical effect. When it is diffused over a large conducting surface the heat is not sufficient to be observable; but if it be confined to the surface of a small wire the heat may be sufficient to melt or even burn it. Various effects can be produced by the heat thus caused by the passage of electricity. Gunpowder may be exploded by it. Alcohol and ether may be readily ignited by it, especially the latter. Gas can sometimes be lighted by pointing the finger to an opened burner after walking across the room two or three times briskly, rubbing the feet upon a thick carpet. [Illustration: Fig. 276.] 392. =Franklin's Discovery.=--It had very early been conjectured that the electricity produced by the electrical machine is identical with lightning; but it was reserved for our countryman Franklin to prove the fact. A tall spire which was being erected in Philadelphia in 1752 he conceived might be used in his investigations, but before it was completed the sight of a boy's kite in the air suggested to him another plan. He made a kite by stretching a silk handkerchief over a frame, and sent it up as he saw a thunder-shower rising, his only companion being his son. Having raised the kite, he attached to the end of the hempen string a key, and also a silk ribbon, by which he insulated his apparatus, as seen in Fig. 276. He now watched with much anxiety the result. A cloud arose, which he supposed, from its appearance, was well charged with electricity, and yet no effect was seen. Franklin began to despair; but he soon saw some loose fibres of the hempen string bristling up, and, applying his knuckle to the key, received just such a spark as he had often received from the conductor of an electrical machine. The discovery was made, and Franklin was at once overcome with emotion at the thought of the immortality which it would give his name. He felt very much as Archimedes did when, after making one of his grand discoveries as he lay in a bath, he went home saying all the way, Εὕρηκα! Εὕρηκα! The fame of the discovery, made in a manner so simple and yet so original, spread every where, and prompted to many experiments by other philosophers. One, Professor Richman of St. Petersburg, fell a victim to his investigations. While he was attending a meeting of the Academy of Sciences he heard the sound of distant thunder, and hastened home to make some observations with an apparatus which he had erected. While doing this a charge of electricity flashed from the conducting rod, and piercing his head killed him instantly. His assistant, who stood near, was struck down, and remained senseless for some time, and the door of the room was torn from its hinges.