This letter also shows that with electricity Franklin had frequently imparted polarity to needles and reversed it at pleasure. Wilson, at London, he said, had failed to produce these results because he had tried it on too large masses and with too small force. The letter also evidences the fact that he had employed the electric spark for the practical purpose of firing gunpowder. Another letter to Collinson dated July 29, 1750, is accompanied by an additional paper on the properties and effects of the Electrical Matter. It acknowledges the debt that Franklin owed to Collinson for the glass tube and the instructions which attended it, and to the Proprietary for the generous present of a complete electrical apparatus which "that bountiful benefactor to our library," as he calls him, had made to it. The telegraph, the Marconi tower, the telephone, the electric bulb, the electric automobile and the trolley car rise up before us when we read this observation in the paper that accompanied the letter: "The beneficial uses of this electric fluid in the creation, we are not yet well acquainted with, though doubtless such there are, and those very considerable." The paper is the most important that Franklin ever wrote on electricity; containing as it does the two suggestions which, when carried into execution, made his name famous throughout the world, that is to say, his suggestion, already quoted by us at length, that houses, churches and ships might be protected by upright rods of iron, and his suggestion, already quoted by us, too, as to how the identity of lightning and electricity could be established. The point of the bodkin and the electrified shot and ball, and the mimic brightness, agility and fury of the lurking fire in the wonderful bottle had led, step by step, to two of the most splendid conceptions in the early history of electrical science.[54] With the discovery that electricity and lightning were the same thing, the real achievements of Franklin in the province of electricity came to an end. But he still continued his electrical experiments with undiminished ardor. We find him on one occasion prostrating with a single shock six persons who were so obliging as to lend themselves to the pursuit of scientific truth. Twice he was the victim of his own inadvertence. Speaking of one of these occasions, in a letter to a friend in Boston, he said: