He explained the laws of hot air and its movements; the Holland stove, which afforded but little ventilation; the German stove, which was simply an iron box fed from outside, with no ventilating properties; and the great open fireplace fed with huge logs, which required such a draft to prevent the smoke from coming back into the room that the outer door had to be left open,--and if the door was shut the draft would draw the outer air whistling and howling through the crevices of the windows. His “Pennsylvania Fireplace” was what we would now call an open-fireplace stove. It was intended to be less wasteful of fuel than the ordinary fireplace and to give ventilation, while combining the heating power of the German and Holland stoves. It continued in common use for nearly a century, and modified forms of it are still called the Franklin stoves. One of its greatest advantages was that it saved wood, which, for some time prior to the introduction of coal, had to be brought such a long distance that it was becoming very expensive. Franklin refused to take out a patent for his invention; for he was on principle opposed to patents, and said that as we enjoyed great advantages from the inventions of others, we should be willing to serve them by inventions of our own. He afterwards learned that a London ironmonger made a few changes in the “Pennsylvania Fireplace” and sold it as his own, gaining a small fortune. Franklin’s invention was undoubtedly an improvement on the old methods of heating and ventilation; but he was not, as has been absurdly claimed, the founder of the “American stove system,” for that system very soon departed from his lines and went back to the air-tight stoves of Germany and Holland. It was not until 1746 or 1747, after he had been making original researches in science for about five years, that he took up the subject of electricity, and he was then forty-one years old. It appears that Mr. Peter Collinson, of London, who was interested in botany and other sciences, and corresponded largely on such subjects, had presented to the Philadelphia Library one of the glass tubes which were used at that time for producing electricity by rubbing them with silk or skin. Franklin began experimenting with this tube, and seems to have been fascinated by the new subject. On March 28, 1747, he wrote to Mr. Collinson thanking him for the tube, and saying that they had observed with its aid some phenomena which they thought to be new. “For my own part, I never was before engaged in any study that so totally engrossed my attention and my time as this has lately done; for what with making experiments when I can be alone, and repeating them to my friends and acquaintance, who from the novelty of the thing, come continually in crowds to see them, I have, during some months past, had little leisure for anything else.”