In the year 1749, Franklin had an idea of making experiments to prove that lightning was really a great electric spark. Before he saw the electric experiments, he had thought that the lightning was due to some kind of sulphurous breath coming from the earth and collecting in the atmosphere. Other people thought that the lightning was due to a gas explosion. The truth of such guesses could be decided only by experiment. For when a thing has to be explained it is not sufficient that it shall be more or less like some other thing. Things are sometimes very similar and still have nothing to do with each other. The flowers of frost on a pane of glass look very much like the leaves of ferns or other delicate plants. But the force which makes the fern grow in the wood is quite different from the frost which makes the crystals grow on the glass, and it would be a great mistake to suppose that the same causes are at work in both cases. For in one case we must have a germ, nourishment, and a certain temperature, whereas in the other case we require moisture and cold only. Benjamin Franklin had the true scientific spirit. When he found that the electric spark and lightning both looked bright and made a noise, he did not jump at once to the conclusion that they had the same nature. By careful study he found that lightning and the electric spark have nearly the same colour. The electric spark looks like a small flash of lightning. Both are equally swift, both are conducted by metals, both are capable of tearing or breaking bodies they pass through, both are capable of killing animals instantly, both can melt metals and fire inflammable substances, and finally both give rise to the same kind of smell. It only remained now for Franklin to draw an electric spark from the clouds to prove that lightning was an electric discharge. To do this he proposed that a pointed iron rod should be mounted on the top of some high steeple as near as possible to the clouds, so as to draw electricity out of them. Near the top of the tower a man was to be placed on a stool with glass legs, and connected with a pointed rod. If the rod collected electricity from the clouds it would be possible to draw sparks from the man. Franklin proposed this experiment in a letter to the Royal Society of London, but the Society did not encourage him. He then tried to start a lottery in order to obtain the money necessary to build a tower for the purpose, since there was no building in Philadelphia high enough for the experiment.