I wonder whether we shall be too deep to-day or not. Remember that we spoke of the attraction by gravitation of _all_ bodies to all bodies by their simple approach. Remember that we spoke of the attraction of particles of the _same_ kind to each other,--that power which keeps them together in masses,--iron attracted to iron, brass to brass, or water to water. Remember that we found, on looking into water, that there were particles of two different kinds attracted to each other; and this was a great step beyond the first simple attraction of gravitation; because here we deal with attraction between _different_ kinds of matter. The hydrogen could attract the oxygen, and reduce it to water, but it could not attract any of its own particles; so that there we obtained a first indication of the existence of _two_ attractions. To-day we come to a kind of attraction even more curious than the last, namely, the attraction which we find to be of a double nature--of a curious and dual nature. And I want first of all to make the nature of this doubleness clear to you. Bodies are sometimes endowed with a wonderful attraction, which is not found in them in their ordinary state. For instance, here is a piece of shell-lac, having the attraction of gravitation, having the attraction of cohesion; and if I set fire to it, it would have the attraction of chemical affinity to the oxygen in the atmosphere. Now, all these powers we find _in_ it as if they were parts of its substance; but there is another property which I will try and make evident by means of this ball, this bubble of air [a light india-rubber ball, inflated and suspended by a thread]. There is no attraction between this ball and this shell-lac at present: there may be a little wind in the room slightly moving the ball about, but there is no attraction. But if I rub the shell-lac with a piece of flannel [rubbing the shell-lac, and then holding it near the ball], look at the attraction which has arisen out of the shell-lac, simply by this friction, and which I may take away as easily by drawing it gently through my hand. [The Lecturer repeated the experiment of exciting the shell-lac, and then removing the attractive power by drawing it through his hand.] Again, you will see I can repeat this experiment with another substance; for if I take a glass rod and rub it with a piece of silk covered with what we call amalgam, look at the attraction which it has, how it draws the ball towards it; and then, as before, by quietly rubbing it through the hand, the attraction will be all removed again, to come back by friction with this silk. [Illustration: Fig. 33.]