And now we will return for a short time to the subject treated of at the commencement of this lecture. You see here (fig. 41) a large machine, arranged for the purpose of rubbing glass with silk, and for obtaining the power called _electricity_; and the moment the handle of the machine is turned, a certain amount of electricity is evolved, as you will see by the rise of the little straw indicator (at A). Now, I know from the appearance of repulsion of the pith ball at the end of the straw, that electricity is present in those brass conductors (B B), and I want you to see the manner in which that electricity can pass away. [Touching the conductor (B) with his finger, the Lecturer drew a spark from it, and the straw electrometer immediately fell.] There, it has all gone; and that I have really taken it away, you shall see by an experiment of this sort. If I hold this cylinder of brass by the glass handle, and touch the conductor with it, I take away a little of the electricity. You see the spark in which it passes, and observe that the pith-ball indicator has fallen a little, which seems to imply that so much electricity is lost; but it is not lost: it is here in this brass; and I can take it away and carry it about, not because it has any substance of its own, but by some strange property which we have not before met with as belonging to any other force. Let us see whether we have it here or not. [The Lecturer brought the charged cylinder to a jet from which gas was issuing; the spark was seen to pass from the cylinder to the jet, but the gas did not light.] Ah! the gas did not light, but you saw the spark; there is, perhaps, some draught in the room which blew the gas on one side, or else it would light. We will try this experiment afterwards. You see from the spark that I can transfer the power from the machine to this cylinder, and then carry it away and give it to some other body. You know very well, as a matter of experiment, that we can transfer the power of heat from one thing to another; for if I put my hand near the fire it becomes hot. I can shew you this by placing before us this ball, which has just been brought red-hot from the fire. If I press this wire to it, some of the heat will be transferred from the ball; and I have only now to touch this piece of gun-cotton with the hot wire, and you see how I can transfer the heat from the ball to the wire, and from the wire to the cotton. So you see that some powers are transferable, and others are not. Observe how long the heat stops in this ball. I might touch it with the wire, or with my finger, and if I did so quickly, I should merely burn the surface of the skin; whereas, if I touch that cylinder, however rapidly, with my finger, the electricity is gone at once--dispersed on the instant, in a manner wonderful to think of. [Illustration: Fig. 42.]