power and is coming into general use for lighting up large spaces. Another transformation is back again into chemical energy, which is shown by the power of the electric current to decompose compound substances. If, for instance, the poles of a battery are plunged into a vessel containing water, the molecules of the water will be decomposed and bubbles of oxygen gas will rise from the positive, and of hydrogen from the negative, pole. Another effect of electrical currents is that of attraction and repulsion on one another. If two parallel wires, free to move, carry currents flowing in the same direction as from positive to negative, or _vice versâ_, they will attract one another; if in opposite directions, they will repel. Electrical currents also work by way of induction, that is, they disturb the electrical equilibrium of bodies brought within their influence and induce currents in them. Thus, if we have two circular coils of insulated wire placed near each other, one on the right hand, the other on the left, and connect the extremities of the right-hand coil with the poles of a battery, when the connection is first made and the current begins to flow, a momentary current in the opposite direction will pass through the left-hand coil. This will cease, and as long as the current continues to flow through the right-hand coil there will be no current through the other; but if we break the contact between the right-hand coil and the battery, there will be again a momentary current through the left-hand coil, but this time in the same direction as the other. The same effect will be produced if, instead of making and breaking contact in the right-hand coil, we keep the current constantly flowing through it, and make the right-hand coil alternately approach and recede from the other coil. In this case, when the right-hand coil approaches, it induces an opposite current in the left-hand one; and when it recedes, one in the same direction as that of the primary. These phenomena of induction prepare us to understand the nature of magnets, and the magnetic effects produced by electrical currents. If an insulated wire is wrapped round a cylinder of soft or unmagnetic iron, and a current passed through the wire, the cylinder is converted into a magnet and becomes able to sustain weights. If the current ceases, the cylinder is no longer a magnet, and drops the weight. A magnet is therefore evidently a substance in which electric currents are circulating at right angles to its axis, and a permanent magnet is one in which such currents permanently circulate from the constitution of the body without being supplied from without. The earth is such a magnet, and also iron and other substances, under certain conditions.