The fact that there is such a space-filling æther is, therefore, only to be deduced by reasoning from experiments and observations, but it is not directly the object of our sense-perceptions in the same way that water or air can be. Nevertheless, there is abundant proof that it is not merely a convenient scientific fiction, but is as much an actuality as ordinary gross, tangible, and ponderable substances. It is, so to speak, matter of a higher order, and occupies a rank in the hierarchy of created things which places it above the materials we can see and touch. The question we have next to discuss is—What are the fundamental properties of this æther? and what are the terms in which we must describe its qualities? In order to answer these questions, we must direct attention to some electrical effects, since it has been shown that most of the electrical phenomena, like those of optics, point to the necessity for the assumption of a similar universal medium, different from ordinary matter. Abundant proof has been gathered in, that the electro-magnetic medium and the luminiferous æther are one and the same. We are met at the very outset of our electrical studies by the term _electric current_. Most of us know that the operation of electric telegraphs and telephones, electric lamps and electric railways, depend upon this employment of an agency called an electric current. The question then arises—What _is_ an electric current? and the answer to this question is not easy to give in a few words. We can, however, begin by explaining what an electric current can do, and how its presence can be recognized. Before me on the table is a spiral of copper wire, and in this wire, by special means, I can create what we call an electric current. I shall ask you to notice that when this is done two effects are immediately produced. In the first place, the wire becomes _hot_, and, secondly, it becomes _magnetic_. The fact that it is hot is evident, because it is now nearly red hot, and is visibly incandescent in the dark. If we dip the wire in iron filings, you will see that these cling to the wire and are taken up by it, just as when an ordinary steel magnet is substituted for the wire. The copper wire, when traversed by the current, also attracts a compass needle, and we thus demonstrate in another way its magnetic quality. Whenever, therefore, we find these two states of heat and magnetism present together in and round a wire, we may take it as an indication that it forms part of a circuit through which an electric current is flowing. An electric current is a physical state or condition which can only exist in or all along a closed path which is called an electric circuit. This electric circuit may consist of a metallic wire, or, as we generally call it, a conductor, or, as we shall see, it may also in part consist of what is usually called a non-conductor.