411. =In what Magnetism is Like Electricity.=--Magnetism is like electricity in several particulars: 1. Its power is on the surface of bodies. 2. It is of two kinds, north and south, or boreal and austral, comparing with the positive and negative electricities. 3. The same rule of attraction and repulsion applies to both; viz., like repel and unlike attract. 4. As electricity can be communicated by induction, so can magnetism. 412. =In what Magnetism is Unlike Electricity.=--The circumstances in which magnetism is unlike electricity are chiefly these: 1. The obvious manifestations of magnetism are to a great extent confined to one class of substances, the ferruginous, and to but a portion of them; while electricity makes its manifestations in connection with all kinds of substances. 2. Magnetism is never transferred as electricity is from one body to another, but a body gains rather than loses in imparting magnetic power to other bodies. 3. The two magnetisms, the boreal and austral, can not be obtained separately as the two electricities can. If a magnet be broken in two, each piece will have in it the two magnetisms and the two poles as the whole did. This is in entire contrast with the electrical experiment noticed in the last of § 379. 4. There are no non-conductors to interrupt magnetic influence. If in the experiments in § 379 a plate of glass or resin were interposed between A and B, the influence would cease, but it would have no effect on the induction of magnetism if interposed between a magnet and a bit of steel or iron. 413. =Electro-Magnetism.=--Though electricity and magnetism differ so much from each other, yet they have intimate relations, and it is now the general opinion among scientific men that they are merely different modes of the same power. Magnetism can produce electricity, and electricity can produce magnetism. The first discovery of facts revealing this connection was made by Professor Oersted of Copenhagen in 1819. Since that time electro-magnetism, or the production of magnetism by electricity, has been a prominent subject of observation and experiment. Oersted's first observation was that a current of electricity passing over a wire near a magnetic needle affected the position of the needle. He found also that iron filings would adhere to a wire over which a current of electricity is passing, just as they do to a magnet, dropping off, however, as soon as the current ceases to pass. Such facts led to a great variety of investigations and arrangements of apparatus by Oersted and others. [Illustration: Fig. 286.]