The magnetism which is produced when iron merely approaches another magnet is said to be induced magnetism. In a treatise on magnetism William Gilbert explained the fact that a magnet always pointed to the north on the grounds that the earth itself was a very large magnet. He knew that there were two kinds of magnetism, and that they produced effects opposite in character. Gilbert appreciated the facts that like poles repel one another, whereas unlike poles attract one another, and that a piece of soft iron when held near to a magnet itself became a magnet. He knew also that the magnetism of a steel bar could be destroyed by heating it to the temperature of red heat, and that when a magnet had been broken into smaller fragments, each fragment remained a small but perfect magnet. Some very important facts for the theory of magnetism were revealed by the discovery made by F. Heusler.