Scientific Lectures. 145 



This is the lodestone,* or leading stone (from the Saxon Iceden, to 

 lead), so called Because it causes iron to follow it. Our forefathers 

 sometimes called it love-stone, just as the French now term it Vaimant, 

 the affectionate, because it has a strong affection for iron, and draws 

 it to itself. We call it magnet, from the province of Magnesia in 

 Lydia, whence the ancient Greeks obtained it and thence gave it the 

 name of the Magnesian stone. 



So much for the name ; but yet, what is a magnet ? Define it. " A 

 definition is the resolution of a complex idea into the simple elements 

 which compose it ;" but, in this case, the complexity of the idea con- 

 veyed by this name, magnet, so increases with our knowledge of its 

 properties, and the simple elements of its characteristics are so far 

 removed from ordinary observation, that I will be forced not to 

 answer the question I have called up, but allow the magnet to speak 

 for itself in its own " writing on the wall " [pointing to the screen], 

 while I will stand by and be its interpreter. 



Over the horizontal condensing lens (C) of this vertical lanternf I 

 place this glass plate, and, on allowing the lime light to fall upon it, 

 you observe on the screen a bright circle of over fifteen feet in 

 diameter. I now place on the plate a few tacks, and there they are 

 in the bright circle, each appearing over two feet long. That huge 

 black mass which you see slowly entering the circle is the lodestone 



* The lodestone is an oxide of iron (FeO + Fe203). We must not, however, infer that all speci- 

 mens of magnetic oxide of iron are magnetic. The lodestone is black or deep brown ; sometimes, 

 however, it is of a lighter color. It is often crystallized in octahedra, more or less modified, or in, 

 rhomboidal dodecahedra. The best are homogeneous, with a fine-grained fracture, and susceptible 

 of a beautiful polish. Their density varies, but is about 1.24. The magnetic ore generally occurs in 

 primary mountains of gneiss, chlonti slate in primitive limestone, and sometimes in considerable 

 masses in serpentine, and in trap. It is found in great quantity and purity at Roselay, in Sweden, 

 in Corsica, on the island of Elba, in Norway, Saxony and Bohemia. A hill in Swedish Lapland, and 

 Mount Pumacharache, in Chili, are said to consist almost entirely of magnetic ore. Extensive beds of 

 magnetic ore occur in various places in the United States, and at some of them are found masses of 

 the mineral possessing polarity ; such as those at Marshall's Island, Maine ; at Magnet's Cave, 

 Arkansas ; at Goshen, Chester County, Pennsylvania ; and at Franklin, New Jersey. " The most 

 powerful native magnets are found in Siberia, and in the Hartz ; they are also obtained on the 

 Island of Elba." [Dana.] 



" Wolf mentions examples of natural magnets which could support, by means of an armature, from 

 sixteen to forty times, and even 320 times their own weight. Duplex had in his possession a magnet 

 of nine pounds in weight, which could hold seventy-six pounds. As a general rule, smaller mag- 

 nets can support comparatively more than larger ones. Such, for example, as weigh twenty to 

 thirty grains will sometimes support fifty times their weight, whilst magnets weighing two pounds 

 scarcely ever sustain ten times their own weight. According to Dr. Martin, Sir Isaac Newton had a 

 magnet which was set in a finger-ring, and which, though only of three grains in weight, could hold 746 

 grains. In the philosophical cabinet of the University of Dorpat there is a magnet weighing forty 

 pounds, including the armature and the copper case, which is able to sustain eighty-seven pounds. A 

 still larger one is found in Tyler's Museum, which weighs 307 pounds, the armature inclusive, and 

 holds more than 320 pounds. Not less considerable was the magnet which John [V], King of Por- 

 tugal, received as a present from the Emperor of China, which weighed a little over thirty-eight 

 pounds, and was able [February, 1781], to support 202 pounds." Smith. Kept., 136, p. 288. 



t See engraving in American Institute Transactions for 1870-1, p. 1019. 



[Inst.] 10 



