THE MAGNETIC NEEDLE. 51 



magnetic and electrical conditions of matter, moreover, es- 

 sentially distinguishes the phenomena of electro-magnetism 

 from those which are influenced by the primitive fundament- 

 al force of matter its molecular attraction and the attrac- 

 tion of masses at definite distances. To establish laws in 

 that which is ever varying is, however, the highest object of 

 every investigation of a physical force. Although it has 

 been shown by the labors of Coulomb and Arago that the 

 electro-magnetic process .may be excited in the most vari- 

 ous substances, it has nevertheless been proved by Faraday's 

 brilliant discovery of diamagnetism (by the differences of the 

 direction of the axes, whether they incline north and south, 

 or east and west) that the heterogeneity of matter exerts an 

 influence distinct from the attraction of masses. Oxygen 

 gas, when inclosed in a thin glass tube, will show itself un- 

 der the action of a magnet to be paramagnetic, inclining 

 north and south like iron; and while nitrogen, hydrogen, 

 and carbonic acid gases remain unaffected, phosphorus, 

 leather, and wood show themselves to be diamagnetic, and 

 arrange themselves equatorially from east to west. 



The. ancient Greeks and Romans were acquainted with 

 the adhesion of iron to the magnet, attraction and repulsion, 

 and the transmission of the attracting action through brass 

 vessels as well as through rings, which were strung together 

 in a chain-like form, as long as one of the rings was kept in 

 contact with the magnet ;* and they were likewise acquaint- 

 ed with the non-attraction of wood and of all metals, except- 

 ing iron. The force of polarity, which the magnet is able 

 to impart to a movable body susceptible of its influence, 

 was entirely unknown to the Western nations (Phoenicians, 

 Tuscans, Greeks, and 'Romans). The first notice which we 

 meet with among the . nations of Western Europe of the 

 knowledge of this force of polarity, which has exerted so im- 

 portant an influence on the improvement and extension of 

 navigation, and which,. from its utilitarian value, has led so 

 continuously to the inquiry after one universally diffused, 

 although previously unobserved force of nature, does not 

 date farther back than the llth and 12th centuries. In the 

 history and enumeration of the principal epochs of a physic- 



* The principal passage referring to the magnetic chain of rings 

 occurs in Plato's Jon., p. 533, D.E, ed. Steph. Mention has been made 

 of this transmission of the attracting action not only by Pliny (xxxiv., 

 14) and Lucretius (vi., 910), but also by Augustine (Da civltate Dei, 

 xx., 4) and Philo (De Mimdi opificio, p. 32 D, ed. 1691). 



