90 PHYSIOLOGICAL PHYSICS. [Chap. ix. 



results shown in the first column, that when the 

 nerve has been exposed for a little time, the same 

 strength of current gives the results shown in the 

 second column, that is, produces the same effects as 

 a stream of medium strength would do in a fresh 

 nerve ; and, when the nerve has been exposed for 

 a still longer time, the results of the third column are 

 obtained, i.e. the results of passing a strong stream 

 to a fresh nerve. Thus is obtained an experimental 

 demonstration of the fact that the excitability of a 

 nerve increases as the nerve dies, and reaches a maxi- 

 mum just before it finally disappears. 



CHAPTER IX. 



MAGNETS, AND THE ACTION ON THEM OF ELECTRIC 



CURRENTS. 



A magnet is a body that has the property of 

 attracting iron. Natural magnets exist as an ore of 

 iron, whose formula is F"e 2 O 3 , and which was known 

 to the ancients, from whom the term magnet (^71/777775) 

 is derived, because the ore was found in the neigh- 

 bourhood of the town of Magnesia in Lydia. Mag- 

 netic properties can be communicated in various ways 

 to iron and steel, and these become artificial magnets. 

 The natural magnet was also called loadstone (Saxon 

 for leading stone), because when freely suspended it 

 always turned in such a direction, that its long axis 

 pointed north and south, the same extremity of the long 

 axis always being to the north, no matter how the 

 magnet was turned, so long as it was free to move. 

 The attractive power of a magnet is found to be 

 greatest at the ends, which are accordingly called the 



