69^ LECTURE LV. 



into close contact; and it might be expected that when these bars are sepa- 

 rated, or when a common magnet has been divided in the middle, the por- 

 tions should possess the properties of the respective poles only. But in fact 

 the ends which have been in contact are found to acquire the properties of 

 the poles opposite to those of their respective pieces, and a certain point in 

 each piece is neutral, which is at first nearer to the newly formed pole than 

 to the other end, but is removed by degrees to a more central situation. lii 

 this case we must suppose, contrarily to the general principles of the theory, 

 that the magnetic fluid has actually escaped by degrees from otie of the 

 ])ieces, and has been received from the atmosphere by the other. 



There is no reason to imagine any immediate connexion between magnet- 

 ism and electricity, except that electricity affects the conducting powers of 

 iron or steel for magnetism, in the same manner as heat or agitation. In some 

 cases a blow, an increase of temperature, or a slwck of electricity, may expe- 

 dite a little the acquisition of polarity ; but more commonly any one of these 

 causes impairs the magnetic power. Professor Robison found, that when a 

 good magnet was struck for three quarters of an hour, and allowed in the 

 mean time to ring, its efficacy was destroyed ; although the same operation 

 had little effect when the ringing was impeded; so that the continued exer- 

 tion of the cohesive and repulsive powers appears to favour the transmission 

 of the magnetic as well as of the electric fluid. The internal agitation, pro- 

 duced in bending a magnetic wire round a cylinder, also destroys its polarity, 

 and the operation of a file has the same effect. Mr. Cavallo has found that 

 brass becomes in general much more capable of beiri'g attracted when it has 

 been hammered, even between two flints; and that this property is again di- 

 minished by fire: in this case it may be conjectured that hammering increases 

 the conducting power of the iron contained in the brass, and thus renders it 

 more susceptible of magnetic action. I\Ir. Cavallo also observed that a mag- 

 netic needle was more powerfully attracted by iron filings during their solu- 

 tion in acids, especially in the sulfuric acid, than either before or after the 

 operation: others have not always succeeded in the experiment; but there is 

 nothing improbable in the circumstance, and there may have been some actual 

 difference in the results, dependent on causes too minute for observation. In 

 subjects so little understood as the theory of magnetism, we are obliged to ad- 



