538 LECTURE LV. 



Iron filings, or the scoriae from a smith's forge, when finely levigated, 

 and formed into a paste with linseed oil, are also capable of being 

 made collectively magnetic. A bar of steel, placed red hot between two 

 magnets, and suddenly quenched by cold water, becomes in some degree 

 magnetic, but not so powerfully as it may be rendered by other means. 

 For preserving magnets, it is usual to place their poles in contact with 

 the opposite poles of other magnets, or with pieces of soft iron, which, in 

 consequence of their own induced magnetism, tend to favour the accumu- 

 lation of the magnetic power in a greater quantity than the metal can 

 retain after they are removed. Hence the ancients imagined that the mag- 

 net fed on iron. 



A single magnet may be made of two bars of steel, with their ends pressed 

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

 separated, or when a common magnet has been divided in the middle, the 

 portions 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. In this case we must suppose, contrarily to the general prin- 

 ciples of the theory, that the magnetic fluid has actually escaped by degrees 

 from one of the pieces, and has been received from the atmosphere by the 

 other. 



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

 netism 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 shock of electricity, 

 may expedite 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 exertion of the cohesive and repulsive powers appears to favour 

 the transmission of the magnetic as well as of the electric fluid. The inter- 

 nal agitation, produced 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 

 being attracted when it has been hammered, even between two flints ; and 

 that this property is again diminished by fire : in this case it may be con- 

 jectured that hammering increases the conducting power of the iron con- 

 tained in the brass, and thus renders it more susceptible of magnetic 

 action. Mr. Cavallo t also observed that a magnetic needle was more 

 powerfully attracted by iron filings during their solution in acids, espe- 

 cially in the sulfuric acid, than either before or after the operation : others 

 have not always succeeded in the experiment ; but there is nothing impro- 

 bable in the circumstance, and there may have been some actual difference 

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

 * Ph. Tr. 1786, p. 62. f Ibid. 1787, p. 6. 



