MAGNETISM. 899 



tance of any part from the poles, so that in the 

 middle between the poles there is no attraction. 

 This may be easily perceived by presenting a 

 piece of iron to various parts of the surface of a 

 magnet. 



The intensity of the attractive power diminishes 

 also, according to the distance from the magnet. 

 The law of diminution of this attraction is not yet 

 known. Some have imagined that it diminished 

 in proportion to the square of the distance, others 

 as the cube of the distance; it is only certain that 

 the attractive force decreases faster than the simple 

 ratio of the distances. 



Soft iron easily acquires magnetism; but it lasts 

 only a short time. With steel the case is very 

 different; and the harder the steel is, the more 

 permament is the magnetism which it acquires 

 from the influence of a magnet; but it will be in 

 the same proportion more difficult to render it 

 magnetic. 



Neither the magnetic attraction nor repulsion is 

 in the least diminished, or at all affected, by the 

 interposition of any sort of bodies, except iron, or 

 such bodies as contain iron. 



The properties of the magnet are not affected 

 either by the presence or by the absence of air. 

 Heat weakens the power of a magnet, and subse- 

 quent cooling restores it, but not quite- to its former 

 degree. A white heat destroys it entirely, or very 

 nearly so; and hence it appears, that the powers 

 of magnets must be varying continually. Cavallo 

 observes, that iron in a full red heat, or white heat, 

 is not attracted by the magnet; but the attraction 

 commences as soon as the redness begins to 

 disappear. 



