352 COMMON ORIGIN OF FOECES. SECT. XXXIV. 



before it comes to it. The action depends much on the form 

 and dimensions of the bar and the magnetic pole, which ought 

 to be flat. The phenomena are due to the high electro-con- 

 ducting power of the copper, and are met with in some of the 

 other pure metals, though in a far inferior degree. 



Great magnetic power is requisite for all these experiments. 

 Dr. Faraday employed a magnet that could sustain a weight of 

 450 Ibs. at each pole, and the poles were either pointed or flat 

 surfaces at pleasure, as the kind of experiment required. 



Heat strongly affects the magnetic properties of bodies. Dr. 

 Faraday found that, when the temperature of nickel is increased, 

 its magnetic force diminishes ; when that of iron is increased its 

 magnetic force remains the same, while that of cobalt increases ; 

 which seems to indicate that there is a temperature at which the 

 magnetic force is a maximum, above and below which it dimi- 

 nishes. Nickel loses its magnetism at the temperature of 

 boiling oil, iron at a red heat, and cobalt near the temperature at 

 which copper melts. Calcareous spar retains its magnetic cha- 

 racter at a very high temperature ; but the same substance when 

 it contains iron, and also oxide of iron, loses it entirely at a dull 

 red heat. A crystal of the ferrocarbonate of lime was absolutely 

 reversed by change of temperature, for at a low heat the optic 

 axis pointed axially, and at a high temperature equatorially. 

 With the exception of these substances, magnecrystals, whether 

 paramagnetic or diamagnetic, are generally all affected alike by 

 heat. The difference between the forces in any two different 

 directions, as for instance the greatest and least principal axes, 

 diminishes as the temperature is raised, increases as the tempera- 

 ture is lowered, and is constant for a given temperature. No 

 unmixed or pure substance has as yet passed by heat from the 

 paramagnetic to the diamagnetic state. No simple magnecrystal 

 has shown any inversion of this kind, nor have any of the chief 

 axes of power changed their characters or relations to one 

 another. 



It appears that, as the molecules of crystals and compressed 

 bodies affect magnetism, so magnetism acts upon the molecules of 

 matter, for torsion diminishes the magnetic force, and the elasticity 

 of iron and steel is altered by magnetism. M. Matteucci has 

 found that the mechanical compression of glass alters the rotatory 

 power of a polarized ray of light transmitted through it, and that 



