SECT. ii. THE AGGREGATION OF MATTER. 77 



cleavages, and upon their power of transmitting elec- 

 tricity. The diamagnetic force is inversely as the con^ 

 ducting power of bodies, and the conducting power of 

 crystals is a maximum in the planes of their principal 

 natural joints. Hence the action of the diamagnetic 

 power is least in the natural joints, and conversely the 

 magnetic force is greatest. In fact, the magnetic 

 phenomena of crystals depends upon unequal conduc- 

 tibility in different directions, and their set is deter- 

 mined by the difference between the forces of attraction 

 and repulsion of the poles, for one pole of the magne- 

 crystallic axis is attracted and the other repelled. It is 

 unnecessary to give more examples to show the action 

 of the magnetic forces upon the atomic structure of 

 crystals. 9 



Magnetism changes the relations and distances be- 

 tween the ultimate atoms of matter, a circumstance 

 which probably depends upon their polarity. It changes 

 steel permanently, iron temporarily, and it elongates a 

 bar of iron, which loses in breadth what it gams in 

 length ; and as heat is developed in one direction and 

 absorbed in the other, the temperature of the bar remains 

 the same. Heat being an expansive force, diminishes 

 the magnetism of iron and nickel in proportion as it 

 increases the distance between their atoms, till at length 

 they lose their cohesive force altogether. But there 

 seems to be a temperature at which the magnetic force 

 is a maximum, above and below which temperature it 

 diminishes. Thus the magnetism of cobalt increases 

 with the temperature up to a certain point; it then 

 decreases as the temperature increases, and it loses its 

 magnetism altogether when the heat amounts to 1996. 



Sir Humphry Davy and M. Arago noticed that the 

 voltaic arc takes a rotatory motion on the approach of a 



9 ' Connection of the Physical Sciences.' 



