SIMPLE AND COMPOUND EVOLUTION. 301 



comes crystalline if exposed to a perpetual jar. The polar 

 forces mutually exercised by the atoms, fail to change the 

 disorderly arrangement into an orderly arrangement while 

 the atoms are relatively quiescent; but these forces succeed 

 in re-arranging them when the atoms are kept in a state of 

 intestine agitation. Similarly, the fact that a«bar of steel 

 suspended in the magnetic meridian and repeatedly struck, 

 becomes magnetized, is ascribed to a re-arrangement of par- 

 ticles that is produced by the magnetic force of the Earth 

 when vibrations are propagated through them, but is not 

 otherwise produced. Now imperfectly as these 



cases parallel the mass of those we are considering, they 

 nevertheless serve roughly to illustrate the effect which 

 adding to the quantity of motion an aggregate contains, has 

 in facilitating re-arrangement of its parts. 



More fully illustrative are the instances in which, by ar- 

 tificially adding to or subtracting from that molecular mo- 

 tion which we call its heat, we give an aggregate increased 

 or diminished facility of re-arranging its molecules. The 

 process of tempering steel or annealing glass, shows us that 

 internal re-distribution is aided by insensible vibrations, as 

 we have just seen it to be by sensible vibrations. When 

 some molten glass is dropped into water, and when its out- 

 side is thus, by sudden solidification, prevented from par- 

 taking in that contraction which the subsequent cooling of 

 the inside tends to produce; the units are left in such a 

 state of tension, that the mass flies into fragments if a small 

 portion of it be broken off. But if this mass be kept for a 

 day or two at a considerable heat, though a heat not suffi- 

 cient to alter its form or produce any sensible diminution of 

 hardness, this extreme brittleness disappears: the com- 

 ponent particles being thrown into greater agitation, the 

 tensile forces are enabled to re-arrange them into a state of 

 equilibrium. Much more conspicuously do we see 



the effect of the insensible motion called heat, where the 

 re-arrangement of parts taking place is that of visible segre- 



