THE LAW OF EVOLUTION 



ternal motion, into ice, the amount of secondary 

 rearrangement which occurs among its particles 

 is comparatively slight, but it is permanent so 

 long as the state of integration lasts. During 

 the continuance of the solid state there is not 

 enough mobility among the particles to admit 

 of further rearrangement to any conspicuous 

 extent. On the other hand, after steam has been 

 integrated into water, the retention of a consid- 

 erable amount of molecular motion allows in- 

 ternal rearrangement to go on so easily and 

 rapidly that no momentary phase of it has a 

 chance to become permanent ; and there can 

 thus be no such stable arrangement of parts as 

 we call structure. The phenomena of crystal- 

 lization supply us with kindred, but slightly 

 different examples. When a crystal is deposited 

 from a solution, there is a certain point up to 

 which the retention of motion keeps the crys- 

 tal's molecules from uniting ; but as soon as this 

 point is passed, the motion is suddenly lost, the* 

 crystal solidifies, and there is no further redis- 

 tribution of its particles. Conversely, when a 

 molten metal is allowed to cool until it assumes 

 a plastic semi-fluid state, its molecular motion 

 is lost so slowly that a perceptible rearrange- 

 ment of parts is possible : currents mav be set 

 up in it, gravity will cause it to spread out 

 wherever it is not confined at the side, and 

 pressure here and there will variously mould it. 

 vol. n 209 



