AND ITS SELF-CONSEKVATION. 187 



C. ELECTEICITY AND MAGNETISM. 



In crystallization, the molecules of certain kinds of 

 matter are seen, under favoring conditions of relation 

 between attraction and repulsion, to undergo arrange- 

 ment, so as to constitute definite geometrical solids. 

 These solid forms possess, too, in most cases, a relatively 

 stable equilibrium. 



Another class of strains, however, is induced by the 

 sudden cooling of a mass of molten matter for example, 

 glass or iron to the solid state. The Prince Eupert 

 drop is a well-known and striking example of extremely 

 unstable equilibrium, produced by the sudden chilling 

 of a small mass of molten glass, so that the surface 

 becomes solid, while the interior is still in a more or 

 less liquid or viscous state. When the whole has become 

 solid, the interior and exterior strains are imperfectly 

 balanced, the density of the central part being less than 

 that of the outer, since an outer rigid shell was formed 

 over an inner nucleus that was still molten, and which, 

 in its solidification, has tended to shrink away from the 

 outer rigid shell. Hence, the slightest break in the 

 exterior portion at once makes way for the complete 

 disruption of the entire mass. 



It is well known, also, that cast car-wheels and edge- 

 tools, when chilled rapidly, possess the same peculiar 

 characteristic of "brittleness." And this, it can scarcely 

 be doubted, is due in reality to the unequally distributed 

 strain characterizing the Prince Rupert's drop, rather 

 than to the additional "hardness" produced by the 

 sudden cooling; for, in truth, the "hardness" must, by 

 the very conditions of rapid cooling, be confined mainly 

 to the superficial portions of the chilled mass. 



