SIMPLE AND COMPOUND EVOLUTION. 303 



may still continue to produce segregation: witness the fact 

 to which attention was first drawn by Mr. Babbage, that 

 when the pasty mixture of ground flints and kaolin, pre- 

 pared for the manufacture of porcelain, is kept some time, 

 it becomes gritty and unfit for use, in consequence of the 

 particles of silica separating themselves from the rest, and 

 uniting together in grains; or witness the fact known to 

 every housewife, that in long-kept currant-jelly the sugar 

 takes the shape of imbedded crystals. 



No matter then under what form the motion contained 

 by an aggregate exists be it mere mechanical agitation, or 

 the mechanical vibrations such as produce sound, be it 

 molecular motion absorbed from without, or the constitu- 

 tional molecular motion of some component liquid, the 

 same truth holds throughout. Incident forces work second- 

 ary re-distributions easily when the contained motion is 

 large in quantity; and w r ork them with increasing diffi- 

 culty as the contained motion diminishes. 



101. Yet another class of facts that fall within the 

 same generalization, little as they seem related to it, must 

 be indicated before proceeding. They are those presented 

 by certain contrasts in chemical stability. Speaking gener- 

 ally, stable compounds contain comparatively little molecu- 

 lar motion; and in proportion as the contained molecular 

 motion is great the instability is great. 



The common and marked illustration of this to be first 

 named, is that chemical stability decreases as temperature 

 increases. Compounds of which the elements are strongly 

 united and compounds of which the elements are feebly 

 united, are alike in this, that raising their heats or increasing 

 the quantities of their contained molecular motion, dimin- 

 ishes the strengths of the unions of their elements; and by 

 continually adding to the quantity of contained molecular 

 motion, a point is in each case reached at which the chemical 

 union is destroyed. That is to say, the re-distribution of 



