304 SIMPLE AND COMPOUND EVOLUTION. 



matter which constitutes simple chemical decomposition, is 

 easy in proportion as the quantity of contained motion is 

 great. The like holds with double decomposi- 



tions. Two compounds, A B and C D, mingled together 

 and kept at a low temperature, may severally remain un- 

 changed the cross affinities between their components 

 may fail to cause re-distribution. Increase the heat of the 

 mixture, or add to the molecular motion throughout it, and 

 re-distribution takes place; ending in the formation of the 

 compounds, A C and B D. 



Another chemical truth having a like implication, is 

 that chemical elements which, as they ordinarily exist, 

 contain much motion, have combinations less stable than 

 those of which the elements, as they ordinarily exist, contain 

 little motion. The gaseous form of matter implies a rela- 

 tively large amount of molecular motion; while the solid 

 form implies a relatively small amount of molecular motion. 

 What are the characters of their respective compounds? 

 The compounds which the permanent gases form with one 

 another, cannot resist high temperatures: most of them 

 are easily decomposed by heat; and at a red heat, even 

 the stronger ones yield up their components. On the 

 other hand, the chemical unions between elements that 

 are solid except at very high temperatures, are ex- 

 tremely stable. In many, if not indeed in most, cases, 

 such combined elements are not separable by any heat we 

 can produce. 



There is, again, the relation, which appears to have a 

 kindred meaning, between instability and amount of com- 

 position. " In general, the molecular heat of a compound 

 increases with the degree of complexity." With increase of 

 complexity there also goes increased facility of decomposi- 

 tion. Whence it follows that molecules which contain 

 much motion in virtue of their complexity, are those of 

 which the components are most readily re-distributed. 

 This holds not only of the complexity resulting from the 



