SIMPLE AND COMPOUND EVOLUTION. 305 



union of several unlike elements; but it holds also of the 

 complexity resulting from the union of the same elements 

 in higher multiples. Matter has two solid- states, distin- 

 guished as crystalloid and colloid; of which the first is due 

 to union of the individual atoms or molecules, and the sec- 

 ond to the union of groups of such individual atoms or mole- 

 cules; and of which the first is stable and the second 

 unstable. 



But the most striking and conclusive illustration is fur- 

 nished by the combinations into which nitrogen enters. 

 These have the two characters of being specially unstable 

 and of containing specially great quantities of motion. A 

 recently-ascertained peculiarity of nitrogen, is, that instead 

 of giving out heat when it combines with other elements, it 

 absorbs heat. That is to say, besides carrying with it into 

 the liquid or solid compound it forms, the motion which 

 previously constituted it a gas, it takes up additional mo- 

 tion; and where the other element with which it unites is 

 gaseous, the molecular motion proper to this, also, is locked 

 up in the compound. Now these nitrogen-compounds 

 are unusually prone to decompositon ; and the decom- 

 positions of many of them take place with extreme vio- 

 lence. All our explosive substances are nitrogenous the 

 most terribly destructive of them all, chloride of nitrogen, 

 being one which contains the immense quantity of motion 

 proper to its component gases, plus a certain further quan- 

 tity of motion. 



Clearly these general chemical truths, are parts of the 

 more general physical truth we are tracing out. We see 

 in them that what holds of sensible aggregates, holds also 

 of the insensible aggregates we call molecules. Like the 

 aggregates formed of them, these ultimate aggregates be- 

 come more or less integrated according as they lose or gain 

 motion ; and like them also, according as they contain much 

 or little motion, they are liable to undergo secondary re-dis- 

 tributions of parts along with the primary re-distribution. 



