306 SIMPLE AND COMPOUND EVOLUTION. 



§ 102. And now having got this general principle clear- 

 ly into view, let lis go on to observe how, in conformity with 

 it, Evolution becomes, according to the conditions, either 

 simple or compound. 



If a little sal-ammoniac, or other volatile solid, be heat- 

 ed, it is disintegrated by the absorbed molecular motion, and 

 rises in gas. When the gas so produced, coming in con- 

 tact with a cold surface, loses its excess of molecular mo- 

 tion, integration takes place — the substance assumes the 

 form of crystals. This is a case of simple evolution. The 

 process of concentration of matter and dissipation of motion 

 does not here proceed in a gradual manner — does not pass 

 through stages occupying considerable periods; but the 

 molecular motion which reduced it to the gaseous state 

 being dissipated, the matter passes suddenly to a completely 

 solid state. The result is that along with this primary re- 

 distribution there go on no appreciable secondary re-dis- 

 tributions. Substantially the same thing holds with crys- 

 tals deposited from solutions. Loss of that molecular mo- 

 tion which, down to a certain point, keeps the molecules 

 from uniting, and sudden solidification when the loss goes 

 below that point, occur here as before; and here as before, 

 the absence of a period during which the molecules are 

 partially free and gradually losing their freedom, is accom- 

 panied by the absence of supplementary re-arrangements. 



Mark, conversely, what happens when the concentration 

 is slow. A gaseous mass losing its heat, and undergoing a 

 consequent decrease of bulk, is not subject only to this 

 change which brings its parts nearer to their common cen- 

 tre, but also to many simultaneous changes. The great 

 quantity of molecular motion contained in it, giving, as we 

 have seen that it must, great molecular mobility, renders 

 every part sensitive to every incident force ; and, as a result, 

 its parts have various motions besides that implied by their 

 progressing integration. Indeed these secondary motions 

 which we know as currents, are so important and conspicu- 



