302 SIMPLE AND COMPOUND EVOLUTION. 



gation. An instance is furnished by the subsidence of fine 

 precipitates. These sink down very slowly from solutions 

 that are cold; while warm solutions deposit them with com- 

 parative rapidity. That is to say, exalting the molecular 

 oscillation throughout the mass, allows the suspended 

 particles to separate more readily from the particles of 

 fluid. The influence of heat on chemical changes 



is so familiar, that examples are scarcely needed. Be the 

 substances concerned gaseous, liquid, or solid, it equally 

 holds that their chemical unions and disunions are aided by 

 rise of temperature. Affinities which do not suffice to effect 

 the re-arrangement of mixed units that are in a state of 

 feeble agitation, suffice to effect it when the agitation is 

 raised to a certain point. And so long as this molecular mo- 

 tion is not great enough to prevent those chemical cohesions 

 which the affinities tend to produce, increase of it gives in- 

 creased facility of chemical re-arrangement. 



Another class of facts may be adduced which, though 

 not apparently, are really illustrative of the same general 

 truth. Other things equal, the liquid form of matter im- 

 plies a greater quantity of contained motion than the solid 

 form — the liquidity is itself a consequence of such greater 

 quantity. Hence, an aggregate made up partly of liquid 

 matter and partly of solid matter, contains a greater quan- 

 tity of motion than one which, otherwise like it, is made up 

 wholly of solid matter. It is inferable, then, that a liquid- 

 solid aggregate, or, as we commonly call it, a plastic aggre- 

 gate, will admit of internal redistribution with comparative 

 facility; and the inference is verified by experience. A 

 magma of unlike substances ground up with water, while it 

 continues thin allows a settlement of its heavier components 

 — a separation of them from the lighter. As the water 

 evaporates this separation is impeded, and ceases when the 

 magma becomes very thick. But even when it has reached 

 the semi-solid state in which gravitation fails to cause 

 further segregation of its mixed components, other forces 



