The nature and causes of Dissolution are thus clearly displayed by theaggregates which so clearly display the nature and causes of Evolution. Oneof these aggregates being made of that peculiar matter to which a large quantityof constitutional motion gives great plasticity, and the ability to evolveinto a highly complex form, (§103); it results that after evolutionhas ceased, a small amount of molecular motion added to that already containedin its peculiar matter, suffices to cause dissolution. Though at death thereis reached an equilibrium among the sensible masses, or organs, which makeup the body; yet, as the insensible units or molecules of which these organsconsist are chemically unstable, small incident forces suffice to overthrowthem, and hence disintegration proceeds rapidly. §180. Most inorganic aggregates, having arrived at dense forms inwhich comparatively little motion is retained, remain long without markedchanges. Each has lost so much motion in passing from the unintegrated tothe integrated state, that much motion must be given to it to cause resumptionof the unintegrated state; and an immense time may elapse before there occurin the environment, changes great enough to communicate to it the requisitequantity of motion. We will look first at those few inorganic aggregateswhich retain much motion, and therefore readily undergo dissolution.
Among these are the liquids and volatile solids which dissipate underordinary conditions -- water that evaporates, camphor that wastes away bythe dispersion of its molecules. In all such cases motion is absorbed; andalways the dissolution is rapid in proportion as the quantity of heat ormotion which the mass receives from its environment is great. Next come thecases in which the molecules of a highly integrated or solid aggregate, aredispersed among the molecules of a less integrated or liquid aggregate; asin aqueous solutions. One evidence that this disintegration of matter hasfor its concomitant the absorption of motion, is that soluble substancesdissolve the more quickly the hotter the water: supposing always that noelective affinity comes into play. Another and still more conclusive evidenceis, that when crystals of a given temperature are placed in water of thesame temperature, the process of solution is accompanied by a fall of temperature-- often a very great one. Omitting instances in which some chemical actiontakes place between the salt and the water, it is a uniform law that themotion which disperses the molecules of the salt through the water, is atthe expense of the molecular motion possessed by the water. An allied andstill better example is furnished by cases in which the dissolution of twosolids results from mixing them, as happens with snow and salt. Here dissolutionnecessitates so great an absorption of molecular motion as greatly to lowerthe temperature of the liquid produced.
Masses of sediment accumulated into strata, afterwards compressed by manythousands of feet of superincumbent strata, and reduced in course of timeto a solid state, may remain for untold millions of years unchanged; butin subsequent millions of years they are inevitably exposed to disintegratingactions. Raised along with other such masses into a continent, denuded andexposed to rain, frost, and the grinding actions of glaciers, they have theirparticles gradually separated, carried away, and widely dispersed. Or when,as otherwise happens, the encroaching sea arrives, the undermined cliffsformed of them fall from time to time; the waves, rolling about the smallpieces, and in storms knocking together the larger blocks, reduce them toboulders and pebbles, and at last to sand and mud. Even if portions of thedisintegrated strata accumulate into shingle banks which afterwards becomesolidified, the process of dissolution, arrested though it may be for someenormous geologic period, is finally resumed. As many a shore shows us, theconglomerate itself is sooner or later subject to the like processes; andits cemented masses of heterogeneous components are broken up and worn awayby impact and attrition -- that is, by communicated mechanical motion.
When not thus effected, the disintegration is effected by communicatedmolecular motion. A consolidated stratum in some area of subsidence, broughtdown nearer and nearer to the regions occupied by molten matter, comes eventuallyto have its particles brought to a plastic state by heat, or finally melteddown into liquid. Whatever may be its subsequent transformations, the transformationthen exhibited by it is an absorption of motion and disintegration of matter.
