We have a group of units acted on by an incident force -- the attractionof the Earth. So long as these units are not agitated, this incident forcecannot change their relative positions; agitate them, and their loose arrangementpasses into a more compact arrangement. Again, so long as they are not agitated,the incident force cannot separate the heavier units from the lighter; agitatethem, and the heavier units begin to segregate. Mechanical disturbances ofmore minute kinds, acting on the parts of much denser masses, produce analogouseffects. A piece of iron which, when it leaves the workshop, is fibrous instructure, becomes crystalline if exposed to a perpetual jar. The polar forcesmutually exercised by the atoms, fail to change their disorderly arrangementinto an orderly arrangement while they are relatively quiescent; but theseforces succeed in rearranging them when they are kept in a state of intestinemotion. Similarly, the fact that a bar of steel suspended in the magneticmeridian and repeatedly struck, becomes magnetized, is ascribed to a re-arrangementof particles produced by the magnetic force of the Earth when vibrationsare propagated through them. Now imperfectly as these cases parallel thosewe are considering. they yet serve roughly to illustrate the effect whichadding to the quantity of motion an aggregate contains, has in facilitatingre-distribution of its components.
More fully illustrative are the instances in which, by artificially addingto or substracting from the molecular motion called its heat, we give anaggregate increased or diminished facility of re-arranging its molecules.
The process of tempering steel or annealing glass, shows us that internalre-distribution is aided by insensible vibrations, as we have just seen itto be by sensible vibrations. When some molten glass is dropped into water,and its outside is thus, by sudden solidification, prevented from participatingin that contraction which subsequent cooling of the inside tends to produce;the units are left in such a state of tension, that the mass flies into fragmentsif a small portion be broken off. But if this mass be kept for a day or twoat a considerable heat, though a heat not sufficient to alter its form, thisextreme brittleness disappears: the component particles being thrown intogreater agitation, the tensile forces are enabled to re-arrange them intoa state of equilibrium. Much more conspicuous is the effect of heat wherethe re-arrangement of parts taking place is that of visible segregation.
An instance is furnished by the subsidence of fine precipitates. These sinkdown very slowly from solutions which are cold; while warm solutions depositthem with comparative rapidity. That is to say, exalting the molecular oscillationthroughout the mass, allows the suspended particles to separate more readilyfrom the particles of fluid. The influence of heat on chemical changes isso familiar that examples are scarcely needed. Be the substances concernedgaseous, liquid, or solid, it equally holds that their chemical unions anddisunions are aided by rise of temperature. Affinities which do not sufficeto effect the re-arrangement of mixed units that are in a state of feebleagitation, suffice to effect it when the agitation is raised to a certainpoint. And so long as this molecular motion is not great enough to preventthose chemical cohesions which the affinities tend to produce, exalting itfacilitates chemical re-arrangement.
Let us pass to illustrations of a different class. Other things equal,the liquid form of matter implies a greater quantity of contained motionthan the solid form: the liquidity being itself a consequence of such greaterquantity. Hence, an aggregate made up partly of liquid matter and partlyof solid matter, contains more motion than one which, otherwise like it,is made up wholly of solid matter. It is inferable, then, that a liquid-solidaggregate, or, as we call it, a plastic aggregate, will admit of internalre-distribution with comparative facility; and the inference is verifiedby experience. While a magma of unlike substances ground up with water continuesthin there goes on a settlement of its heavier components -- a separationof them from the lighter. As the water evaporates this separation is impeded,and ceases when the magma becomes thick. But even when it has reached thesemi-solid state in which gravitation fails to cause further segregationof its mixed components, other forces may still produce segregation: witnessthe fact that when the pasty mixture of ground flints and kaolin, preparedfor making porcelain, is kept some time, it becomes gritty and unfit foruse -- the particles of silica separate themselves from the rest and uniteinto grains; or witness the fact known to every housewife, that in long-keptcurrant-jelly the sugar takes the shape of imbedded crystals.
No matter then under what form the motion contained by an aggregate exists-- be it visible agitation, or such vibrations as produce sound, be it molecularmotion absorbed from without, or the constitutional molecular motion of somecomponent liquid, the same truth holds. Incident forces work secondary re-distributionseasily when the contained motion is large in quantity; and work them withincreasing difficulty as the contained motion diminishes. §101. Yet another class of facts which fall within the same generalizationmust be named before proceeding. They are those presented by certain contrastsin chemical stability. Speaking generally, stable compounds contain but littlemolecular motion, and in proportion as the contained molecular motion isgreat the instability is great.
