The most common and marked illustration of this, is that chemical stabilitydecreases as temperature increases. Compounds of which the elements are stronglyunited and compounds of which the elements are feebly united, are alike inthis, that heating them or adding to the quantities of their contained molecularmotion, diminishes the strengths of the unions of their elements; and bycontinually augmenting the contained molecular motion, a point is in eachcase reached at which the union is destroyed. That is to say the re-distributionof matter which constitutes simple chemical decomposition, is easy in proportionas the quantity of contained motion is great. The like holds with doubledecompositions. Two compounds, A B and C D, mingled together and kept ata low temperature, may severally remain unchanged: the cross-affinities betweentheir components may fail to cause re-distribution. Raise the heat of themixture, and re-distribution takes place; ending in the formation of thecompounds A C and B D.
Another truth having a like implication, is that chemical elements which,as they ordinarily exist, contain much motion, have combinations less stablethan those of which the elements, as they ordinarily exist, contain littlemotion. The gaseous form of matter implies a relatively large amount of molecularmotion, while the solid form implies a relatively small amount. What arethe traits of their respective compounds? Those which the permanent gasesform with one another, cannot resist high temperatures: most of them areeasily decomposed by heat; and at a red heat, even the stronger ones yieldup their components. On the other hand, the chemical unions between elementsthat are solid except at high temperatures, are very stable. In many, ifnot indeed in most, cases, such unions are not destroyed by any heat we canproduce.
There is, again, the relation, which appears to have a kindred meaning,between instability and amount of composition. "In general, the molecularheat of a compound increases with the degree of complexity." With increaseof complexity there also goes increased facility of decomposition. Whenceit follows that molecules with contain much motion in virtue of their complexity,are those of which the components are most easily re-distributed. This holdsnot only of the complexity arising from the union of several unlike elements;it holds also of the complexity arising from the union of the same elementsin higher multiples. Matter has two solid states, distinguished as crystalloidand colloid; of which the first is due to union of the individual atoms ormolecules, and the second to the union of groups of such individual atomsor molecules; and of which the first is stable and the second unstable.
But the most conclusive illustration is furnished by the combinationsinto which nitrogen enters. These are specially unstable and contain speciallygreat quantities of motion. A peculiarity of nitrogen is that, instead ofgiving out heat when it combines with other elements, it absorbs heat. Besidescarrying with it into the liquid or solid compound it forms, the motion whichpreviously constituted it a gas, it takes up additional motion; and wherethe other element with which it unites is gaseous, the molecular motion properto this, also, is locked up in the compound. Now these nitrogen-compoundsare unusually prone to decomposition; and the decompositions of many of themtake place with extreme violence. All our explosive substances are nitrogenous-- the most destructive of them all, chloride of nitrogen, being one whichcontains the immense quantity of motion proper to its component gases, plusa further quantity of motion.
Evidently these general chemical truths are parts of the more generalphysical truth we are tracing out. We see in them that what holds of sensiblemasses, holds also of the insensible masses we call molecules. Like the aggregatesformed of them, these ultimate aggregates become more or less integratedaccording as they lose or gain motion; and like them also, according as theycontain much or little motion, they are more or less liable to undergo secondaryre-distributions along with the primary re-distribution. §102. And now having brought this general principle clearly intoview, let us observe how, in conformity with it, Evolution becomes, accordingto the conditions, either simple or compound.
If a little sal-ammoniac or other volatile solid be heated, it is disintegratedby the absorbed molecular motion and rises in gas. If this gas comes in contactwith a cold surface, and loses it excess of molecular motion, integrationtakes place -- the substance assumes the form of crystals. This is a caseof simple evolution. The concentration of matter and dissipation of motiondo not here proceed gradually -- do not pass through stages; but the molecularmotion which caused assumption of the gaseous state being dissipated, thematter passes suddenly to a solid state. The result is that along with thisprimary re-distribution there go on no appreciable secondary re-distributions.
Substantially the same thing holds with crystals deposited from solutions.
Loss of that molecular motion which, down to a certain point, keeps the moleculesfrom uniting, and sudden solidification when the loss goes below that point,occur here as before; and here as before, the absence of a period duringwhich the molecules are partially free and gradually losing their freedom,is accompanied by the absence of minor re-arrangements.
