Mark, conversely, what happens when the concentration is slow. A gaseousmass losing its heat and undergoing a consequent decrease of bulk, undergoesalso many simultaneous changes. The great quantity of molecular motion containedin it, giving great molecular freedom, renders every part sensitive to everyincident force; and, as a result, its parts have various motions besidesthat implied by their progressing integration. Indeed these secondary motionswhich we know as currents, are so conspicuous as quite to subordinate theprimary motion. Suppose that, presently, the loss of molecular motion hasreached the point at which the gaseous state can no longer be maintained,and condensation follows. Under their more closely-united form, the partsof the aggregate display, to a considerable degree, the same phenomena asbefore. The molecular motion and accompanying molecular mobility impliedby the liquid state, permit easy re-arrangement; and hence there go on rapidand marked changes in the relative positions of parts -- local streams producedby slight disturbing forces. But now, if instead of a mobile liquid we takea sluggish one such as molten pitch or asphalte, what happens as the molecularmotion decreases? The liquid thickens -- its parts cease to be movable amongone another with ease; and the transpositions caused by feeble incident forcesbecome slow. Little by little the currents are stopped, but the mass stillcontinues modifiable by stronger incident forces. Gravitation makes it bendor spread out when not supported on all sides, and it may easily be indented.
As it cools, it continues to grow stiffer; and eventually, further loss ofheat renders it quite hard: its parts are no longer appreciably re-arrangeableby any save violent actions.
Among inorganic aggregates, then, secondary redistributions accompanythe primary re-distributions where this is gradual. During the gaseous andliquid stages, the secondary re-distributions, rapid and extensive as theyare, leave no traces: the molecular mobility being such as to negative thefixed arrangement of parts we call structure. On approaching solidity wearrive at a plastic condition in which re-distributions can still be made,though much less easily; and in which they have a certain persistence --a persistence which can, however, become decided only where solidificationstops further re-distribution.
Here we see what are the conditions under which Evolution becomes compound,while we see how the compounding of it can be carried far only in cases morespecial than any hitherto contemplated; since, on the one hand, extensivesecondary re-distributions are possible only where there is a great quantityof contained motion, and, on the other hand, such re-distributions can havepermanence only where the contained motion has become small: opposing conditionswhich seem to negative any large amount of permanent secondary re-distribution. §103. And now we are in a position to see how these apparently contradictoryconditions are reconciled. We shall appreciate the peculiarity of the aggregatesclassed as organic, in which Evolution becomes so high; and shall see thatthis peculiarity consists in the combination of matter into forms embodyingenormous amounts of motion at the same time that they have a great degreeof concentration.
For notwithstanding its semi-solid consistence, organic matter containsmolecular motion locked up in each of the ways above contemplated separately.
Let us note its distinctive traits. Three out of its four chief componentsare gaseous; and in their uncombined states these gases united in it haveso much molecular motion that they are condensible only with extreme difficulty.
Hence it is to be inferred that the proteid-molecule concentrates an immenseamount of motion in a small space. And since many equivalents of these gaseouselements unite in one of these proteid-molecules, there must be in it a largequantity of relative motion in addition to that which the ultimate atomspossess. Moreover, organic matter has the peculiarity that its moleculesare aggregated into the colloid and not into the crystalloid arrangement;forming, as is supposed, clusters of clusters which have movements in relationto one another. Here, then, is a further mode in which molecular motion isincluded. Yet again, these compounds of which the essential parts of organismsare built, are nitrogenous; and we have lately seen it to be a peculiarityof nitrogenous compounds that, instead of giving out heat during their formation,they absorb heat. To all the molecular motion possessed by gaseous nitrogen,is added more motion; and the whole is concentrated in semi-solid protein.
Organic aggregates are very generally distinguished, too, by having muchinsensible motion in a free state -- the motion we call heat. Though in manycases the quantity of this contained insensible motion is inconsiderable,in other cases a temperature much above that of the environment is constantlymaintained. Once more, there is the vast quantity of motion embodied in thewater that permeates organic matter. It is this which, giving to the waterits high molecular mobility, gives mobility to the organic molecules partiallysuspended in it; and preserves that plastic state which so greatly facilitatesre-distribution.
