Page:Popular Science Monthly Volume 48.djvu/548

506 or the molecule be made up of two or more atoms, as in the case of most of the elements, it is undisputed, not only that the uniting parts are likes to each other, but that the molecules which they form are also likes to each other. That which, moreover, is especially obvious in the case of the elements, where the molecule is always made up of the same kind of atoms, is none the less true of compounds; for, however superficially unlike the atoms or molecules may be that enter into combination to form such compounds, and whether the matter formed be what we know as inorganic or what we know as organic—whether it be extremely simple, as in the case of water, or highly complex, as in the case of albumin—the resultant system is in every instance a system whose unit parts all possess the same general character.

The power of material systems to dissociate unlikes is best seen when the dissociating stress is exerted by matter in the liquid or gaseous state—in a form, that is to say, in which the power to dissociate is at its maximum and the resistance to dissociation at a minimum. If we thrust a ball of wood into a volume of water, the entering mass is immediately expelled to the surface, and it is thus expelled for the reason that it possesses less density than water, and is therefore an unlike: the act of dissociation, moreover, is manifestly an act of the water system; for, if we remove the water, our wooden ball will descend freely through the space previously occupied by the fluid. So a balloon ascends through air because, being less dense than air, it appears in the air system as an unlike, and is by that system expelled to those regions of the atmosphere which resemble it in density: in this case, as well, the act of dissociation is an act of the air system, for, if the air could be removed, the balloon would not rise at all.

Unlikes, sufficiently free to move, often repel one another when forced into association. Zinc and lead, or zinc and bismuth, may be melted together, but they separate more or less completely during the process. Chloroform and water may be mechanically mixed, but when they are no longer shaken the two fluids part into distinct layers. If, upon oil placed in a receptacle, water be poured, the water will displace the oil; while, if mercury be added, it will displace both oil and water; the final result of the experiment—three layers occupying positions in the order of their density—being reached by acts involving both association and dissociation association of the like parts of each layer, and dissociation of the unlike layers.

How likes are associated and unlikes dissociated may also be shown in most of the phenomena of cohesion, and especially in those wherein particles of matter assume the spherical or globular form. Where the medium is not apparent, as in the case of the sun, moon, and planets, the association of likes is alone displayed;