REPULSION, like caloric, luminous corpuscles, and other crude hypotheses of medieval times appears to be doomed to speedy extinction. The apparent repulsion between the particles of a gas, in virtue of which it exerts pressure on the containing vessel, is now known to be due to motion (see HEAT). A wet cork and an oiled one, floating on water, repel each other—a phenomenon fully accounted for by capillary 4? attract as is that of the apparent repulsion of mercury by glass, which is shown to be due to the fact, that mercury attracts itself more than it attracts glass. No one now believes that a balloon rises while a stone falls because the former is repelled, aidthe latter attracted, by the earth. The last is a very good example, because it clearly shows how apparent repulsion may be the result of attraction. The earth attracts the balloon less than it attracts an equal bulk of the medium (air) in which it floats; and, conse quently, the pressure of the air on the balloon is more than sufficient to support its weight. The moon raises tidos not only ou the side of the earth nearest her, but also on that furthest from her. No one imagines that she attracts the nearer water, and repels the further. We know that she attracts the nearer water more, and the further less,
than she attracts the earth; and that the apparent repulsion is thus merely a difference of attractions.
It is not quite so clear how we are to account generally for repulsion in electricity (q.v.), magnetism (q.v.), and electromagnetism (q.v.), though many of these phenomena are known (especially by the beautiful experimental researches of Faraday) to bear explanations precisely analogous to that of the balloon above alluded to. There are also very curious problems, apparently involving repulsion, connected with the behav ior of the tails of comets. But it is reasonable to suppose that, in all probability, we shall soon be able to account for all these phenomena by simple differences of attraction on the body influenced and the medium which surrounds it. Our real difficulty will thus be reduced to the explanation of attraction itself, 'which promises to he a problem of a far higher order of complexity. For an account of some of the modern specula tions on this subject, see FORCE.