Page:Popular Science Monthly Volume 16.djvu/649

Rh muscular impressions which are (so to speak) accidental, that (as it seems to me) we find the real basis of our cognition of the "ponderosity" of matter.

But "ponderosity" can not be considered an essential property of matter, being merely the "accident" of the earth's attraction for bodies lying within its range. This attraction varies with the distance of a body from the center of the earth; and a body occupying the common center of gravity of the earth and sun would be equally drawn toward both, and would consequently have no "weight." We must, therefore, seek a satisfactory definition of matter elsewhere; and we find the clew to it in the consideration that the sense of effort we experience in antagonizing the downward pressure of a body is but a particular case of our more general cognition of resistance. When we project our hand against a hard and fixed solid body, our consciousness of its resistance to our pressure is exactly that which we experience when we try to raise a weight that we have not strength to lift; while, if that solid be either yielding in its parts or movable as a whole, we measure its resistance, as in lifting a weight, by our sense of the effort necessary to overcome it. When we move our hand through a liquid, we are conscious of a resistance to its motion, which is greater or less according to the "viscosity" of the liquid. And, when we move our open hand through air at rest, we are still conscious of a resistance, our sense of it being augmented by an extension of the surface moved, as in the act of fanning; while, if the air is in motion, we feel its pressure on the sail of a boat by the "pull" of the sheet we hold in our hand, or on the sails of a windmill by the rotation it imparts, the force of which we can estimate by the effort we must put forth to resist it. Attenuate any kind of air or gas as we may, its resistance can still be made apparent by the like communication of its own motion to solid bodies. Thus, in Mr. Crookes's wonderful radiometer, a set of vanes, poised on a pivot within a globe of glass exhausted to a millionth of its ordinary gaseous contents, is whirled round by the movement excited in the molecules of that residual millionth, either by the heat of the radiant beam falling on the surface of the globe or by the passage of an electric current across its interior; and the mechanical force required to impart that motion can be measured with precision, by bringing it into comparison with some other force (as that of gravity) of which we can take immediate cognizance. And thus, as Herbert Spencer remarks, by the decomposition of our knowledge of any form of matter into simpler and simpler components, we must come at last to the simplest, to the ultimate material, to the substratum; and this we find in the impression of resistance we receive through what we may call our "force-sense."