THE FORCE BEHIND NATURE. 619 



muscular impressions which are (so to speak) accidental, that (as it 

 seems to me) we find the real basis of our cognition of the " ponderos- 

 ity " 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 ex- 

 perience 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 con- 

 sciousness 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, \he 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 re- 

 sistance can still be made apparent by the like communication of its 

 own motion to solid bodies. Thus, in Mr. Crookes's wonderful radi- 

 ometer, a set of vanes, poised on a pivot within a globe of glass ex- 

 hausted to a millionth of its ordinary gaseous contents, is whirled 

 round by the movement excited in the molecules of that residual mil- 

 lionth, 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 mea- 

 sured 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 im2)ression of resistance we re- 

 ceive through what we may call our " force-sense." * 



* Herbert Spencer considers the congnition of resistance to be essentially derived 

 from the sense of muscular tension. I have already expressed my reason for now dis- 

 senting from this view, which I myself formerly held. 



