356 NATURE AND MAN, 



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 centre of the earth ; and a body 

 occupying the common centre of gravity of the earth and sun 

 would be equally drawn towards both, and would consequently 

 have no " weight." We must, therefore, seek a satisfactory 

 definition of matter elsewhere ; and we find the clue 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 ; 

 whilst 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 ; whilst 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 

 mUionth, 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 



