ULTIMATE SCIENTIFIC IDEAS. 63 



this something, we find the perplexity is not got rid of but 

 only postponed. We are obliged to conclude that matter, 

 whether ponderable or imponderable, and whether aggre- 

 gated or in its hypothetical units, acts upon matter through 

 absolutely vacant space ; and yet this conclusion is positively 

 unthinkable. 



Yet another difficulty of conception, converse in nature 

 but equally insurmountable, must be added. If, on the 

 one hand, we cannot in thought see matter acting upon 

 matter through a vast interval of space which is absolutely 

 void; on the other hand, that the gravitation of one particle 

 of matter towards another, and towards all others, should 

 be absolutely the same whether the intervening space is 

 filled with matter or not, is incomprehensible. I lift from 

 the ground, and continue to hold, a pound weight. Now, 

 into the vacancy between it and the ground, is introduced 

 a mass of. matter of any kind whatever, in any state what- 

 ever hot or cold, liquid or solid, transparent or opaque, 

 light or dense ; and the gravitation of the weight is entirely 

 unaffected. . The whole Earth, as well as each individual 

 of the infinity of particles composing the Earth, acts on the 

 pound in absolutely the same way, whatever intervenes, or 

 if nothing intervenes. Through eight thousand miles of the 

 Earth's substance, each molecule at the antipodes affects 

 each molecule of the weight I hold, in utter indifference to 

 the fulness or emptiness of the space between them. So 

 that each portion of matter in its dealings with remote por- 

 tions, treats all intervening portions as though they did not 

 exist ; and yet, at the same time it recognizes their existence 

 with scrupulous exactness in its direct dealings with them. 

 We have to regard gravitation as a force to which every- 

 thing in the Universe is at once perfectly opaque in respect of 

 itself and perfectly transparent in respect of other things. 



While then it is impossible to form any idea of Force 

 in itself, it is equally impossible to comprehend its mode 

 of exercise. 



