THE BELFAST ADDRESS 201 



of new conceptions. The idea of the attraction of gravita- 

 tion was preceded by the observation of the attraction of 

 iron by a magnet, and of light bodies by rubbed amber. 

 The polarity of magnetism and electricity also appealed to 

 the senses. It thus became the substratum of the concep- 

 tion that atoms and molecules are endowed with attractive 

 and repellent poles, by the play of which definite forms 

 of crystalline architecture are produced. Thus molecular 

 force becomes structural. 1 It required no great boldness 

 of thought to extend its play into organic nature, and to 

 recognize in molecular force the agency by which both 

 plants and animals are built up. In this way, out of ex- 

 perience arise conceptions which are wholly ultra-experi- 

 ential. None of the atomists of antiquity had any notion 

 of this play of molecular polar force, but they had experi- 

 ence of gravity, as manifested by falling bodies. Abstract- 

 ing from this, they permitted their atoms to fall eternally 

 through empty space. Democritus assumed that the larger 

 atoms moved more rapidly than the smaller ones, which 

 they therefore could overtake, and with which they could 

 combine. Epicurus, holding that empty space could offer 

 no resistance to motion, ascribed to all the atoms the same 

 velocity; but he seems to have overlooked the conse- 

 quence that under such circumstances the atoms could 

 never combine. Lucretius cut the knot by quitting the 

 domain of physics altogether, and causing the atoms to 

 move together by a kind of volition. 



Was the instinct utterly at fault which caused Lucre- 

 tius thus to swerve from his own principles? Diminish- 

 ing gradually the number of progenitors, Mr. Darwin 



1 See Art. oil Matter and Force, or "Lectures on Light," No. III. 



