PHILOSOPHICAL SOCIETY OF WASHINGTON. 127 



long waged (and so long practically decided) between realism and 

 nominalism in the field of mind, bids fair to be revived (though 

 under quite other auspices) in the field of matter. These two modes 

 of thought may be conveniently designated the dynamic and the 

 kinematic theories of physics. In the terminology of the Philo- 

 sophie Positive, the dynamic theory still lingers in the shaded 

 vale of " metaphysics," while the kinematic theory has reached the 

 sunny hill of" positivism."* An attempt to examine and compare 

 these divergent lines of interpretation may be a not unprofitable ex- 

 ercise. 



The Cohesion of Matter. — Among the earliest of our experiences is 

 the perception that the bodies around us possess in varying degrees a 

 quality of "hardness;" and the child who gathers a rounded pebble 

 on the beach, (if perchance inspired by its inquisitive instinct to 

 see what the interior looks like,) discovers that to break the pebble 

 requires the heavy and repeated strokes of a stone much larger than 

 itself. Whence this remarkable tenacity of coherence? Whence 

 the striking physical difference between the pebble and an equiva- 

 lent mass of very fine sand ? 



From a large variety of facts observed in the actions of solution, 

 of fusion, of evaporation, of the very existence of a kinetic tempera- 

 ture in bodies, in the phenomena of crystallization, of isomorphism, 

 of definite and unvarying numerical mass-ratios in chemical com- 

 binations, of polymerism or serial groupings in multiple proportion, 

 of isomerism, of allotropy, and of other more recondite habitudes 

 of matter, the general conviction has been reached (by what has 

 been called " a consilience of inductions ") that all substance is a 

 collection of constituent molecules of probably uniform magnitudes 

 held together by some powerful agency. A few it is true have 

 asserted their superiority to such popular weakness as the admission 

 of the atomic theory ; but as their vague suggestion of some con- 

 tinuous or colloidal form of substance has not even pretended to 

 interpret any of the classes of phenomena just alluded to, such dis- 



*AuGtrsTE Comte, in his Positive Philosophy, maintains that "Forces 

 are only motions produced or tending to be produced. - - - We hear 

 too much still of the old metaphysical language about forces and the like ; 

 and it would be wise to suit our terms to our positive philosophy." (Har- 

 riet Martineau's Translation. London, 1853: book I, chap. 4.) Even in- 

 ertia is treated as a metaphysical fiction. 



