MAN THE INTERPRETER OF NATURE. 207 



were merely higher generalizafiofis in which Kepler's are in- 

 cluded ; to me they seem to possess an altogether different 

 character. For starting with the conception of two forces, one 

 of them tending to produce continuous uniform motion in a 

 straight line, the other tending to produce a uniformly accele- 

 rated motion towards a fixed point, Newton's wonderful mastery 

 of geometrical reasoning enabled him to show that, if these 

 ^//rtw/Va/ assumptions be granted, Kepler's //2<?/itfw^««/ ." laws," 

 being necessary consequences of them, must be universally true. 

 And while that demonstration would have been alone sufficient 

 to give him an imperishable renown, it was his still greater glory 

 to divine that the fall of the moon towards the earth — that is, the 

 deflection of her path from a tangential line to an ellipse — is a 

 phenomenon of the same order as the fall, of a stone to the ground ; 

 and thus to show the applicability to the entire universe, of those 

 simple dynamical conceptions which constitute the basis of the 

 geometry of the Principia. 



Thus, then, whilst no "law" which is simply d, generalization 

 of phenotnena can be considered as having any coercive action, we 

 may assign that value to laws which express the universal con- 

 ditions of the action of a force whose existence we learn from the 

 testimony of our own consciousness. The assurance we feel that 

 the attraction of gravitation tnust act under all circumstances ac- 

 cording to those simple laws which arise immediately out of our 

 dynamical conception of it, is of a very different order from that 

 which we have in regard (for example) to the laws of chemical 

 attraction, which are as yet only generalizations of phenomena. 

 And yet even in that strong assurance, we are required by our 

 examination of the basis on which it rests, to admit a reserve of 

 the possibility of something different ; a reserve which we may 

 well believe that Newton himself must have entertained. 



A most valuable lesson as to the allowance we ought always 

 to make for the unknown " possibilities of Nature," is taught us 

 by an exceptional phenomenon so familiar that it does not attract 

 the notice it has a right to claim. Next to the law of the uni- 

 versal attraction of masses of matter, there is none that seems to 

 have a wider range than that of the expansion of bodies by heat and 

 their contraction by cold. Excluding water and one or two other 



