MAN AS THE INTERPRETER OF NATURE. 699 



a uniformly accelerated motion toward a fixed point, Newton's won- 

 derful mastery of geometrical reasoning enabled him to show that, if 

 these dynamical assumptions be granted, Kepler's phenomenal " laws," 

 being a necessary consequence of them, must b'e 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 toward 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 con- 

 ceptions which constitute the basis of the geometry of the " Principia." 



Thus, then, while no "law" which is simply a generalization of 

 phenomena can be considered as having any coercive action, we may 

 assign that value to laws which express the universal conditions 

 of the action of a force, the existence of which we learn from the tes- 

 timony of our own consciousness. The assurance we feel, that the 

 attraction of gravitation must act under all circumstances according 

 to its one simple law, 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 some- 

 thing different a reserve which we may well believe that Newton 

 himself must have entertained. 



A most valuable lesson as to the allowance we ought 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 

 riobt to claim. Next to the law of the universal attraction of masses 

 of matter, there is none that has a wider range than that of the ex- 

 pansion of bodies by heat. Excluding water and one or two other 

 substances, the fact of such expansion might be said to be invariable, 

 and, as regards bodies whose gaseous condition is known, the law of 

 expansion can be stated in a form no less simple and definite than the 

 law of gravitation. Supposing those exceptions then to be unknown, 

 the law would be universal in its range. But it comes to be discov- 

 ered that water, while conforming to it in its expansion from 39 

 upward to its boiling-point, as also, when it passes into steam, to the 

 special law of expansion of vapors, is exceptional in its expansion also 

 from 39-J downward to its freezing-point ; and of this failure in the 

 universality of the law no rationale can be given. Still more strange 

 is it that by dissolving a little salt in water we should remove this 

 exceptional peculiarity, for sea-water continues to contract from 39 

 downward to its freezing-point 12 or 14 lower, just as it does with 

 reduction of temperature at higher ranges. 



Thus, from our study of the mode in which we arrive at those con- 

 ceptions of the orderly sequence observable in the phenomena of Na- 



