"DIRECTIVITY" A WITNESS OF MIND 75 



I have alluded to an important paper by the late 

 Mr. James Croll in the Philosophical Magazine for July, 

 1872 ; and as this is not readily accessible to every one, 

 I propose making extracts from it ; as they convey 

 precisely what I could have wished said on the present 

 occasion. The title of the paper I have already given, 

 but will here repeat — What Deten^iines Molecular Motion ? 

 — the Fundamental Problem of Nature (1872). 



" Physical inquiry in every direction is converging 

 towards Molecular Physics, is resolving itself into ques- 

 tions regarding the dynamical action of the ultimate 

 particles of matter. 



"To determine (i) the constitution of the ultimate 

 atoms and molecules of matter, what they really are, and 

 (2) the laws of their motions, are two great problems of 

 Molecular Physics. 



" With regard to all physical change or motion . . . 

 there are at the very outset two fundamental questions : 

 (i) what produces the change — causes motion? (2) what 

 determines or directs it ? 



" All agree that what produces change or causes 

 motion is Force. 



" What determines or directs the motion, is not so 

 easily answered. 



'• In the production of all physical phenomena we 

 have two distinct elements, viz., force, and the way or 

 manner in which force acts ; in other words — Force and 

 the Laws of Force ; or as Mr. Lewes expresses them — 

 ' the paths along which they travel to their particular 

 results '. What determines the paths along which force 

 acts? It is not what gives existence to the motion, but 

 what detennines its direction, is the great question." 



Mr. Croll thus arrives at the first most important dis- 

 tinction ; — 



