SCIENCE IN THE LAST HALF CENTURY. 79 



contemporary, of great acquirements and remarkable intellectual pow- 

 ers, to read the signs of the times, is a lesson and a warning worthy of 

 being deeply pondered by any one who attempts to prognosticate the 

 course of scieutitic progress. 



I have poiilted out that the growth of clear and definite views re- 

 specting the constitution of matter lias led to the conclusion that so far 

 as natural agencies are concerned, it is ingenerable and indestructible. 

 In so far as matter may be conceived to exist in a purely ])assi^ e state, 

 it is. imaginably, older than motion. But as it must be assumed to be 

 susceptible of motion, a particle of bare matter at rest must be endowed 

 with the potentiality of motion. Such a i)article however, by the sup- 

 l)ositiou can have no energy, for there is uo cause why it should move. 

 Suppose now that it receives an imjiulse, it will begin to move with a 

 velocity inversely proportional to its mass on the one hand, and directly 

 proportional to the strength of the impulse on the other, and will pos- 

 sess Jdnetic energy, in virtue of which it will not only continue to move 

 forever if unimpeded, but if it impinges on another such particle it will 

 impart more or less of its motion to the latter. Let it be conceived that 

 the particle acquires a tendency to move, and that nevertheless it does 

 not move. It is then in a condition totally difi'erent from that in which 

 it was at first. A cause competent to ])roduce motion is operating upon 

 it, but, for some reason or other, is unable to give rise to motion. If the 

 obstacle is removed, the energy which was there but could not manifest 

 itself, at once gives rise to motion. ^Yhile the lestraint lasts, the en- 

 ergy of the imrticle is merely potential ; and the case supposed illus- 

 trates what is meant by potential energy. In this contrast of the ])0- 

 tcntial with the actual, modern physics is turning to account the most 

 familiar of Aristotelian distinctions — that between duva/u:; and hipyt'.a. 



That kinetic energy appears to be imparted by impact is a fact ot 

 daily and hourly experience: we see bodies set in motion by bodies, 

 already in motion, which seem to come into contact with them. It is 

 a truth which could have been learned by nothing but experience, and 

 which can not be exi)lained, but must be taken as an ultimate fact 

 about which, explicable or inexi)licable, there can be no doubt. Strictly 

 speaking, we have no direct apprehension of any other cause of mo- 

 tion. But experience furnishes innumerable examples of the produc- 

 tion of kinetic energy in a body previously at rest, when no impact 

 is discernible as the cause of that energy. In all such cases, the 

 presence of a second body is a necessary condition ; and the amount of 

 kinetic energy, which its presence enables the first to gain, is strictly de- 

 pendent on the relative positions of the two. Ilence the phrase energy of 

 position, which isfrequently used as equivalent to potential energy. If a 

 stone is picked up and held, say, C feet above the ground, ithiis potential 

 energy, because, if let go, it will immediately begin to move towards the 

 earth ; and this energ;^ may be said to be energy of position, because it 

 depends upon the relative position of the earth and the stone. The 



