THE PERSISTENCE OF FORCE. 195 



know. The mode of force which is revealed to us only by 

 opposition to our own powers, may be in essence the same 

 with the mode of force which reveals itself by the changes it 

 initiates in our consciousness. That the space a body occu- 

 pies is in part determined by the degree of that activity pos- 

 sessed by its molecules which we call heat, is a familiar 

 truth. Moreover, we know that such molecular re-arrange- 

 ment as occurs in the change of water into ice, is accom- 

 panied by an evolution of force which may burst the con- 

 taining vessel and give motion to the fragments. Never- 

 theless, the forms of our experience oblige us to distinguish 

 between two modes of force ; the one not a worker of change 

 and the other a worker of change, actual or potential. The 

 first of these the space-occupying kind of force has no 

 specific name. 



For the second kind of force, distinguishable as that by 

 which change is either being caused or will be caused if 

 counterbalancing forces are overcome, the specific name 

 now accepted is " Energy." That which in the last chapter 

 was spoken of as perceptible activity, is called by physicists, 

 " actual energy "; and that which was called latent activ- 

 ity, is called " potential energy.'' While including the 

 mode of activity shown in molar motion, Energy includes 

 also the several modes of activity into which molar motion is 

 transformable heat, light, etc. It is the common name for 

 the power shown alike in the movements of masses and in 

 the movements of molecules. To our perceptions this sec- 

 ond kind of force differs from the first kind as being not 

 intrinsic but extrinsic. 



In aggregated matter as presented to sight and touch, 

 this antithesis is, as above implied, much obscured. Espe- 

 cially in a compound substance, both the potential energy 

 locked up in the chemically-combined molecules, and the 

 actual energy made perceptible to us as heat, complicate 

 the manifestations of intrinsic force by the manifestations 

 of extrinsic force. But the antithesis here partially hidden, 



