Force, Energy, and Will 235 



sense of the transformation of activities, is what everybody 

 knew of since man first made fire by rubbing sticks together. 

 The word ' energy/ however, is, at the least, too often used 

 in such a way as to be in fact our old friend ' force ' in a new 

 costume. Thus Professor Tait ^ gives as examples of energy, 

 the energy of any current of water capable of driving an 

 undershot wheel ; of wmds which also are used for driving 

 machinery; the energy of water-waves or of sound-waves; 

 the radiant energy which comes to us from the sun,' etc. 

 By these expressions Professor Tait cannot evidently mean 

 merely the activity of running water, but its ' force ' as that 

 word is commonly understood, and so of his other forms of 

 energy. To the expression the * transformation of energy ' in 

 this sense all the objections apply which have already been 

 made to the expression 'transformation of force.' I con- 

 tend then that care should be taken not so to speak of force 

 and energy as to lead to the belief in their substantial exist- 

 ence,- as force and energy, and I object to the phrases 'unity 



^ Lectures on Some Recent Advances in Physical Science, p. 359. 



2 Professor Tait {l. c. p. 55) tells us that Mayer 'actually says it {i.e. 

 motion) must cease to be motion in order to become heat,' thus implying 

 that in his (Professor Tait's) opinion, motion does not cease to be motion in 

 becoming heat. Nevertheless he is very severe (by implication) upon Pro- 

 fessor Tyndall for simply saying that heat is a mode of motion. Professor 

 Tait (p. 362) observes : ' The conception of kinetic energy is a very simple 

 one, at least when visible motion alone is involved. And from motion of 

 visible masses to those motions of the particles of bodies whose energy we 

 call heat, is by no means a very difficult mental transition. Mark, however, 

 that heat is not the mere motions, but the energy of these motions — a very 

 different thing, for heat and kinetic energy in general are no more modes 

 of motion than potential energy of every kind (including that of unfired 

 gunpowder) is a mode of rest ! ' But surely this is a piece of hypercriticism, 

 and one moreover very obscurely and even incorrectly expressed. Much as I 

 deprecate as false and misleading the phrase 'heat a mode of motion,' yet 

 Professor Tait has no right to object to it as a popular way of speaking, 

 which is how Professor Tyndall used it. Professor Tait seems to object to the 

 use of the term ' motion ' as denoting ' motion in the abstract ' ; biit if so, then 

 he should not have objected (as above quoted) that ' heat is not the mere 

 motions, but the energy of these motions ' ; he should have objected 'heat is 

 not the mere motions in the ahatract, but the motions of the molecules.'' 



