Ill 



which we are at present concerned, we shall use that word 

 instead. The explanation then is this, that the amount of 

 latent and actual motion before the explosion, was exactly equal 

 to the amount after. That if we express the amount of latent 

 motion before, by 9, and the actual by 1, we must, after the 

 change, express the latent by 1, and the actual by 9. The 

 phrase latent motion may appear so strange as to cause it to be 

 doubted whether we are at liberty to use it. We must remember, 

 however, that motion, energy, and working power are under- 

 stood as interchangeable terms by most of the writers of whom 

 I am speaking. Professor Tyndall, while calling heat a mode of 

 motion, speaks of latent heat, that is a latent mode of motion. 

 Latent motion, therefore, is motion at rest, remaining motion 

 still. The apprehension of this is somewhat difficult, if not impos- 

 sible. Power in exercise and power latent are perfectly compre- 

 hensible, but motion that is motionless is quite a different concep- 

 tion, if it may indeed be called a conception. Mr. Grove, in 

 controverting the hypothesis of latent matter, in the material 

 theory of heat, rightly asks, Is not ' invisible light ^ a contra- 

 diction in terms ? Has not light ever been regarded as that 

 agent which affects our visual organs ? Invisible light, then, is 

 darkness : and if it exist, then is darkness light." In like manner 

 I ask, is not motionless motion a contradiction in terms ? Is it 

 not rest ? And if it exist, then is rest, motion ? If rest and 

 motion be one and the same ; if matter always possess latent 

 motion, when it has not actual, then, indeed, the explanation is 

 sound, — the origination of motion is an absurdity. But if latent 

 motion be not motion, but rest, then the explanation is the 

 absurdity, and motion has a commencement. The statement 

 that " throughout the universe the sum of these two energies is 

 constant,'^ has been shown by Sir John Herschel to be a mere 

 truism, whether expressed in so many words, or by saying that 

 the potential together with the actual energy of the system is 

 invariable ; or, again, in other words, that when certain changes 

 have taken place in the relative situations of the parts of the 

 system, what it has lost in actual it has gained in potential 

 energy/' This must be evident to all; for if we are at liberty 

 to say that the energy which has disappeared as actual still exists 

 as potential ; and that which comes into manifestation as actual, 

 previously existed as potential, it follows as a matter of course, 

 that the sum of the two must be always the same. 



26. Putting aside this fiction of the hypothetical measurement 

 of the unknown by the elimination of the known, the conser- 

 vation of energy, motion, working power, is at once seen to 

 have no existence. As Sir John Herschel says, — No such 

 conservation in the sense of an identity of total arnount of energy 



