16 Prof. S. P. Thompson on the Conservation of Electricity , 



itself.-" But here again the reasoning is at fault ; for the 

 kinetic energy of a moving body is equal to the half-pro- 

 duct of its mass into the square of the velocity in space of its 

 centre of inertia, a velocity which must be reckoned as rela- 

 tive to something else, and which depends therefore upon the 

 external conditions*. 



It would appear, then, that kinetic energy may be of either 

 of two kinds — ponderokinetic and electrokinetic ; and that 

 potential energy may also be of two kinds — namely, either the 

 mutual energy of two masses of self-attracting matter, or the 

 mutual energy of two quantities of self-repelling electricity 

 (see below, § 5). 



4. Relations between Matter } Energy, and Electricity. — Of 

 the three conserved physical quantities, Matter, Energy, and 

 Electricity, it would therefore appear that two, namely Matter 

 and Electricity, stand each in very similar relation to the third, 

 Energy. The following facts bring out this relation: — 

 (a) To set matter in motion requires the expenditure of 



energy. 

 (a) To set electricity in motion requires the expenditure of 



energy. 

 (h) When the motion of matter is arrested its (pondero-) 

 kinetic energy reappears in the form of heat (or some 

 other equivalent form). 

 (/3) When the motion of electricity is arrested, its (electro-) 

 kinetic energy reappears in the form of heat (or some 

 equivalent form). 

 (c) Energy spent upon an elastic body, when it suffers a 

 strain under the action of a stress, passes from the (pon- 

 dero-)kinetic to the (pondero-)potential state. 

 (7) Energy spent upon a medium which possesses electric 

 elasticity (see Maxwell, Art. 50), such as glass, air, 

 vacuum, or any non-conducting dielectric, causes it to 

 experience a strain which is partly electrical (i. e. an 

 electrical displacement) and partly mechanical, the 

 (electro-) kinetic energy passing over into potential 

 energy, part of it consisting in the displacement of 

 electricity against an electric reaction (i. e. an electro- 

 motive force), part of it consisting in the displacement 

 of matter against a mechanical reaction (i. e. a force). 

 In the preceding case (c) of mechanical stress producing 

 mechanical strain, as when an elastic metal rod is struck, 

 there is also (as Yolpicelli has shown t) a partial passing 



* See also Clerk Maxwell, ' Matter and Motion/ art. ex. 

 t Volpicelli, Comptes Rendus, t. xxxviii. 15 Mai, 1854. 



