ti.] TRANSFORMATIONS OF ENERGY. 27 



very little force, and nearly all their energy of motion 

 disappears. We now, however, know that the energy of 

 motion which is lost by the masses is transferred to the 

 molecules, being transformed into heat ; so that the 

 elasticity in which the masses are deficient is shown to 

 belong to the molecules. Thus, what appears to be 

 inelasticity is really a form of elasticity. 



When energy of motion disappears, it is generally trans- Motion 

 formed into heat, but under special circumstances it is formed 

 transformed into electricity. This is done by the common into elec 

 electrical machine, in which, by a suitable arrangement, 

 a large part of the energy of motion that is usually trans- 

 formed by friction into heat is transformed into electricity 

 instead. The law on this subject appears to be, that 

 similar substances rubbing against each other produce 

 heat, but dissimilar ones produce electricity ; and the more 

 unlike the substances are to each other, the more of the 

 energy of motion that disappears is transformed into 

 electricity, and the less into heat.^ 



In the electrical machine, artificial means are used to 

 retain the electricity when it is produced. But when these 

 are not used, the electricity nearly always escapes as soon 

 as produced, and, in escaping, is transformed into heat. 

 We may consequently lay it down as a law which is prac- 

 tically true throughout nature, that when energy of motion 

 seems to be lost, it is really transformed into heat. 



Energy of motion is also transformed into electricity by 

 a totally different process in the magneto-electric machine. 



The nature of electricity is not nearly so well understood 

 as that of heat and light. I state some experiments and 

 reasonings bearing on the subject in a note to Chapter III. 



Equal quantities of energy of motion, as already stated, Quantita- 

 are capable of transformation into equal quantities of valence of 

 heat ; and equal quantities of energy of motion are capa- ^^ ^°^'™®^ 

 ble of transformation also into equal q;iantities of electric 

 energy.^ Equal quantities of electric energy and of heat 



1 Grove on the Correlation of Physical Forces. 



^ I say that equal quantities of heat or of energy of motion are capable 

 of transformation into equal quantities, not of electricity, but of electric 



