276 Mr. Rankine on (he Transformation of Energy. 



XXXIV. — On the General Law of the Transformation of Energy. 

 By W. J. Macquorn Rankine. 



Actual, or Sensible Energv, is a measurable, transmissible, and 

 transformable condition, Vbose presence causes a substance to tend to 

 cbange its state in one or more respects. By tbe occurrence of such 

 changes, actual energy disappears, and is replaced by 



Potential or Latent Energy ; which is measured by the product of a 

 change of state into the resistance against which that change is made. 



(The vis viva of matter in motion, thermometric heat, radiant heat, 

 light, chemical action, and electric currents, are forms of actual energy; 

 amongst those of potential energy are the mechanical powers of gravita- 

 tion, elasticity, chemical affinity, statical electricity, and magnetism.) 



The law of the Conservation of Energy is already known, viz. : — that 

 the sum of all the energies of the universe, actual and potential, is 

 unchangeable. 



The object of the present paper is to investigate the law according to 

 which all transformations of energy, between the actual and potential 

 forms, take place. 



Let V be the magnitude of a measurable state of a substance ; 



U, the species of potential energy which is developed when the state 

 V increases ; 



P, the common magnitude of the tendency of the state V to increase, 

 and of the equal and opposite resistance against which it increases ; so 

 that— 



dU=PdV; andP=^ (A.) 



Let Q be the quantity which the substance possesses, of a species of 

 actual energy whose presence produces a tendency of the state V to 

 increase. 



It is required to find how much energy is transformed from the^actual 

 form Q to the potential form U, during the increment dV ; that is to 

 say, the magnitude of the portion of dU, the potential energy developed, 

 which is due to the disappearance of an equivalent 'portion of actual 

 energy of the species Q. 



The development of this portion of potential energy is the immediate 

 efi'ect of the presence in the substance of the total quantity Q of actual 

 energy. 



Let this quantity be conceived to be divided into indefinitely small 

 equal parts dQ. As those parts are not only equal, but altogether alike 

 in nature and similarly circumstanced, their effects must be equal ; there- 

 fore, the efi'ect of the total energy Q must be equal simply to the efi\ict 



of one of its small parts dQ, multiplied by the ratio — 



