ENERGY AND ITS TRANSFORMATIONS. 29 



If two energies only, as thernnal and mechanical, are 

 affected, and if the original stock were simply heat- 

 energy, we should have a change, dE^ in the initial 

 stock of heat-energy, which would be the precise 

 equivalent of the sum of the two changes taking place, 

 simultaneously, in the initial store and in the tempera- 

 ture, T, of the system, and in work by the change of 

 volume, F, against a pressure of, say, the intensity /. 

 Then, obviously, 



dE^f^dT+%dV; .... (I) 



and, since f^^) measures the specific heat, K^^ for 

 \dri V 



— - ) must measure the in- 

 dViT 



tensity of pressure producing, or resisting, the change 

 of volume, 



dE^K,dT-\-pdV. (2) 



If but one kind of transformation occurs, as by con- 

 version of any original form of energy, E, into work, 

 a process illustrated in every purely thermodynamic, 

 thermo-electric, or electro-dynamic operation, 



dE = pdV\ or, dE = RdS. ... (3) 



Whatever the method of transformation of actual 

 energy, it is simply a process of transfer of the energy 

 of motion from one kind of matter to another, or from 

 a mass to a collection of molecules, with, usually, modi- 

 fication of the mode of motion, as from rectilinear to 

 vibratory, or from motion in an orbit of one form to 

 m.cvement in a path of different character. 



