ENERGY AND ITS TRANSFORMATIONS. 27 



ently worded statement of Newton's three laws com- 

 bined. 



To assert that every moving body tends to persist in 

 its rate of motion, exerting an effort always equal to 

 the retarding or accelerating force, and exerting such 

 effort in the line of action of such force, is to assert 

 that its energy can only be altered by the performance 

 of an equivalent amount of work, and an equal amount 

 of energy of opposite sign ; and this latter assertion is a 

 declaration of the indestructibility of energy. To assert 

 that all bodies, w^hether masses or molecules, when in 

 motion tend to move in rectilinear paths, is to assert a 

 tendency to unlimited dissipation of energy through 

 space. To assert that all matter in motion is subject to 

 Newton's laws is to assert the laws of universal conserva- 

 tion of energy, and of the quantivalence of all transforma- 

 tions, as stated in the third general law. Whenever it 

 becomes established that any phenomenon, as the trans- 

 fer of heat, of light, of electricity, or of sound, is a mode 

 of motion affecting bodies of whatever class, Newton's 

 laws bring that phenomenon within the scope of the 

 general laws of energy. Every phenomenon, molecu- 

 lar or other, which involves relative motion of masses, 

 vibrations of parts, or pulsations in fluid media, is now 

 well understood to be subject to these laws. 



9. Algebraic Expressions of the transformability 

 of the energies are now readily deduced. If in any 

 isolated system a certain quantity of energy exists, 

 homogeneous in character and heterogeneously dis- 

 tributed ; and if, by any process, other and various 

 forms of energy appear in that system, these latter 

 must be the result of transformations of parts of the 

 initial stock of energy by conversion into the new 



