14 Elementary Biology. 



ear and the source of the sound. Sound is therefore a 

 mode of motion. Light, according to the undulatory theory 

 first propounded_ by-Huyghens, is to be considered as due 

 to the vibrations of an imponderable medium termed ether, 

 which pervades all space between the particles of matter, and 

 which is by some eminent physicists regarded as in all proba- 

 bility forming the fundamental basis of matter itself. Light 

 may therefore be considered as a mode of motion, though 

 not precisely in the same sense as sound is considered to be 

 so. Lastly, heat is undoubtedly due to the vibration ot 

 molecules of matter. Of course, on the assumption that the 

 . space between all molecules is filled by an ethereal medium, 

 :the vibrations of molecules mean also the simultaneous 

 vibration of their ethereal surroundings. Now, we have seen 

 that the potential energy of the gunpowder became trans- 

 formed into kinetic energy, manifesting sufficient force to 

 cause a considerable alteration in the distance relationships 

 of surrounding objects. But the heat produced during the 

 explosion, which we have just seen to be a form of kinetic 

 energy, must have resulted from the transformation of a 

 portion of the original potential energy of the gunpowder ; 

 similarly for the light and sound. We conclude, there- 

 fore, that all the energy of the gunpowder is not available 

 for the production of ordinary motion of translation amongst 

 masses. Part is transformed into forms of kinetic energy, 

 probably not desired, but nevertheless invariably accompany- 

 ing the chief manifestation. An extended course of obser- 

 vation and experiment teaches us that every transformation 

 of potential energy into kinetic, or of kinetic into potential, 

 is attended by one or more, what might be termed, by- 

 manifestations of energy, the chief of these, and that form 

 of kinetic energy into which all others ultimately become 

 transformed, being heat. 1 



Heat, of course, as kinetic energy, can itself be trans- 



1 The rough determination of the source, destination, and propor- 

 tional amount of the various forms in which energy manifests itself in 



