ON THE PHYSICAL VIKW OK NATURK. 147 



vation of the sum-total of the energy. TIiIh energy 

 could exist as motion (actual or kinetic energy), Ixiing 

 either motion of electricity as in the current controlled 

 by the law of Ohm, or motion of ponderable masses, such 

 as magnets or electric conductors ; or it might be dis- 

 sipated energy — i.e., energy apparently lost in the form 

 of heat — controlled by the law of Joule, or, to complete 

 the summation, it might be stored-up energy — potential 

 energy. Faraday's researches had suggested where this 

 store was : it was in the surrounding space, which must 

 be considered as capable of being strained or put int(j a 

 condition of stress, as elastic bodies are capalile of being 

 strained. Thomson and Tait had shortly before shown 

 how to submit the properties of elastic systems to cal- 

 culation in the most general manner, by studying the 

 modes in which energy, actual and potential, was dis- 

 tributed in them, whether at rest or in motion. The 

 way seemed then paved for Maxwell to consider with the 

 greatest generality the properties of the electro-magnetic 

 field, reducing them all to mechanical measures. This he 

 did b}' introducing the generalised conception of a dis- 

 placement or strain wliich exists in the field, and which 

 is commimicated as a periodic or vibratory motion with 

 a velocity dependent on the properties or so-called con- 

 stants of the medium. It is known how he succeeded in 

 identifying very completely all the various experimentally 

 ascertained electric and magnetic phenomena, fixing their 

 nature and quantities in conformity with experience, 

 and arriving finally at the suggestion that the velocity of 

 tlie transmission of the electro-magnetic displacement in 

 air must l)e the same as that of light, the latter being. 



