OF NATURE. 577 



feature which distinguishes all physical (actual) pro- 

 cesses from merely mechanical (artificial) contrivances ; 

 secondly, in a very different direction he had already 

 (1845) taken a great step in advance by showing how 

 two seemingly quite different ways of approaching 

 electrical and magnetic phenomena — the " action-at-a- 

 distance theory " of Continental mathematicians, such 

 as Poisson, and Faraday's " Lines of force," filling space 

 continuously, — led, through mathematical language, to the 

 same results.^ Tait carried on a lifelong battle with 

 the older conceptions of attractive and repulsive forces, 

 assisted in replacing in physics the conception of particles 

 moving about in empty space by the conception of a 

 plenum, and ended by suggesting that the word " force " 

 should be discarded as an unnecessary and misleading 

 term. Maxwell worked in the same direction, though 

 with more caution and impartiality, through his small 

 tract on ' Matter and Motion,' and still more by building 

 up a large portion of the sciences of electricity and mag- 

 netism on the basis of the conception of Energy and its 

 distribution in space, discarding latterly the mechanical 

 models which he had previously invented as illustra- 

 tions of Faraday's " lines and tubes of force." Inci- 

 dentally a controversy arose between Tait and Herbert 

 Spencer as to the illegitimate use which the latter 



less, of larger or smaller ; there is 

 no differeuee of degree iu any other 

 sense. Another sense or meaning 

 is introduced only with reference 

 to the observing or thinking mind 

 which derives more pleasure, more 

 use, from some sensations than from 

 others, and accordingly puts a 

 greater value on the former than 



VOL. III. '2 



on the latter. The well-known 

 " Demon " of Clerk Maxwell shows 

 by a fiction how, for beings other- 

 wise constituted than we are, the 

 most degraded forms of energy or 

 motion might be of the same value 

 for practical purposes as molar 

 motions are for us. 



^ See ante, vol. ii. p. 72. 



