444 On the Conservation of Force. [1857. 



changeable in its manifestation, it offers an unchanging test of 

 the matter which we recognize by it. 



Agreeing with those who admit the conservation of force to 

 be a principle in physics as large and sure as that of the inde- 

 structibility of matter, or the invariability of gravity, I think 

 that no particular idea of force has a right to unlimited or un- 

 qualified acceptance, that does not include assent to it ; and 

 also, to definite amount and definite disposition of the force, 

 either in one effect or another, for these are necessary conse- 

 quences : therefore, I urge, that the conservation of force ought 

 to be admitted as a physical principle in all our hypotheses, 

 whether partial or general, regarding the actions of matter. I 

 have had doubts in my own mind whether the considerations I 

 am about to advance are not rather metaphysical than physical. 

 I am unable to define what is metaphysical in physical science ; 

 and am exceedingly adverse to the easy and unconsidered ad- 

 mission of one supposition upon another, suggested as they often 

 are by very imperfect induction from a small number of facts, 

 or by a very imperfect observation of the facts themselves : 

 but, on the other hand, I think the philosopher may be bold in 

 his application of principles which have been developed by 

 close inquiry, have stood through much investigation, and con- 

 tinually increase in force. For instance, time is growing up 

 daily into importance as an element in the exercise offeree. The 

 earth moves in its orbit in time ; the crust of the earth moves 

 in time; light moves in time; an electro-magnet requires time 

 for its charge by an electric current : to inquire, therefore, 

 whether power, acting either at sensible or insensible distances, 

 always acts in time, is not to be metaphysical ; if it acts in time 

 and across space, it must act by physical lines of force ; and 

 our view of the nature of the force may be affected to the ex- 

 tremest degree by the conclusions, which experiment and ob- 

 servation on time may supply ; being, perhaps, finally determi- 

 nable only by them. To inquire after the possible time in 

 which gravitating, magnetic, or electric force is exerted, is no 

 more metaphysical than to mark the times of the hands of a 

 clock in their progress ; or that of the temple of Serapis in its 

 ascents and descents ; or the periods of the occupations of 

 Jupiter's satellites ; or that in which the light from them comes 

 to the earth. Again, in some of the known cases of action in 



