HIS FIRST LAW OF MOTION. 35 



ever since maintained, without any renewal of 

 the cause.* 



But in all the operations of nature with which 

 we are best acquainted, force is always expended 

 in producing motion ; and if not as constantly re- 

 newed, is very soon exhausted. Nor have we the 

 slightest proof, that motion is ever generated 

 without the immediate agency of some active 

 principle. The power of steam is created by the 

 immediate agency of caloric, and ceases when- 

 ever the acting principle is withdrawn. The force 

 with which a horse shoe magnet of soft iron holds 

 on to the armature, is generated by the passage of 

 an electric current through the wires which are 

 coiled around it ; but ceases the moment the cur- 

 rent is arrested. The metallic wire that connects 

 the extremities of a voltaic battery, attracts iron 

 filings, and deflects a magnetic needle in its 

 vicinity, so long as the electric fluid is disen- 

 gaged by chemical action, but ceases whenever 

 that action is at an end. The powers of digestion, 

 secretion, nutrition, and muscular motion, are 

 generated by means of an active principle which 

 is obtained from the atmosphere by respiration ; 



* Newton observes, " that the motions of the planets may 

 subsist an exceeding long time, it is necessary that the heavens 

 should be void of all resisting matter." And he adds in another 

 place, " by the vis insita of bodies, I mean nothing but their 

 vis inertia." (Princip. book iii.) 



