MOTION. 21 



in masses, as with the planetary spheres, but also mole- 

 cularly, or throughout its most intimate structure : thus every 

 alteration of temperature produces a molecular change 

 throughout the whole substance heated or cooled ; slow 

 chemical or electrical actions, actions of light or invisible 

 radiant forces, are always at play, so that as a fact we can- 

 not predicate of any portion of matter that it is absolutely at 

 rest. Supposing, however, that motion is not an indispensa- 

 ble function of matter, but that matter can be at rest, matter 

 at rest would never of itself cease to be at rest ; it would not 

 move unless impelled to such motion by some other moving 

 body, or body which has moved. This proposition applies 

 not merely to impulsive motion, as when a ball at rest is 

 struck by a moving body, or pressed by a spring which has 

 previously been moved, but to motion caused by attractions 

 such as magnetism or gravitation. Suppose a piece of iron 

 at rest in contact with a magnet at rest ; if it be desired to 

 move the iron by the attraction of the magnet, the magnet or 

 the iron must first be moved ; so before a body falls it must 

 first be raised. A body at rest would therefore continue so 

 for ever, and a body once in motion would continue so for 

 ever, in the same direction and with the same velocity, un- 

 less impeded by some other body, or affected by some other 

 force than that which originally impelled it. These propo- 

 sitions may seem somewhat arbitrary, and it has been doubted 

 whether they are necessary truths ; they have for a long time 

 been received as axioms, and there can at all events be no 

 harm in accepting them as postulates. It is however very 

 generally believed that if the visible or palpable motion of 

 one body be arrested by impact on another body, the mo- 

 tion ceases, and the force which produced it is annihi- 

 lated. 



Now the view which I venture to submit is, that force 

 cannot be annihilated, but is merely subdivided or altered in 

 direction or character. First, as to direction. Wave your 



