THE CONSTITUTION OF NATURE. 19 



out having added, during the infinitely small duration 

 of its action, a due amount of motion to that previously 

 possessed by D. The nearer D approaches to F, the 

 smaller is the sum of the tensions remaining, but the 

 greater is the vis viva ; the farther D is from F, the 

 greater is the sum of the unconsumed tensions, and 

 the less is the living force. Now the principle of con- 

 servation affirms not the constancy of the value of the 

 tensions of gravity, nor yet the constancy of the vis 

 viva, taken separately, but the absolute constancy of 

 the value of both taken together. At the beginning 

 the vis viva was zero, and the tension area was a maxi- 

 mum ; close to F the vis viva is a maximum, while the 

 tension area is zero. At every other point, the work- 

 producing power of the particle D consists in part of 

 vis viva, and in part of tensions. 



If gravity, instead of being attraction, were repulsion, 

 then, with the particles in contact, the sum of the tensions 

 between D and F would be a maximum, and the vis viva 

 zero. If, in obedience to the repulsion, D moved away 

 from F, vis viva would be generated ^ and the farther D 

 retreated from F the greater would be its vis viva, and 

 the less the amount of tension still available for producing 

 motion. Taking repulsion as well as attraction into 

 account, the principle of the conservation of force affirms 

 that the mechanical value of the tensions and vires 

 vivce of the material universe, so far as we know it, is 

 a constant quantity. The universe, in short, possesses 

 two kinds of property which are mutually convertible. 

 The diminution of either carries with it the enhance- 

 ment of the other, the total value of the property 

 remaining unchanged. 



The considerations here applied to gravity apply 

 equally to chemical affinity. In a mixture of oxygen and 

 hydrogen the atoms exist apart, but by the application 



c 2 



