THE CONSTITUTION OF NATURE. 25 



though it has ceased to be a source of energy ', the at- 

 traction of gravity still acts as a force, which holds the 

 earth and weight together. 



The same remarks apply to attracting atoms and 

 molecules. As long as distance separates them, they 

 can move across it in obedience to the attraction ; and 

 the motion thus produced may, by proper appliances, 

 be caused to perform mechanical work. When, for ex- 

 ample, two atoms of hydrogen unite with one of oxygen, 

 to form water, the atoms are first drawn towards each 

 other they move, they clash, and then by virtue of 

 their resiliency, they recoil and quiver. To this quiver- 

 ing motion we give the name of heat. This atomic 

 vibration is merely the redistribution of the motion 

 produced by the chemical affinity ; and this is the only 

 sense in which chemical affinity can be said to be con- 

 verted into heat. We must not imagine the chemical 

 attraction destroyed, or converted into anything else. 

 For the atoms, when mutually clasped to form a mole- 

 cule of water, are held together by the very attraction 

 which first drew them towards each other. That which 

 has really been expended is the pull exerted through 

 the space by which the distance between the atoms has 

 been diminished. 



If this be understood, it will be at once seen that 

 gravity, as before insisted on, may, in this sense, be said 

 to be convertible into heat ; that it is in reality no more 

 an outstanding and inconvertible agent, as it is some- 

 times stated to be, than is chemical affinity. By the 

 exertion of a certain pull through a certain space, a 

 body is caused to clash with a certain definite velocity 

 against the earth. Heat is thereby developed, and this 

 is the only sense in which gravity can be said to be con- 

 verted into heat. In no case is the force which produces 

 the motion annihilated or changed into anything else. 



