30 FRAGMENTS OF SCIENCE 



the attraction, and in falling can perform work. "When 

 it rests at its lowest level it is not a source of power or 

 energy, because it can fall no further. But though it has 

 ceased to be a source of energy, the attraction of gravity 

 still acts as a force, which holds the earth and weight 

 together. 



The same remarks apply to attracting atoms and mole- 

 cules. 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 example, two atoms 

 of hydrogen unite with one of oxygen, to form water, the 

 atoms are first drawn toward each other — they move, they 

 clash, and then, by virtue of their resiliency, they recoil 

 and quiver. To this quivering motion we give the name 

 of heat. This atomic vibration is merely the redistribu- 

 tion of the motion produced by the chemical afiQ.nity; and 

 this is the only sense in which chemical affinity can be 

 said to be converted into heat. We must not imagine the 

 chemical attraction destroyed, or converted into anything 

 else. For the atoms, when mutually clasped to form a 

 molecule of water, are held together by the very attrac- 

 tion which first drew them toward 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 sometimes 

 stated to be, than is chemical affinity. By the exertion of 

 a certain pull through a certain space, a body is caused 



