18 FRAGMENTS OF SCIENCE. 



and wants of an individual, but exceedingly minute in com- 

 parison with the earth's primitive store. 



To sum up. The whole stock of energy or working- 

 power in the world consists of attractions, repulsions, and 

 motions. If the attractions and repulsions be so circum- 

 stanced as to be able to produce motion, they are sources of 

 working-power, but not otherwise. As stated a moment 

 ago, the attraction exerted between the earth and a body 

 at a distance from the earth's surface, is a source of work- 

 ing-power; because the body can be moved by the attrac- 

 tion, 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 farther. 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 redistribution 

 of the motion produced by the chemical affinity; and this 

 is the only sense in which chemical affinity can be said to 

 be converted into heat. We must not imagine the chem- 

 ical 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 attraction 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 dimin- 

 ished. 



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

 as before insisted on, may, in this sense, be said to be con- 

 vertible into heat; that it is in reality no more an outstand- 

 ing 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 to clash with a 

 certain definite velocity against the earth. Heat is there- 

 by developed, and this is the only sense in which gravity 



