30 FRAGMENTS OF SCIENCE. 



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. Now this atomic vibration is merely the redistribu- 

 tion 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 

 chemical attraction destroyed, or converted into any thing 

 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 diminished. 



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

 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 chemical affin- 

 ity. 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 converted into heat. In no case is the force which pro- 

 duces the motion annihilated or changed into any thing 

 else. The mutual attraction of the earth and weight exists 

 when they are in contact as when they were separate ; 

 but the ability of that attraction to employ itself in the 

 production of motion does not exist. 



The transformation, in this case, is easily followed by 

 the mind's eye. First, the weight as a whole is set in 

 motion by the attraction of gravity. This motion of the 

 mass is arrested by collision with the earth, being broken up 

 into molecular tremors, to which we give the name of heat. 



And when we reverse the process, and employ those 



