THE CONSTITUTION OF NATVRE. 19 
can be said to be converted into heat. In no. case is the 
force which produces the motion annihilated or changed 
into anything 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 
tremors of heat to raise a weight which is done through 
the intermediation of an elastic fluid in the steam-engine 
a certain definite portion of the molecular motion is 
consumed. In this sense, and in this sense only, can the 
heat be said to be converted into gravitv ; or, more cor- 
rectly, into potential energy of gravity. Here the destruc- 
tion of the heat has created no new attraction ; but the old 
attraction has conferred upon it a power of exerting a cer- 
tain definite pull, between the starting-point of the falling 
weight and the earth. 
When, therefore, writers on the conservation of energy 
speak of tensions being "consumed " and " generated," they 
do not mean thereby that old attractions have been anni- 
hilated, and new ones brought into existence, but that, in 
the one case, the power of the attraction to produce 
motion has been diminished by the shortening of the dis- 
tance between the attracting bodies, while, in the other 
case, the power of producing motion has been augmented 
by the increase of the distance. These remarks apply to 
all bodies, whether they be sensible masses or molecules. 
Of the inner quality that enables mutter to attract mat- 
ter we know nothing ; and the law of conservation makes 
no statement regarding that quality. It takes the facts of 
attraction as they stand, and affirms only the constancy of 
working-power. That power may exist in the form of 
MOTION ; or it may exist in the form of FORCE, with dis- 
tance to act through. The former is dynamic energy, the 
latter is potential energy, the constancy of the sum of both 
being affirmed by the law of conservation. The converti- 
bility of natural forces consists solely in transformations of 
dynamic into potential, and of potential into dynamic 
