THti CONSTITUTION OF NA TURE. 7 
It is sometimes stated that gravity is distinguished 
from all other forces by the fact of its resisting conver- 
sion into other forms of force. Chemical affinity, it is 
said, can be converted into heat and light, and these again 
into magnetism and electricity: but gravity refuses to be so 
converted; being a force maintaining itself under all cir- 
cumstances, and not capable of disappearing to give place 
to another. The statement arises from vagueness of 
thought. If by it be meant that a particle of matter can 
never be deprived of its weight, the assertion is correct; 
but the law which affirms the convertibility of natural 
forces was never intended, in the minds o those who un- 
derstood it, to affirm that such a conversion as that here 
implied occurs in any case whatever. As regards convert- 
ibility into heat, gravity and chemical affinity stand on 
precisely the same footing. The attraction in the one case 
is as indestructible as in the other. Nobody affirms that 
when a stone rests upon the surface of the earth, the 
mutual attraction of the earth and stone is abolished; 
nobody means to affirm that the mutual attraction of 
oxygen for hydrogen ceases, after the atoms have com- 
bined to form water. What is meant, in the case of 
chemical affinity, is, that the pull of that affinity, acting 
through a certain space, imparts a motion of translation of 
the one atom toward the other. This motion is not heat, 
nor is the force that produces it heat. But when the 
atoms strike and recoil, the motion of translation is con- 
verted into a motion of vibration, which is heat. The 
vibration, however, so fur from causing the extinction of 
the original, attraction, is in part carried on by that at- 
traction. The atoms recoil, in virtue of the elastic force 
which opposes actual contact, and in the recoil they are 
driven too far back. The original attraction then 
triumphs over the force of recoil, and urges the atoms once 
more together. Thus, like a pendulum, they oscillate, 
until their motion is imparted to the surrounding ether; 
or, in other words, until their heat becomes radiant 
heat. 
In this sense, and in this sense only, is chemical affinity 
converted into heat. There is, first of all, the attraction 
between the atoms; there is, secondly, space between them. 
Across this space the attraction urges them. They collide, 
they recoil, they oscillate. There is here a change in the 
