4 FRA GMENTS OF SCIENCE. 
a stone falls to the ground and is warmed by the 
shock; under its operation meteors plunge into our atmos- 
phere and rise to incandescence. Showers of such meteors 
doubtless fall incessantly upon the sun. Acted on by this 
force, the earth, were it stopped in its orbit to-morrow, 
would rush toward, and finally combine with the sun. 
Heat would also be developed by this collision. Mayer 
first, and Helmholtz and Thomson afterward, have calcu- 
lated its amount. It would equal that produced by the com- 
bustion of more than 5,000 worlds of solid coal, all this 
heat being generated at the instant of collision. In the 
attraction of gravity, therefore, acting upon non-luminous 
matter, we have a source of heat more powerful than 
could be derived from any terrestial combustion. And 
were the matter of the universe thrown in cold detached 
fragments into space, and there abandoned to the mutual 
gravitation of its own parts, the collision of the fragments 
would in the end produce the fires of the stars. 
The action of gravity upon matter originally cold may, 
in fact, be the origin of all light and heat, and also the 
proximate source of such other powers as are generated by 
light and heat. But we have now to inquire what is the 
light and what is the heat thus produced? This question 
has already been answered in a general way. Both light 
and heat are modes of motion. Two planets clash and 
come to rest; their motion, considered as that of masses, 
is destroyed, but it is in great part continued as a motion 
of their ultimate particles. It is this latter motion, 
taken up by the ether, and propagated through it with a 
velocity of 186,000 miles a second, that comes to us as the 
light and heat of suns and stars. The atoms of a hot 
body swing with inconceivable rapidity billions of times 
in a second but this power of vibration necessarily implies 
the operation of forces between the atoms themselves. It 
reveals to us that while they are held together by one 
force, they are kept asunder by another, their position at 
any moment depending on the equilibrium of attraction 
and repulsion. The atoms behave as if connected by 
elastic springs, which oppose at the same time their ap- 
proach and their retreat, but which tolerate the vibration 
called heat. The molecular vibration once set up is in- 
stantly shared with the ether, and diffused bv it through- 
out space. 
