THE CONSTITUTION OF NATURE 11 



of their component parts. Under the operation of this 

 force 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 

 combustion 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-lumi- 

 nous matter, we have a source of heat more powerful than 

 could be derived from any terrestrial 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 ques- 

 tion 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 mo- 

 tion, 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 



