4 PR A OMENT8 OP 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 by it through- 

 out space. 



