THE CONSTITUTION OF NATUBE. 7 



developed by this collision. Mayer first, and Helm- 

 holtz and Thomson afterwards, have calculated 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 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 pro- 

 duce 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 

 enquire 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 velo- 

 city 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 neces- 

 sarily 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 

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