THE CONSTITUTION OF NATURE. 13 



The action of gravity upon matter originally cold may, 

 in fact, be the origin of all light and heat, and the proxi- 

 mate 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 masses, is de- 

 stroyed, but it is really continued as a motion of their 

 ultimate particles. It is this motion, taken up by the 

 ether, and propagated through it with a velocity of one 

 hundred and eighty-five thousand 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, 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 de- 

 pending on the equilibrium of attraction and repulsion. The 

 atoms are virtually connected by elastic springs, which op- 

 pose at the same time their approach and their retreat, but 

 which tolerate the vibration called heat. When two bod- 

 ies drawn together by the force of gravity strike each other, 

 the intensity of the ultimate vibration, or, in other words, 

 the amount of heat generated, is proportional to the vis 

 viva destroyed by the collision. The molecular motion 

 once set up is instantly shared with the ether, and diffused 

 by it throughout space. 



We on the earth's surface live night and day in the 

 midst of ethereal commotion. The medium is never still. 

 The cloud-canopy above us may be thick enough to shut 

 out the light of the stars, but this canopy is itself a warm 

 body, which radiates its motion through the ether. The 

 earth also is warm, and sends its heat-pulses incessantly 

 forth. It is the waste of its molecular motion in space 



