TH/3 CONSTITUTION OP' NA TUUE. 15 



beams also wrench the atoms of carbonic acid asunder, and 

 sacrifice themselves in the act; but when the plants are 

 burned, the amount of heat consumed in their production 

 is restored. 



This, then, is the rhythmic play of Nature as regards 

 her forces. Throughout all her regions she oscillates from 

 tension to vis viva, from vis viva to tension. We have the 

 same play in the planetary system. The earth's orbit is 

 an ellipse, one of the foci of which is occupied by the sun. 

 Imagine the earth at the most distant part of the orbit. 

 Her motion, and consequently her vis viva, is then a 

 minimum. The planet rounds the curve, and begins its 

 approach to the sun. In front it has a store of tensions, 

 which are gradually consumed, an equivalent amount of 

 vis viva being generated. When nearest to the sun the 

 motion, and consequently the vis viva, reach a maximum. 

 But here the available tensions have been used up. The 

 earth rounds this portion of the curve and retreats from 

 the sun. Tensions are now stored up, bukvis viva is lost, 

 to be again restored at the expense of the complementary 

 force on the opposite side of the curve. Thus beats the 

 heart of the universe, but without increase or diminution 

 of its total stock of force. 



I have thus far tried to steer clear amid confusion, by 

 fixing the mind of the reader upon things rather than upon 

 names. But good names are essential; and here, as yet, 

 we are not provided with such. We have had the force of 

 gravity and living force two utterly distinct things. We 

 have had pulls and tensions; and we might have had 

 the force of heat, the force of light, the force of magnet- 

 ism, or the force of electricity all of which terms have 

 been employed more or less loosely by writers on physics. 

 This confusion is happily avoided by the introduction of 

 the term "energy," which embraces both tension and vis 

 viva. Energy is possessed by bodies already in motion; 

 it is then actual, and we agree to call it actual or dynamic 

 energy. It is our old vis viva. On the other hand, 

 energy is possible to bodies not in motion, but which, in 

 virtue of attraction or repulsion, possess a power of motion 

 which would realize itself if all hindrances were removed. 

 Looking, for example, at gravity; a body on the earth's 

 surface in a DOsition from which it cannot fall to a lower 

 one possesses no energy. It has neither motion nor power 



