ON THE ORIGIN OF THE PLANETARY SYSTEM. 177 



radiation is equal to the constant exertion of 7,000 

 horse-power for each square foot of the sun's surface. 



For a long time experience had impressed on our 

 mechanicians that a working force cannot be produced 

 from nothing ; that it can only be taken from the 

 stores which nature possesses ; which are strictly limited 

 and which cannot be increased at pleasure whether it 

 be taken from the rushing water or from the wind ; 

 whether from the layers of coal, or from men and from 

 animals, which cannot work without the consumption 

 of food. Modern physics has attempted to prove the 

 universality of this experience, to show that it applies 

 to the great whole of all natural processes, and is inde- 

 pendent of the special interests of man. These have 

 been generalised and comprehended in the all-ruling 

 natural law of the Conservation of Force. No natural 

 process, and no series of natural processes, can be 

 found, however manifold may be the changes which 

 take place among them, by which a motive force can 

 be continuously produced without a corresponding con- 

 sumption. Just as the human race finds on earth but 

 a limited supply of motive forces, capable of producing 

 work, which it can utilise but not increase, so also 

 must this be the case in the great whole of nature. 

 The universe has its definite store of force, which 

 works in it under ever varying forms ; is indestructible, 

 not to be increased, everlasting and unchangeable lifca 



II. N 



