ON THE INTERACTION OF NATURAL FORCES. 157 



gravity, acts in the heavenly spaces as gravitation. As terrestrial 

 gravity when it draws a weight downwards performs work and 

 generates vis viva, so also the heavenly bodies do the same when 

 they draw two portions of matter from distant regions of- space 

 towards each other. 



The chemical forces must have been also present, ready to 

 act ; but as these forces can only come into operation by the 

 most intimate contact of the different masses, condensation 

 must have taken place before the play of chemical forces began. 



Whether a still further supply of force in the shape of heat 

 was present at the commencement we do not know. At all 

 events, by aid of the law of the equivalence of heat and work, 

 we find in the mechanical forces existing at the time to which 

 we refer such a rich source of heat and light, that there is no 

 necessity whatever to take refuge in the idea of a store of these 

 forces originally existing. When, through condensation of the 

 masses, their particles came into collision and clung to each 

 other, the vis viva of their motion would be thereby annihilated, 

 and must reappear as heat. Already in old theories it has been 

 calculated that cosmical masses must generate heat by their col- 

 lision, but it was far from anybody's thought to make even a 

 guess at the amount of heat to be generated in this way. At 

 present we can give definite numerical values with certainty. 



Let us make this addition to our assumption that, at the 

 commencement, the density of the nebulous matter was a van- 

 ishing quantity as compared with the present density of the sun 

 and planets : we can then calculate how much work has been 

 performed by the condensation ; we can further calculate how 

 much of this work still exists in the form of mechanical force, 

 as attraction of the planets towards the sun, and as vis viva of 

 their motion, and find by this how much of the force has been 

 converted into heat. 



The result of this calculation 1 is, that only about the 454th 



part of the original mechanical force remains as such, and that 



the remainder, converted into heat, would be sufficient to raise 



a mass of water equal to the sun and planets taken together, 



1 See note on page 172. 



