ON THE ORIGIN OF THE PLANETARY SYSTEM. 179 



known, and also the quantity of heat produced by the 

 union of known weights of oxygen and hydrogen. 

 Calculation shows that under the above supposition, 

 the heat resulting from their combustion would be 

 sufficient to keep up the radiation of heat from the 

 sun for 3,021 years. That, it is true, is a long time, 

 but even profane history teaches that the sun has 

 lighted and warmed us for 3,000 years, and geology 

 puts it beyond doubt that this period must be ex- 

 tended to millions of years. 



Known chemical forces are thus so completely in- 

 adequate, even on the most favourable assumption, to 

 explain the production of heat which takes place in 

 the sun, that we must' quite drop this hypothesis. 



We must seek for forces of far greater magnitude, 

 and these we can only find in cosmical attraction. We 

 have already seen that the comparatively small masses 

 of shooting-stars and meteorites can produce extra- 

 ordinarily large amounts of heat when their cosmical 

 velocities are arrested by our atmosphere. Now the 

 force which has produced these great velocities is 

 gravitation. We know of this force as one acting on 

 the surface of our planet when it appears as terrestrial 

 gravity. We know that a weight raised from the 

 earth can drive our clocks, and that in like manner 

 the gravity of the water rushing down from the moun- 

 tains works our mills. 



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