168 OX THE INTERACTION OF NATURAL FORCES. 



bodies, in the time of their fiery fluid condition, were sub- 

 jected. 



I would not have brought forward these conclusions, which 

 again plunge us in the most distant future, if they were not 

 unavoidable. Physico-mechanical laws are, as it were, the 

 telescopes of our spiritual eye, which can penetrate into the 

 deepest night of time, past and to come. 



Another essential question as regards the future of our 

 planetary system has reference to its future temperature and 

 illumination. As the internal heat of the earth has but little 

 influence on the temperature of the surface, the heat of the sun 

 is the only thing which essentially affects the question. The 

 quantity of heat falling from the sun during a given time upon 

 a given portion of the earth's surface may be measured, and 

 from this it can be calculated how much heat in a given time is 

 sent out from the entire sun. Such measurements have been 

 made by the French physicist Pouillet, and it has been found 

 that the sun gives out a quantity of heat per hour equal to that 

 which a layer of the densest coal 10 feet thick would give out 

 by its combustion ; and hence in a year a quantity equal to the 

 combustion of a layer of 17 miles. If this heat were drawn 

 uniformly from the entire mass of the sun, its temperature 

 would only be diminished thereby 1^ of a degree Centigrade 

 per year, assuming its capacity for heat to be equal to that of 

 water. These results can give us an idea of the magnitude of 

 the emission, in relation to the surface and mass of the sun ; 

 but they cannot inform us whether the sun radiates heat as a 

 glowing body, which since its formation has its heat accumulated 

 within it, or whether a new generation of heat by chemical 

 processes is continually taking place at the sun's surface. At 

 all events, the law of the conservation of force teaches us that 

 no process analogous to those known at the surface of the earth 

 can supply for eternity an inexhaustible amount of light and 

 heat to the sun. But the same law also teaches that the store 

 of force at present existing as heat, or as what may become 

 heat, is sufficient for an immeasurable time. With regard to 

 the store of chemical force in the sun, we can form no conjee- 



