300 CLASSICS OF MODERN SCIENCE 

 as regards their planets ; it is, perhaps, due to the action of the mighty 

 ebb and flow to which these bodies, in the time of their fiery fluid 

 condition, were subjected. 



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 tempera- 

 ture of the surface, the heat of the sun is the only thing which es- 

 sentially 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 ten feet thick would give 

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

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

 uniformly from the entire mass of the sun, its temperature would only 

 be diminished thereby one and one-third 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 in- 

 form 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 takes 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 conjecture, and the store 

 of heat there existing can only be determined by very uncertain 

 estimations. If, however, we adopt the very probable view, that the 



