HEAT. 81 



terns of planets may be so constituted as to communicate 

 forces, inter se, so that forces which have hitherto escaped 

 detection may be in a continuous or recurring state of inter- 

 change. 



The movements produced by mutual gravitation may be 

 the means of calling into existence molecular forces within 

 the substances of the planets themselves. As neither from 

 observation, nor from deduction, can we fix or conjecture any 

 boundary to the universe of stellar orbs, as each advance in 

 telescopic power gives us a new shell, so to speak, of 

 stars, we may regard our globe, in the limit, as surrounded by 

 a sphere of matter radiating heat, light, and possibly other, 

 forces. 



Such stellar radiations would not, from the evidence we 

 have at present, appear sufficient to supply the loss of heat 

 by terrestrial radiations ; but it is quite conceivable that the 

 whole solar system may pass through portions of space hav- 

 ing different temperatures, as was suggested, I believe, by 

 Poisson ; that as we have a terrestrial summer and winter, 

 so there may be a solar or systematic summer and winter, in 

 which case the heat lost during the latter period might be re- 

 stored during the former. The amount of the radiations of 

 the celestial bodies may again, from changes in their positions, 

 vary through epochs which are of enormous duration as re- 

 gards the existence of the human species. 



The views of Mr. Thompson differ from those of Laplace, 

 recently enforced by M. Babinet, which suppose the planets 

 to have been formed by a gradual condensation of nebulous 

 matter. A modification of this view might, perhaps, be sug- 

 gested, viz. that worlds or systems, instead of being created 

 as wholes at definite periods, are gradually changing by at- 

 mospheric additions or subtractions, or by accretions or dim- 

 inutions arising from nebulous substance or from meteoric 

 bodies, so that no star or planet could at any time be said to 

 be created or destroyed, or to be in a state of absolute stabil- 

 4* 



