Date of the Glacial and the Upper Miocene Period. 371 



there would of course be no external force or pressure exerted 

 on the mass analogous to that of the piston on the air ; but there 

 would be, what is equivalent to the same, the gravitation of the 

 particles to each other. There would be the pressure of the whole 

 mass towards the centre of convergence. In the case of air, and 

 all perfect gases cooling under pressure, about 234 foot-pounds 

 of the original heat possessed by the gas is given out while 95 

 foot-pounds is generated by condensation. We have, however, 

 no reason whatever to believe that in the case of the cooling of 

 the sun the same proportions would hold true. The propor- 

 tion of original heat possessed by the mass of the sun to that 

 produced by condensation may have been much greater than 

 234 to 95, or it may have been much less. In the absence of 

 all knowledge on this point, we may in the meantime assume 

 that to be the proportion. The total quantity of heat given out 

 by the sun resulting from the condensation of his mass, on the 

 supposition that the density of the sun is uniform throughout, 

 we have seen to be equal to 20,237,500 years 7 sun-heat. Then 

 the quantity of heat given out, which previously existed in the 

 mass as original temperature, must have been 49,850,000 years' 

 heat. In all, 70,087,500 years'' heat as the total amount. 



The above quantity represents, of course, the total amount of 

 heat given out by the mass since it began to condense. But 

 the geological history of our globe must date its beginning to a 

 period posterior to that. For at that time the mass would pro- 

 bably occupy a much greater amount of space than is presently 

 possessed by the entire solar system ; and consequently, before it 

 had cooled down to within the limits of the earth's present orbit, 

 our earth could not have had an existence as a separate planet. 

 Previously to that time it must have existed as a portion of the 

 sun's fiery mass. If we assume that it existed as a globe pre- 

 viously to that, and came in from space after the condensation of 

 the sun, then it is difficult to conceive how its orbit should be 

 so nearly circular as it is at present. 



Let us assume that by the time that the mass of the sun had 

 condensed to within the space encircled by the orbit of the pla- 

 net Mercury (that is, to a sphere having, say, aradius of 18,000,000 

 miles) the earth's crust began to form ; and let this be the time 

 when the geological history of our globe dates its commence- 

 ment. The total amount of heat generated by the condensation 

 of the sun's mass from a sphere of this size to its present volume 

 would equal 19,740,000 years' sun-heat. The amount of ori- 

 ginal heat given out during that time would equal 48,625,000 

 years' sun-heat, — thus giving a total of 68,365,000 years' sun- 

 heat enjoyed by our globe since that period. The total quantity 

 may possibly, of course, be considerably more than that, owing 



