708 



SCIENCE. 



[N. S. Vol. IX. No. 229. 



break the crust above it and burst up 

 thi'ougb it. 



§ 37. But in reality the upper crust can- 

 not have been infinitely strong ; and, judg- 

 ing alone from what we know of properties 

 of matter, we should expect gigantic cracks 

 to occur from time to time in the upper 

 crust tending to shrink as it cools and pre- 

 vented from lateral shrinkage by the non- 

 shrinking uncooled solid below it. When 

 any such crack extends downwards as far 

 as a pocket of mother liquor underlying the 

 wholly solidified crust, we should have an 

 outburst of trap rock or of volcanic lava 

 just such as have been discovered by geolo- 

 gists in great abundance in many parts of 

 the world. We might even have compara- 

 tively small portions of high plateaus of the 

 primitive solid earth raised still higher by 

 outbursts of the mother liquor squeezed out 

 from below them in virtue of the pressure 

 of large surrounding portions of the super- 

 incumbent crust. In any such action, due 

 to purely gravitational energy, the center of 

 gravity of all tlie material concerned must 

 sink, although portions of the matter may 

 be raised to greater heights ; but we must 

 leave these large questions of geological dy- 

 namics, having been only bi-ought to think 

 of them at all just now by our consideration 

 of the earth antecedent to life upon it. 



§ 38. The temperature to which the 

 earth's surface cooled within a few years 

 after the solidification reached it must have 

 been, as it is now, such that the temperature 

 at which heat radiated into space during the 

 night exceeds that received from the sun 

 during the day by the small diflference due 

 to heat conducted outwards from within. * 

 One year after the freezing of the granitic 



* Suppose, for example, the cooling and thiokening 

 of the upper crust has preceeded so far that at the 

 surface, and, therefore, approximately for a few deci- 

 metres below the surface, the rate of augmentation of 

 temperature downwards is one degree per centimeter. 

 Taking as a rougli average 005 c. g. s. as the thermal 

 conductivity of the surface rock, we should have for 



interstitial mother liquor at the earth's sur- 

 face in any locality the average tempera- 

 ture at the surface might be warmer, by 60° 

 or 80° Cent., than if the whole interior had 

 the same average temperature as the sur- 

 face. To fix our ideas, let us suppose at 

 the end of one year the surface to be 80° 

 warmer than it would be with no under- 

 ground heat ; then at the end of 100 years 

 it would be 8° warmer, and at the end of 

 10,000 years it would be -8 of a degree 

 warmer, and at the end of 25 million years 

 it would be 016 of a degree warmer, than 

 if there were no underground heat. 



§ 39. When the surface of the earth was 

 still white-hot liquid all round, at a tem- 

 perature fallen to about 1200° Cent., there 

 must have been hot gases and vapor of 

 water above it in all parts, and possibly 

 vapors of some of the more volatile of the 

 present known terrestrial solids and liquids, 

 such as zinc, mercury, sulphur, phosphorus. 

 The verjr rapid cooling which followed in- 

 stantly on the solidification at the surface 



the heat conducted outwards -005 of a gramme water 

 thermal unit Centigrade per sq. cm. per sec. ( Kelvin 

 Math, and Phys. Papers, Vol. III., p. 226). Hence, 

 if (ibid. p. 223) we take -^-^ji as the radiational em- 

 issivity of rock and atmosphere of gases and watery 

 vapor above it radiating heat into the surrounding 

 vacuous space (iEther), we find 8000 X -005 or 40 de- 

 grees Cent, as the excess of the mean surface tempera- 

 ture above what it would be if no heat were conduc- 

 ted from within'outwards. The present augmentation 

 of temperature downwards may be taken as 1 degree 

 Cent, per 27 meters as a rough average derived from 

 observations in all parts of the earth where under- 

 ground temperature has been observed. (See British 

 Association Reports from 1868 to 1895. The very 

 valuable work of this Committee has been carried on 

 for these twenty-seven years, with great skill, perse- 

 verance and success, by Professor Everett, and he 

 promises a continuation of his reports from time to 

 time.) This, with the same data for conductivity and 

 radiational emissivity as in the preceding calculation, 

 makes 40°/2700 or 0.0148° Cent, per centimeter as the 

 amount by which the average temperature of the 

 earth's surface is at present kept up by underground 

 heat. 



