230 THE AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 



limitations of time urged upon us. At tir.st thought it would per- 

 haps seem that this alternative would but intensify the limitations. 

 Since the argument for a short history is based on the degree to which 

 the earth is cooled, an original cold state should but hasten the present 

 status. But this neglects an essential factor. The question really 

 hinges on the proportion of potential energy converti))le into heat 

 which remained within the earth when full grown. There is no great 

 difference between the alternative hypotheses so far as the amount of 

 sensible heat at the beginning of the habitable stage is concerned; 

 for, on the one hand, the white-hot earth must have become relatively 

 cool on the exterior before life could begin, and, on the other, it is 

 necessary to assume a sufficiency of internal heat coming from impact 

 and internal compression, or other changes, to produce the igneous 

 and crystalline phenomena which the lowest rocks present. The super- 

 ficial and subsuperficial temperatures in the two cases could not, there- 

 fore, have been widely difierent. 



So far as the temperatures of the deep interior are concerned there 

 is onl}^ recourse to hypothesis. It is probable that there would be a 

 notable rise of temperature toward the center of the earth in either 

 case. In a persistently liquid earth this high central temperature 

 would be lost through convection, but if central crystallization took 

 place at an early stage through pressure much of the high central 

 heat might be retained. In a meteor-built earth, solid from the begin- 

 ning, very much less convectional loss would be suffered, and the cen- 

 tral temperature would probably correspond somewhat closely to the 

 density. The probabilities, therefore, seem somewhat to favor a 

 higher thermal gradient toward the center in the case of the solid 

 meteor-built earth. 



But if we turn to the consideration of potential energy there is a 

 notable difference between the two hypothetical earths. In the liquid 

 earth the material must be presumed to have arranged itself accord- 

 ing to its specific gravity and, therefore, to have adopted a nearly 

 complete adjustment to gravitative demands; in other words, to have 

 exhausted, as nearly as possible, its potential energy — i. e., its "energy 

 of position." On the other hand, in an earth built up by the accretion 

 of meteorites without free readjustment there must have been. initially 

 a heterogeneous arrangement of the heavier and lighter material 

 throughout the whole body of the earth, except only so far as the par- 

 tial liquefaction and the very slow, plastic, viscous, and diffusive 

 rearrangement of the material permitted an incipient adjustment to 

 gi-avitative demands. A large amount of potential energy was there- 

 fore restrained, for the time being, from passing into sensible thermal 

 energy. This potential energy thus restrained is supposed to have 

 gradually become converted into heat as local liquefaction and viscous, 

 molecular, and massive movements permitted the sinking of the heavier 



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