Thenomena in the Solar System, 447 



cal and simultaneous point of departure we can understand that 

 their cooling has taken place at a rate nearly proportioned to 

 their volumes. That of the moon being about two hundredths the 

 volume of the earth, its temperature, if we admit an equal con- 

 ductibility, will have decreased with a rapidity fifty times greater, 

 so that the geological epochs of the moon will have been in the 

 same proportion shorter than the corresponding epochs on the 

 earth, up to the time when the solar heat began to be an appre- 

 ciable element. The moon has then advanced much more 

 rapidly than the earth in the series of phenomena through which 

 both must pass, and we may therefore logically suppose that our 

 globe will one day offer the same general characters as are now 

 presented by the moon. 



We believe then that the water which covers the surface of 

 the earth, and the air which surrounds it will one day disappear, 

 as a necessary consequence of the complete cooling of the inte- 

 rior of our planet. Rocks, with few exceptions, readily absorb 

 moisture, and the more crystalline varieties are the most porous ; 

 we need not, however, consider the quantity of water which rocks 

 may imbibe in this way, for the total amount of this element on 

 the earth's surface is so small when compared with the whole 

 mass of the globe, that the ordinary processes of chemical analysis 

 would not detect its presence. If we take the mean depth of the 

 ocean at 600 meters (= 1968 feet), its weight will be equal to 

 one twenty-four-thousancith of the earth, which being reduced to 

 decimals, would give for 100 parts, 



Earth, 99-9958 



Water, -0042 



In the Bulletin of the Geol. Society of France, (2d series, vol. 

 X, p. 131,) Durocher has published a series of experiments made 

 to determine the quantity of water in those minerals which enter 

 into the structure of rocks, such as the feldspars, micas, horn- 

 blende and pyroxene, and which are regarded as anhydrous in 

 composition. These minerals were reduced to coarse powder and 

 exposed to moist air, the proportion of water being determined 

 both before and after ; it will be sufficient for our purpose to 

 give the amount of water found after exposure. The orthoclase 

 of Utoe absorbed in this way 0*41 for 100 parts, while the mean 

 of seven other varieties of the same species was 1*28, and that of 

 thirty specimens of various substances 1*2 7. We have already 

 seen that if the whole of the ocean were to be equally distrib- 



