November 16, 1900.] 



SCIENCE. 



749 



hankered after a metallic nucleus for the 

 earth, composed chiefly, by analogy with 

 meteories, of iron. Lord Kelvin has ad- 

 mitted the probable existence of some such 

 nucleus, and lately Professor Wiechert has 

 furnished us with arguments — ' powerful ' 

 arguments Professor Darwin terms them — 

 in support of its existence. The interior 

 of the earth for four-fifths of the radius is 

 composed, according to Professor Wiechert, 

 chiefly of metallic iron, with a density of 

 8.2 ; the outer envelope, one-fifth of the 

 radius, or about 400 miles in thickness, 

 consists of silicates, such as we are familiar 

 with in igneous rocks and meteorites, and 

 possesses a density of 3.2. It was from 

 this outer envelop when molten that the 

 moon was trundled ofi", twenty- seven miles 

 in depth going to its formation. The den- 

 sity of this material, as we have just seen, 

 is supposed to be 3.2 ; the density of the 

 moon is 3.39, a close approximation, such 

 difference as exists being completely ex- 

 plicable by the comparatively low tempera- 

 ture of the moon. 



The outer envelope of the earth which 

 was drawn off to form the moon was, as 

 we have seen, charged with steam and 

 other gases under a pressure of 5,000 lb. to 

 the square inch ; but as the satellite wan- 

 dered away from the parent planet this 

 pressure continuously diminished. Under 

 these circumstances the moon would be- 

 come as explosive as a charged bomb, steam 

 would burst forth from numberless vol- 

 canoes, and while the face of the moon 

 might thus have acquired its existing 

 features, the ejected material might possibly 

 have been shot so far away from its origin 

 as to have acquired an independent orbit. 

 If so we may ask whether it may not be 

 possible that the meteorites, which some- 

 times descend upon our planet, are but 

 portions of its own envelope returning to 

 it. The facts that the average specific 

 gravity of those meteorites which have 



been seen to fall is not much above 3.2, and 

 that they have passed through a stage of 

 fusion, are consistent with this suggestion. 



SECOND CRITICAL PEEIOD. ' CONSISTENTIOR 

 STATUS.' 



The solidification of the earth probably 

 became completed soon after the birth of 

 the moon. The temperature of its surface 

 at the time of consolidation was about 

 1,170° C, and it was therefore still sur- 

 rounded by its primitive deep atmosphere 

 of steam and other gases. This was the 

 second critical period in the history of the 

 earth, the stage of the ' consistentior status,' 

 the date of which Lord Kelvin would 

 rather know than that of the Norman Con- 

 quest, though he thinks it lies between 

 twenty and forty millions of years ago, 

 probably nearer twenty than forty. 



Now that the crust was solid there was 

 less reason why movements of the atmos- 

 phere should be unsteady, and definite re- 

 gions of high and low pressure might have 

 been established. Under the high-pressure 

 areas the surface of the crust would be de- 

 pressed ; correspondingly under the low- 

 pressure areas it would be raised ; and thus 

 from the first the surface of the solid earth 

 might be dimpled and embossed.* 



THIRD CRITICAL PERIOD. ORIGIN OP THE 



OCEANS. 



The cooling of the earth would continu- 

 ously progress, till the temperature of the 

 surface fell to 370° C, when that part of 

 the atmosphere which consisted of steam 

 would begin to liquefy ; then the dimples 

 on the surface would soon become filled 

 with superheated water, and the pools so 

 formed would expand and deepen, till they 

 formed the oceans. This is the third crit- 



■^It would be difficult to discuss with sufficient 

 brevity the probable distribution o£ these inequali- 

 ties, but it may be pointed out that the moon is pos- 

 sibly responsible, and that in more ways than one, 

 for much of the existing geographical asymmetry. 



