354 POPULAR SCIENCE REVIEW. 
and above this again the vapour of water in enormous 
quantity. 
The third stage in the history of the earth may now be said 
to have commenced, when the earth as a molten sphere sur- 
rounded by a furnace- like atmosphere began to cool down owing 
to the loss of heat radiated from its exterior into space ; by 
degrees a thin crust would commence to form on the surface of 
the molten rock which soon consolidated and extended over the 
exterior of the entire sphere, becoming thicker and thicker over 
the nucleus of molten matter until it offered more and more 
resistance to the passage of heat from within outwards, and thus 
caused the rate of further cooling to diminish greatly, and the 
more so from its being composed of a highly nonconducting 
material. In time, therefore, the external surface of the 
earth would come to be barely red hot, and as soon as this was 
the case we should find it become coated with a layer or incrus- 
tation of the chlorides and other vapours hitherto held in sus- 
pension in the heated atmosphere, but which now owing to the 
lowering of temperature would be condensed and precipitated 
on to the now consolidated crust of the earth. From the 
amount of the salts contained in the ocean and known deposits, 
it has been estimated that the quantity of common salt alone 
would be sufficient to cover the entire sphere with a layer some 
ten feet in thickness. 
As the process of cooling went on, as soon as the tempera- 
ture of the atmosphere had become so lowered as to be below 
that of the boiling point of water, the enormous amount of 
steam hitherto pervading its uppermost regions would naturally 
become condensed into water, and at once fall down from the 
heavens as a deluge of hot rain upon the saline crust covering 
the sphere which it would instantly dissolve, forming the ocean 
which would thus be salt from the very first appearance of 
water upon the face of the globe. 
The atmosphere now freed from the vapours previously 
diffused throughout it, would still be very different from 
what it now is, as although it might contain precisely the same 
gases, these would, however, be present in vastly different pro- 
portions ; it would mainly be composed of an admixture of 
nitrogen and carbonic acid gas, free oxygen if present at all 
being but in very small amount, for it must be remembered that 
the total amount of nitrogen and carbon contained in the entire 
animal, vegetable, and mineral kingdoms of the future, were at 
this moment held suspended in the atmosphere in the gaseous 
form. 
This state of things brings us down to the fourth or last 
stage of the prozoic history of the earth since it required but 
a comparatively short period to lower its temperature sufficiently 
