Geological History of the Atmosphere. 405 



Instead of saying that there must be a nearly exact balancing 

 of oxygen and oxidizable matter on the earth if there is an 

 excess of oxygen at all, we should have to say that the total 

 quantity of oxygen and of halogens has been adjusted with 

 great exactness to the total amount of other elements — a 

 position which is very similar to the one already discussed. 



It may, however, be argued that though there should be a 

 deficiency of oxygen on the earth, relatively to the other 

 elements, it does not necessarily follow that there ever was a 

 time when there was no free oxygen in the earth's atmosphere. 

 According to usually accepted theories, the temperature of the 

 earth was at one time very high — red, or possibly even white hot 

 throughout. The temperature could therefore quite well have 

 been above the dissociation-point of most oxides or oxygen 

 compounds, and therefore the earth's supply of oxygen, or a 

 large proportion of it, may have existed in the free condition. 

 On cooling down it would unite with those elements for 

 which it had the greatest affinity, and with which it came 

 into most frequent contact ; but being a gas it would all 

 exist in the atmosphere, and would not come into intimate 

 contact with the original liquid nucleus of the earth, which 

 would probably consist principally of heavy metals, or heavy 

 metals combined with more or less sulphur, silicon, carbon, 

 nitrogen, &c. The oxygen would probably therefore combine 

 first with the lighter and more oxidizable elements, e. g. silicon, 

 carbon, aluminium, calcium, magnesium, potassium, and 

 sodium ; and owing to the great abundance of silicon, and its 

 strong affinity for oxygen, a large quantity of silica, or of 

 silicates, would be formed. It is quite possible, then, that 

 before all the oxygen had passed into the combined condition, 

 a thick layer of silicates would be formed which would prevent 

 the remaining oxygen from getting access to the still oxi- 

 dizable iron and other elements lower down. 



This is quite a plausible theory ; but still it does not quite 

 surmount the balancing difficulty. Any one holding it will 

 still have to admit that our total free oxygen bears only a 

 very small ratio to the amount of combined oxygen on the 

 earth ; for the latter is, as we have seen, on the lowest possible 

 computation, about 6000 times as much as the free. The 

 earth's supply of oxygen has therefore combined to the extent 

 of at least 99*98 per cent, of its total amount with oxidizable 

 matters, and as there is still a considerable quantity of oxi- 

 dizable matter within easy reach (geologically speaking) of 

 the remaining oxygen, it is difficult to see why the oxygen 

 should not all have gone into combination by this time, if it 

 were not for the operation of some counteracting influence. It 

 Phil, Mag. S. 5. Vol. 50. No. 305. Oct. 1900. 2 F 



