348 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1908. 



atmosphere was many times larger than at present, was rich in car- 

 bonic acid, and had no free oxygen, may be inapplicable to any part 

 of geological time ; though they may very likely be true for the first 

 formed atmosphere, long before the date of the oldest known sedi- 

 mentary rocks. From the earth's surface we look up through zones 

 of atmosphere, in which the oxygen and carbonic acid steadily di- 

 minish, and the minute proportion of hydrogen at sea level increases, 

 until 50 miles high the air consists practically of hj-drogen alone." 

 The aurora flares above us in a mixture of hydrogen and neon ; and 

 as there is evidence of such fundamental variations in the atmos- 

 phere in space, there may well have been marked changes in time. 

 There are so many agents pouring carbonic acid into the air and so 

 many others withdrawing it that it would be strange if the present 

 equilibrium had always been maintained. 



The oceanic control. 



Nevertheless it must not be forgotten that the ocean, as shown by 

 Schloesing,^ supported by the weighty experiments of Dittmar, con- 

 trols the amount of carbonic acid in the atmosphere. If the amount 

 of carbonic acid in the atmosphere is diminished, the bicarbonates in 

 the sea are dissociated ; the gas thus liberated is poured into the air 

 until the former equilibrium between the tension of the carbonic acid 

 in the atmosphere and in the sea is reestablished. Hence, a reduction 

 of carbonic acid in the air is automatically followed by the discharge 

 of nearly as large a quantity from the sea; so that any reduction is 

 distributed between the air and the ocean. Any increase of carbonic 

 acid in the atmosphere is followed by the reverse change, and only 

 one-sixth of the amount poured into the atmosphere is retained there. 

 It is true that great variations in the relative extent of sea and land 

 would affect the dissociation pressure of the bicarbonates in the sea, 

 but it would require a great reduction in the area of sea surface to 

 affect the equilibrium appreciably. 



Possible evidence from paleontology. 



Efforts may be made to ascertain from paleontological evidence 

 whether the atmosphere has recently altered its composition. This 

 line of inquiry does not promise reliable conclusions, owing to the 

 powers of adaptation of both animals and plants to changes in the 

 atmosphere. An increase in carbonic acid, provided it be not accom- 



'^ Sir J. D. Dewar, " The problems of the atmosphere," Proc. Roy. Inst., Vol. 

 XVII, 1902, p. 226. 



^ Schloesing, " Sur la Constance de la proportion d'acide carbouique dans 

 Fair," Compt. lieud,, Vol. 90, 1880, p. 140. 



