164 LIFE ON THE EARTH. 



in chemical composition, and always on the average 

 charged to the same degree with aqueous vapour. 

 On the contrary, to take an example which appears 

 decisive, from the accumulation of the Coal Strata, 

 there is good reason to adopt positively the opinion 

 that the chemical constitution (if that may be 

 termed a chemical constitution which is only a me- 

 chanical mixture) of the atmosphere has been greatly 

 altered. For if the Carbon, fixed in the thick and 

 extensive beds of coal since the Palaeozoic ages, 

 were again restored to the atmosphere from which 

 it was taken, the weight of Carbonic acid now in 

 the atmosphere (_J^.th part) would be more than 

 doubled. Those who think the proportion of the 

 three main constituents of the atmosphere must ever 

 have been as they are now, may if they please 

 double also the Oxygen and Nitrogen, and thus aug- 

 ment the total barometric pressure of the early 

 Palaeozoic ages to 60 inches ! But without adopting 

 such an extreme view, there is really no reason to 

 limit our theory of the ancient atmosphere in respect 

 of Oxygen or Nitrogen any more than in regard to 

 Carbonic acid. The whole atmosphere may have 

 weighed more ; if so its measured depth must have 

 been greater, and its effect in restraining the waste 

 of heat, and, what is equally important, in reducing 

 the extremes of climatal difference, also greater. 



