( 2$ ) 



There is every reason to believe that the character of 

 the atmosphere has gradually changed during geologic 

 time, and that various constituents of the mixture have 

 been successively removed from it, and been stored in 

 the solid material of the earth's crust in a state of com- 

 bination. Geological chemistry has shown that the 

 cooling of the earth has been accompanied by the pre- 

 cipitation of many substances only gaseous at high tem- 

 peratures. Hydrochloric and sulphuric acids have been 

 transferred to mineral deposits or aqueous solutions. 

 The removal of carbonic acid gas and the vapor of 

 water has been a process of much slower progress, and 

 after the expiration of all the ages a proportion of both 

 yet remains. Evidence of the abundance of the former 

 in the earliest periods is seen in the vast deposits of 

 limestone rock ; later, in the prodigious quantities of 

 shells which have been elaborated from the same in so- 

 lution. Proof of its abundance in the atmosphere in 

 later periods is seen in the extensive deposits of coal of 

 the Carboniferous, Triassic and Jurassic periods. If the 

 most luxuriant vegetation of the present day takes but 

 fifty tons of carbon from the atmosphere in a century, 

 per acre, thus producing a layer over that extent of less 

 than a third of an inch in thickness, what amount of 

 carbon must be abstracted in order to produce strata of 

 thirty-five feet in depth ? No doubt it occupied a long 

 period, but the atmosphere, thus deprived of a large 

 proportion of carbonic acid, would in subsequent periods 

 undoubtedly possess an improved capacity for the sup- 

 port of animal life. 



The successively higher degree of oxidization of the 

 blood in the organs designed for that function, whether 

 performing it in water or air, would certainly accelerate 



