190 CHEMISTRY OF THE EARTH. 



The first effect of these dissolved carbonates would be to precipitate 

 the dissolved alumina and the heavy metals, after which came the de- 

 composition of the chloride of calcium and the formation of carbonate of 

 lime and chloride of sodium. This action of carbonic acid is still going 

 on at the earth's surface, slowly breaking down and destroying the 

 hardest rocks, and, aided by mechanical processes, transforming them 

 into clays ; although the action, from the comparative rarity of carbonic 

 acid in the atmosphere, is now less energetic than in earlier times, when 

 the abundance of this gas, and a higher temperature, favored the 

 chemical decomposition of the rocks. But now, as then, every clod of 

 clay formed from the decay of a crystalline rock corresponded to an 

 equivalent of carbonic acid abstracted from the atmosphere, and to 

 equivalents of carbonate of lime and common salt formed from the 

 chloride of calcium of the sea-watei\ 



It is very instructive, in this connection, to compare the composition 

 of the waters of the modern ocean with that of the sea in ancient times, 

 whose composition we learn from the fossil sea- waters which are still to 

 be found in certain regions, imprisoned in the pores of the older stratified 

 rocks, and are the source of many of our saline mineral waters*. These 

 are vastly richer in salts of lime and magnesia than those of the present 

 sea, from which have been separated, by chemical ]>rocesses, all the car- 

 bonate of lime of our limestones, with the exception of that derived from 

 the sub-aerial decay of calcareous and magnesian silicates belonging to 

 the i^rimitive crust. 



§ 18. The gradual removal, in the form of carbonate of lime, of the 

 carbonic acid from the primeval atmosphere, has been connected with 

 great changes in the organic life of the globe. Tlie air was doubtless 

 at first unfit for the respiration of warm-blooded animals, and we find 

 the higher forms of life coming gradually into existence as we approach 

 the present period of a i)urer air. Calculations based upon the proba- 

 ble amount of limestone in the earth's crust, lead us to conclude that 

 the amount of carbon thus removed in the form of carbonic acid has 

 been so enormous, that we must suppose the earlier forms of air-breath- 

 ing animals to have been peculiarly adapted to live in an atmosphere 

 which would probably be too impure to sup})ort modern rei)tilian life. 



Growing plants under the stimulus of light possess, as is well 

 known, the power to absorb carbonic acid, appropriating the carbon and 

 liberating oxygen. The importance of this agencj" in purifying the 

 primitive atmosphere was long since pointed out by Brongniart, and 

 our great stores of fossil fuel have been derived from the decomposition, 

 by the ancient vegetation, of the excess of carbonic acid of the early 

 atmosphere, which, through this agency, was exchanged for oxygen gas. 

 In this connection the vegetation of former i^eriods presents the curious 

 phenomenon of plants allied to those now growing beneath the tropics 

 llourishing within the polar circles. Many ingenious hypotheses have 

 been proposed to account for the warmer climate of earlier times, but 

 are at best unsatisfactory, and it would appear that the true solution 

 of the i^roblem may be found in the constitution of the early atmosphere, 

 w hen considered in the light of Dr. Tyndal's beautiful researches on 

 radiant heat. He has found that the presence of a few hundredths of 

 carbonic acid gas in the atmosphere, while offering almost no obstacle 

 to the passage of the solar rays, would suffice to prevent almost entirely 

 the loss by radiation of obscure heat. 



The aqueous vapor which our atmosphere contains exerts a powerful 



* T. S. Hunt. Contributions to the Chemistry of Natural Waters, Americau Journal 

 of Science, [2. J XXXIX, 184. 



