CHEMISTRY OF THE EARTH. 191 



influence of tlie same kind, allowing tlie sun's rays to reach tbe earth, 

 but preventing to a great extent the loss by radiation of the heat thus 

 comuuinicated. When, however, the supply of heat from the sun is in-* 

 terrupted at night, the radiation which goes on into space causes the 

 precipitation of a great part of the watery vapor from the air, and the 

 earth, being thus deprived of its protecting shield, becomes moie and 

 more rapidly cooled. If now we could suppose the atmosphere to be 

 mingled with some permanent gas which should possess an absorptive 

 power like that of aqueous vapor, this cooling process would be in a 

 great measure arrested, and an effect would be produced similar to that 

 of a screen of glass, which keeps up the temperature beneath it, both 

 directly by preventing the escape of radiant heat, and indirectly by 

 hindering the condensation of the aqueous vapor in the air conlined be- 

 neatli. Such a gas is carbonic acid, and the large amount of it which 

 existed in the atmosphere during former geological periods must have 

 aided greatly to maintain the elevated temperatures which then existed 

 at tlie earth's surface. Without doubt the greater extent of sea and the 

 absence or rarity of high mountains contributed much to the mikl cli- 

 mate of former geologic ages; but to these must be added the iulluence 

 of the whole of the carbon since condensed in the forms of carbonate of 

 lime and coal, which then existed as a transparent and permanent gas 

 mingled with the atmosphere surrounding the earth, and protecting it 

 like a dome of glass. To this effect of carbonic acid it is possible that 

 other gases may have contributed. The ozone which is mingled with 

 the oxygen set free from growing plants, and the marsh-gas which is now 

 evolved from decomposing vegetation, may, by their absorptive powers, 

 which are far greater than that of carbdtiic acid, have contributed 

 greatly to maintain a high temperature at the earth's surface in early 

 times.* 



§ 11). The part which vegetation has played in the chemical history of the 

 globe has not been limited to the purification of the atmosphere. It 

 seems to have been the great agent through which solar force has effected 

 a partial deoxidation of the thoroughly burned or oxidized materials of 

 the primitive world. By means of growing plants- carbonic acid and 

 water are reduced, giving rise to the various forms of carbon and to 

 hydrocarbonaceous bodies, and these have been the agents by which the 

 sulphates of the metals have been deoxidized, and sulphur, native 

 metals, and metallic sulphides produced. It is moreover by the reducing 

 action of decaying organic matters that the peroxide of iron is partially re- 

 duced and removed in a soluble form from sediments, to b6 afterwards 

 deposited in the form of iron ore. The evidences of this reducing and 

 dissolving action of organic matter are met with not only in the fire- 

 clays and iron- stones of the carboniferous system, and among secondary, 

 tertiary, and modern deposits, but on a grand scale in the Laurentiau 

 system, where great thicknesses of sediments are found almost destitute 

 of iron, while beds of iron ore more extensive than at any subsequent 

 periods are evidences of the abundance of organic matters at that early 

 time. If these are not more frequently preserved in the form of an- 

 thracite and graphite, it is because the amount of peroxide of iron dif- 

 fused through the sediments of tbe period furnished the oxygen neces- 

 sary for the oxidation of the carbon. Inasmuch as the ores of these old 

 rocks, in their present forms of hematite and magnetite, are very insolu- 

 ble, aud represent so much iron withdrawn from the terrestrial circula- 

 tion, it is evident that the proportion of this element, existing in a dif- 



* T. S. Hunt, Ou the Earth's Climate, etc., American Journal of Science, [2,] xxxvi, 396, 

 1863. 



