IV.] THE CHEMISTRY OF THE PRIMEVAL EARTH. 47 



ordinary temperature absorbs more than five hundredths of the heat 

 radiated from a metallic vessel filled with boiling water, and Tyndall 

 calculates that of the heat radiated from the earth's surface warmed 

 by the sun's rays, one tenth is intercepted by the aqueous vapor 

 within ten feet of the surface. Hence the powerful influence of 

 moist air upon the climate of the globe. Like a covering of glass, 

 it allows the sun's rays to reach the earth, but prevents to a great 

 extent the loss by radiation of the heat thus communicated. 



When, however, the supply of heat from the sun is interrupted 

 during long nights, the radiation which goes on into space causes 

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

 and the earth, thus deprived of this protecting shield, becomes 

 more and more rapidly cooled. If now we could suppose the at- 

 mosphere to be mingled with some permanent gas, which should 

 possess an absorptive power like that of the vapor of water, 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, directly, by preventing the escape of 

 radiant heat, and indirectly by hindering the condensation of the 

 aqueous vapor in the air confined beneath. 



Now we have only to bear in mind that there are the best of 

 reasons for believing that, during the earliest geological periods, all 

 of the carbon since deposited in the forms of limestone and of 

 mineral coal existed in the atmosphere in the state of carbonic acid, 

 and we see at once an agency which must have aided greatly to 

 maintain the elevated temperature that then prevailed at the earth's 

 surface.* Without doubt the great extent of sea, and the absence 



* [The carbonic acid contained in a layer of pure carbonate of lime or mar- 

 He, covering the entire surface of the globe, and having a thickness of 8.61 

 metres, would, if set free, double the weight of the atmosphere. (Canadian 

 Naturalist (2), III. 119.) It is probable that the amount of carbonic acid 

 thus fixed in the earth's crust must surpass this many times, but from the 

 activity of chemical forces then prevailing, the greater part of this was doubt- 

 less fixed in the form of carbonate of lime at a very early period in the history 

 of the globe, so that the atmosphere in the palaeozoic age may not have con- 

 tained more than a few hundredths of carbonic acid. It must not be sup- 

 posed that the whole of the vast deposits of limestone which have since been 

 formed are directly and immediately due to the reaction of carbonic acid on 

 the alkaline and earthy silicates of the rocks. A large part of the carbonate 

 of lime deposited in later times was doubtless derived from the solution of 

 the limestones of pre-existing formations. It nevertheless remains true that 

 a reaction between the carbonic acid of the atmosphere and mineral silicates, 

 similar to that of early times, though small in amount, is still going on at 

 the earth's surface. (Ante, pages 10 and 20.)] 



