THE GEOLOGIST. 



its absorptive power from 1 to 85, and even to 136 ; and watery va- 

 pour present in the air at ordinary temperatures produces an absorp- 

 tion of heat represented by 70 or 80. Air saturated with moisture 

 at the ordinary temperature absorbs more" than 5-100ths 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 vapour 

 within 10 feet of the surface. The influence of moist air upon the 

 climate of the globe is like that of 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. During the long 

 nights, however, the radiation which goes on into space causes the 

 precipitation of a great part of the watery vapour of the air ; and the 

 earth, deprived of its protecting shield, becomes rapidly cooled. " If 

 now," says Mr. Hunt, "we could suppose the atmosphere to be min- 

 gled with some permanent gas, which should possess an absorptive 

 power like the vapour 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 in the temperature beneath it 

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

 hindering the condensation of the aqueous vapour in the air beneath." 

 Such a heat-absorbing gas might have existed, and, if geologists are 

 right in their old notions about the abundance of carbonic acid gas 

 in the days of the luxuriant coal-plants, did so ; and Mr. Hunt seizes 

 on this idea at once, and considers there are "the best of reasons for 

 believing that during the earlier 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 he also considers 

 other gases may have aided. " The ozone which is mingled with the 

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

 evolved from decomposing vegetation under conditions similar to 

 those then presented by the coal-fields, may by their very great ab- 

 sorptive power have very well aided to maintain on the earth's sur- 

 face that high temperature, the cause of which has been one of the 

 enigmas of geology." 



So far, very good ; Mr. Hunt seemingly prefers carbonic acid in the 

 air to the supposititious molten mass inside our globe. If there were 

 a former higher climatal temperature of our planet, and such abun- 

 dance of free carbonic acid in our atmosphere, doubtless all would 

 happen just as Mr. Hunt indicates. But was there such an amount 



