2 
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-lOOths 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. Huut, "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 o£ 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 
