324 earth's climate in paleozoic times. 



olefiant gas 970, and by ammonia 1195. The diffusion of defiant 

 gas of one inch tension in a vacuum produces an absorption of 90, 

 and the same amount of carbonic acid gas, an absorption of 5 # 6. 

 The small quantities of ozone present in electrolytic oxygen were 

 found to raise its absorptive power from 1 to 85 , and even to 136 ; 

 and the watery vapor present in the air at ordinary temperatures 

 in like manner produces an absorption of heat represented by 70 

 or 80. Air saturated with moisture at the 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 gr^at 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 atmosphere to be mingled with some permanent gas, which 

 should posses 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 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 we see at once an agency which must have aided greatly 

 to produce the elevated temperature that prevailed at the earth's 

 surface in former geological periods. Without doubt the great 

 extent of sea, and the absence or rarity of high mountains, contri- 

 buted much towards the mild climate of the carboniferous age, for 

 example, when a vegetation as luxuriant as that now found in the 

 tropics flourished within the frigid zones ; but to these causes must 

 be added the influence of the whole of the carbon which was after- 

 wards condensed in the form of coal and carbonate of lime, and 



