GEOLOGY. 



THE EARTH'S CLIMATE IN PALEOZOIC TIMES. 



The following interesting suggestion relative to the climate that pre- 

 vailed upon our earth during the paleozoic epoch, is communicated to 

 the American Journal of Science by Prof. T. S. Hunt. 



The late researches of Tyndall on the relation of gases and vapors 

 to radiant heat are important in their bearing iipon the temperature of 

 the earth's surface in former geological periods. He has shown that 

 heat, from whatever source, passes through hydrogen, oxygen and ni- 

 trogen gases, or through dry air, with nearly the same facility as 

 through a vacuum. These gases are thus to radiant heat what rock- 

 salt is among solids. Glass and some other solid substances, which are 

 readily permeable to light and to solar heat, offer, as is well-known, 

 great obstacles to the passage of radiant heat from non-luminous bod- 

 ies ; and Tyndall has recently shown that many colorless vapors and 

 gases have a similar effect, intercepting the heat from such sources, 

 by which they become warmed, and in their turn radiate heat. Thus 

 while for a vacuum the absorption of heat from a body at 212 F. is 

 represented by 0, and that for dry air is 1 , the absorption by an at- 

 mosphere of carbonic acid gas equals 90, by marsh gas 403, by olefiant 

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

 ties of ozone present in electrolytic oxygen were found to raise its ab- 

 sorptive 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-hun- 

 dredths 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 in- 

 fluence 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 dur- 

 ing long nights, the radiation which goes on into space causes the pre- 

 cipitation 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 atmosphere to be min- 

 gled 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 

 18* 209 



t 



