736 COSMICAL PHYSICS 



are radiation, heat conduction is too small a factor to come here 

 into consideration, and the addition of heat through convection 

 currents from warmer surroundings; the horizontal currents, so 

 far as our observations go, become more significant the greater the 

 altitude. 



Here, again, is a field for the application of physics. This shows 

 that the gases which are the chief constituents of the atmosphere, 

 oxygen, nitrogen, and argon, absorb practically no heat, oxygen 

 shows some weak, so-called telluric lines, in the sun's spectrum. On 

 the other hand, carbonic acid gas, and, in a still higher degree, 

 water-vapor, which enter to some extent into the constitution of 

 the atmosphere, possess a remarkable capacity for absorption of 

 the non-luminous heat radiation. They thus effect a moderating 

 influence upon the climate. This is a well-known fact, which is quite 

 evident when the daily variation in temperature at a dry place, 

 e. g., in the desert, is compared with that at a damp place, e. g., on 

 an oceanic island. About a hundred years ago, Fourier and Pouillet 

 showed that the air acts in a similar way to the glass of a hothouse 

 bed. This is true for water-vapor, and carbonic acid gas, and also 

 for a few other gases which enter to a less degree into the constitu- 

 tion of the atmosphere, namely, ammoniac, and the hydrocarbons. If 

 the quantity of these gases in the air increases, the hothouse action 

 also increases, and the temperature of the earth's surface is increased. 

 Furthermore, the warming of the earth's surface through direct 

 radiation from the sun is diminished, while that of the air is in- 

 creased. The vertical circulation in the lowest layers of the atmo- 

 sphere would, by virtue of the absorption, be decreased; on the other 

 hand, the horizontal circulation in the higher layers would be in- 

 creased, whereby the differences between the temperatures of the 

 air at various places of the earth would decrease. 



As a matter of fact, geology teaches us that, in earlier times, for 

 the last time in the Tertiary period, the temperature of the air was 

 not only much higher than now (in the Tertiary period about 10 C.), 

 but also that it was much more uniformly distributed. In order 

 to explain this, there has been previously found no more plausible 

 ground than the assumption that there has been a change in the 

 content of the atmosphere with respect to the heat-absorbing gases, 

 and, in this connection, one thinks first of carbonic acid gas. 

 Through the increase of heat, the content of the air with respect 

 to water- vapor is greater, and the effect is increased. In a similar 

 way, the lower temperature of the ice age, through the decrease in 

 the heat-absorbing constituents of the atmosphere, may be explained. 



Before accurate calculations can be made, ' there is needed an 

 accurate spectrum analysis investigation, particularly in the ultra- 

 red spectrum, of the gases which are important in this connection. 



