THE PERMIAN PERIOD. 671 



almost the only ones that are radiated from the earth at its ordinary tempera- 

 tures, and it is the retention of these upon which the surface temperature of 

 the earth chiefly depends. It is only by the masterly work of Langley and others, 

 that instrumental devices have been invented capable of exploring the lower 

 end of the spectrum which is formed of these rays, and even these devices have 

 not yet reached a sufficient degree of refinement to completely separate the indi- 

 vidual effects of the gases. There is a further difficulty in the feebleness of the 

 rays emanating from bodies at the temperature of the earth's surface. Higher 

 temperatures, as that of boiling water, have therefore usually been employed 

 in experiments; but as the proportion of short wave-lengths increases with the 

 temperature, most of the experiments are not directly applicable to the prob- 

 lem in hand, and, besides, the results are not very consistent at best. 1 It is 

 known that the same amount of moisture absorbs more radiation when near 

 saturation than when much below it, which is perhaps due to a larger propor- 

 tion of carbonic acid (H 2 COj) in proportion to carbonic anhydride and water- 

 vapor (C0 2 +H 2 0). The thermal efficiency of carbonic acid proper (H 2 C0 3 ) 

 has not been determined. For the present it is therefore necessary to accept 

 rude approximations only. 



Two modes of absorption of radiation are recognized, general and selective. 

 When a molecule takes up those vibrations which it itself gives forth, when 

 agitated, the absorption is selective. Gas composed of a single kind of mole- 

 cules takes up the vibrations proper to it, as the string of a musical instrument 

 takes up and reproduces the vibrations to which it is attuned. Other vibra- 

 tions, for the greater part, pass through it without interference, but not wholly 

 so. In partial and irregular ways, not fully understood, some parts of the vibra- 

 tions of other lengths are absorbed. This incidental or irregular action con- 

 stitutes general absorption. The nitrogen of the air does not absorb selectively 

 much of the radiation of the sun or of the earth's surface, both of which pass 

 through it with little modification, but not wholly without general absorption. 

 Oxygen absorbs a larger number of known wave-lengths in the visible spectrum, 

 but the amount of heat energy involved is not very large. Oxygen does not 

 appear to absorb the long wave-lengths of the heat radiated from the earth to 

 any notable extent. Carbon dioxide, on the other hand, absorbs certain bands 

 of long wave-length rays whose energy is large. Arrhenius has lately deter- 

 mined experimentally the absorptive capacity of carbon dioxide for rays emitted 

 by a body at 15° C, about the average temperature of the earth's surface, as 

 follows : 2 



Amount of absorption 1% 2% 5% 10% 15% 20% 25% 30% 



Amount of C0 2 traversed in cm. (at 



15° C. and 760 mm. air pressure)... 0.6 1.3 5 20.7 60 142 300 580 



1 Paschen, Wied. Annalen, Vol. LIII, 1894; Angstrom, Ann. Phys. u. Chem. 3, 

 1900; Rubens and Aschkinass, Ann. Phys. u. Chem., 64,1898; Arrhenius, Kosmische 

 Physik, II, 1903. 



2 Kosmische Physik, II, p. 503. 1903. 



