in the Air upon the lemperature of the Ground, 23i) 



The selective absorption of the atmosphere is, according to 

 the researches of JTyndall, Lecher and Pernter, Rontgen, 

 Heine, Langley, Angstrom, Paschen, and others *, of a 

 wholly different kind. It is not exerted by the chief mass of 

 the air, bnt in a high degree by aqueous vapour and carbonic 

 acid, which are present in the air in small quantities. 

 Further, this absorption is not continuous over the whole 

 spectrum, but nearly insensible in the light part of it, and 

 chiefly limited to the long-waved part, where it manifests 

 itself in very well-defined absorption-bands, which fall off 

 rapidly on both sides f. The influence of this absorption 

 is comparatively small on the heat from the sun, but must 

 be of great importance in the transmission of rays from 

 the earth. Tyndall held the opinion that the water- vapour 

 has the greatest influence, whilst other authors, for instance 

 Lecher and Pernter, are inclined to think that the carbonic 

 acid plays the more important part. The researches of 

 Paschen show that these gases are both very effective, so that 

 probably sometimes the one, sometimes the other, may have 

 the greater effect according to the circumstances. 



In order to get an idea of how strongly the radiation of the 

 earth (or any other body of the temperature -j-15° C.) is 

 absorbed by quantities of water-vapour or carbonic acid in 

 the proportions in which these gases are present in our 

 atmosphere, one should, strictly speaking, arrange experi- 

 ments on the absorption of heat from a body at 15° by means 

 of appropriate quantities of both gases. But such experiments 

 have not been made as yet, and, as they would require very 

 expensive apparatus beyond that at my disposal, I have not 

 been in a position to execute them. Fortunately there are 

 other researches by Langley in his work on ' The Temperature 



For ultra-violet rays the absorption becomes extremely great in 

 accordance with facts. 



As one may see from the probable errors which I have placed alongside 

 for the least concordant values and also for one value (1*50 ji), where 

 the probable error is extremely small, the differences are just of the 

 magnitude that one might expect in an exactly fitting formula. The 

 curves for the formula and for the experimental values cut each other at 

 four points (l/A=2-43, 1-88, 1-28, and 0'82 respectively). From the 

 formula we may estimate the value of the selective reflexion for those 

 parts of the spectrum that prevail in the heat from the moon and the 

 earth (angle of d3viation=38 -36°, \=104-24-4 /*). We find that the 

 absorption from this cause varies betweeen 0*5 and 1 p. c. for air-mass 1. 

 This insensible action, which is wholly covered by the experimental 

 errors, I have neglected in the following calculations. 



* Vide Winkelmann. Handbuch der Physik. 



t Of., e.g., Trabert, Meteorologische Zeitschrift, Bd. ii, p. 238 (1894). 



S 2 



