jflection. 



Absorption, 



o 





 21-5 

 60-4 





 17-20 

 100-0 



Prof. Tyndall's Contributions to Molecular Physics, 519 



I thought it likely that the accord between its periods of vibra- 

 tion and those of the cool aqueous vapour of the atmosphere 

 would still be such as to cause the atmospheric vapour to exert 

 a special absorbent power upon the radiation. The following 

 experiments test this surmise : — 



Table XXVL — Radiation through Atmospheric Air. Source, 

 a hydrogen-flame. 



Dry air . . . 



Undried air . . 

 Total heat . . 



Thus, in a polished tube 4 feet long, the aqueous vapour of our 

 laboratory air absorbed 17 per cent, of the radiation from the 

 hydrogen-flame. A platinum spiral, raised by electricity to a 

 degree of incandescence not greater than that obtainable by 

 plunging a wire into the hydrogen-flame, was used as a source of 

 heat ; of its radiation, the undried air of the laboratory absorbed 



5*8 per cent., 



or one-third of the quantity absorbed when the flame of hydrogen 

 was employed. 



The plunging of a spiral of platinum wire into the flame 

 reduces its temperature; but it at the same time introduces 

 vibrations which are not in accord with those of aqueous vapour : 

 the absorption by ordinary undried air of heat emitted by this 

 composite source amounted to 



8*6 per cent. 



On humid days the absorption of the rays emitted by a hy- 

 drogen-flame exceeds even the above large figure. Employing 

 the same experimental tube and a new burner, the experiments 

 were repeated some days subsequently, with the following 

 result : — 



Table XXVII. — Radiation through Air. Source, hydrogen- 

 flame. 



Absorption. 



Dry air 



Undried air .... 20'3 



The undried air here made use of embraced the carbonic acid 

 of the atmosphere ; after the foregoing experiments, the air was 

 conducted through a tube containing a solution of caustic pot- 

 ash, in which the carbonic acid was intercepted, while the air 

 charged itself with a little additional moisture. The absorption 



