RADIATION. 201 



the radiative power of every vapor being found proportional 

 to its absorptive power. 



The method of experiment here pursued, though not of 

 the simplest character, is still within your grasp. When 

 air is permitted to rush into an exhausted tube, the tem- 

 perature of the air is raised to a degree equivalent to the 

 vis viva extinguished. 1 Such air is said to be dynamically 

 heated, and if pure, it shows itself incompetent to radiate, 

 even when a rock-salt window is provided for the passage 

 of its rays. But if instead of being empty the tube contain 

 a small quantity of vapor, then the warmed air will com- 

 municate heat by contact to the vapor, which will be thus 

 enabled to radiate. Thus the molecules of the vapor con- 

 vert into the radiant form the heat imparted dynamically 

 to the atoms of the air. By this process, which has been 

 called dynamic radiation, the radiative power of both vapors 

 and gases has been determined, and the reciprocity of their 

 radiation and absorption proved. 2 



In the excellent researches of Leslie, De la Provostaye 

 and Desains, and Balfour Stewart, the reciprocity of radia- 

 tion and absorption as regards solid bodies has been vari- 

 ously illustrated ; while the labors, theoretical and experi- 

 mental, of Kirchhoff have given this subject a wonderful 

 expansion, and enriched it by applications of the highest 

 kind. To their results are now to be added the foregoing, 

 whereby gases and vapors which have been hitherto thought 

 inaccessible to experiments of this kind are proved to ex- 

 hibit the duality of radiation and absorption, the influence 

 on both of chemical combination being exhibited in the 

 most decisive and extraordinary way. 



1 See page 20 for a definition of vis viva. 



2 When heated, air imparts its motion to another gas or vapor; the 

 transference of heat is accompanied by a change of vibrating period. The 

 dynamic radiation of vapors is rendered possible by the transmutation of 

 vibrations. 



