64 FKAGMENTS OF SCIENCE. 



gases proved themselves unable to emit any sensible 

 amount of radiant heat, the molecules of compound 

 gases were shown to be capable of powerfully disturbing 

 the surrounding ether. By special modes of experiment 

 the same was proved to hold good for the vapours of 

 volatile liquids, the radiative power of every vapour 

 being found proportional to its absorptive power. 



The method of experiment here pursued, though 

 not of the simplest character, is still easy to grasp. 

 When air is permitted to rush into an exhausted tube, 

 the temperature of the air is raised to a degree equi- 

 valent to the vis viva extinguished.* 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 

 vapour, the warmed air communicates its heat by con- 

 tact to the vapour, the molecules of which convert into 

 the radiant form the heat imparted to them by the 

 atoms of the air. By this process also, which I have 

 called Dynamic Eadiation, the reciprocity of radiation 

 and absorption has been conclusively proved.f 



In the excellent researches of Leslie, De la Pro- 

 vostaye and Desains, and Balfour Stewart, the same 

 reciprocity, as regards solid bodies, has been variously 

 illustrated; while the labours, theoretical and experi- 

 mental, of Kirchhoff have given this subject a won- 

 derful expansion, and enriched it by applications of the 

 highest kind. To their results are now to be added 

 the foregoing, whereby gases and vapours, which have 



* See page 15 for a definition of vis viva. 



f When heated air imparts its motion to another gas or va- 

 pour, the transference of heat is accompanied by a change of 

 vibrating period. The Dynamic Radiation of vapours is ren- 

 dered possible by this transmutation of vibrations. 



