RADIATION. 47 



object now is to inquire whether the act of chemical com- 

 bination, which proves so potent as regards the phenomena 

 of absorption, does not also manifest its power in the 

 phenomena of radiation. For the examination of this 

 question it is necessary, in the first place, to heat our gases 

 and vapors to the same temperature, and then examine 

 their power of discharging the motion thus imparted to 

 them upon the ether in which they swing. 



A heated copper ball was placed above a ring gas-burner 

 possessing a great number of small apertures, the burner 

 being connected by a tube with vessels containing the vari- 

 ous gases to be examined. By. gentle pressure the gases 

 were forced through the orifices of the burner against the 

 copper ball, where each of them, being heated, rose in an 

 ascending column. A. thermo-electric pile, entirely 

 screened from the hot ball, was exposed to the radiation 

 of the warm gas, while the deflection of a magnetic needle 

 connected with the pile declared the energy of the radia- 

 tion. 



By this mode of experiment it was proved that the self- 

 same molecular arrangement which renders a gas a power- 

 ful absorber, renders it a powerful radiator that the atom 

 or molecule which is competent to intercept the calorific 

 waves is, in the same degree, competent to send them 

 forth. Thus, while the atoms of elementary 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 vapors of volatile liquids, 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 easy to grasp. When air is 

 permitted to rush into an exhausted tube, the temperature 

 of the air is raised to a degree equivalent 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 



\ See page 10 for a definition of vis viva. 



