RADIATION 69 



the phenomena of absorption, does not also manifest its 

 power in the phenomena of radiation. For the exami- 

 nation 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 various 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 radia- 

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

 needle connected with the pile declared the energy of the 

 radiation. 



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 them- 

 selves 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 spe- 

 cial 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 



