RADIATION. 47 
object now is to inquire whether the act of chemical com- 
bi nation, 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, 
