RADIATION. 63 



all cases it was the transference of motion from the 

 aether to the comparatively quiescent molecules of the 

 gas or vapour that occupied our thoughts. We have now 

 to change the form of our conception, and to figure these 

 molecules not as absorbers but as radiators, not as the re- 

 cipients but as the originators of wave-motion. That is 

 to say, we must figure them vibrating, and generating in 

 the surrounding aether undulations which speed through 

 it with the velocity of light. Our object now is to 

 enquire whether the act of chemical combination, 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 

 vapours to the same temperature, and then examine 

 their power of discharging the motion thus imparted to 

 them upon the aether 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 con- 

 taining 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 radiation 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 

 selfsame molecular arrangement which renders a gas a 

 powerful 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 



