RADIATION. G3 



intercepted by those molecules in various degrees. lu 

 all cases it was the transference of motion from the 

 ether 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 recipients but as the originators of wave-motion. 

 That is to say, we must figure them vibrating, and gen- 

 erating in the surrounding ether undulations which 

 speed through it with the velocity of light. Our object 

 now is to enquire whether the act of chemical combina- 

 tion, 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 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 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 



