2 4 o THE POPULAR SCIENCE MONTHLY 



THE ABSOBPTION AND EMISSION CENTEKS OF 

 LIGHT AND HEAT. 



By Dr. W. W. STRONG 



UNIVERSITY OF PITTSBURGH 



THE mechanical motions of nature are transmitted by solids and 

 fluids from sources that consist of more or less well known me- 

 chanical systems. "Waves on a pond may be due to a boat moving over 

 the surface of the water. Sound waves in air may be due to the vibra- 

 tions of a tuning fork. Wireless telegraph waves may be due to high 

 frequency electromotive force and current waves in electrical circuits. 

 In general the source of the above type of wave motion is a kind of 

 mechanism that can be made in the laboratory or in the shop — a mechan- 

 ism that is man-made and whose operation is quite obvious to us. 



The phenomena of light and radiant heat introduce to us a type of 

 wave motion that is altogether different. Not only may the medium 

 that transmits this wave motion possess entirely different properties 

 from that of matter, but the mechanisms that take part in the emission 

 and the absorption of the wave motion are altogether different from any 

 that we have been able to make in our laboratories. No one has suc- 

 ceeded in producing radiant heat, much less visible light and ultraviolet 

 radiations by means of electromagnetic oscillators, although such a feat 

 may be possible. 



Inasmuch as matter is the source of all heat and light radiations, 

 the mechanism responsible for the emission and absorption of these 

 radiations must be intimately related to the nature and constitution of 

 matter itself and therefore theories of emission and absorption systems 

 depend to a large extent upon our theories of the nature of atoms and 

 molecules. It must be remembered, however, that the nature and con- 

 stitution of atoms and molecules that explain chemical and many other 

 phenomena need not necessarily be at all related to the systems taking 

 part in heat and light radiations. 



In the past many different hypotheses have been advanced to extend 

 the atomic and molecular conceptions of Dalton, Clausius, Maxwell 

 and others. As long as the elastic solid theory of the ether prevailed, 

 it was frequently assumed that the vibrating systems emitting light and 

 radiant heat were of a mechanical nature due to the development of 

 stresses and strains. The electromagnetic theory of Maxwell, while 

 not even suggesting the nature of the mechanism, metamorphosed our 

 views of radiant energy and indicated the whole phenomenon to be an 

 electromagnetic one. 



