LIGHT AND HEAT 243 



enormous absorbing or emitting power, so that only a small part of the 

 matter in a given region is concerned with the liglit and heat emitted 

 or absorbed in the region. Light beams that are sufliciently intense to 

 study are apparently emitted by a very large number of light centers 

 and for this reason it has been found impossible to isolate individual 

 centers; and even if this were possible it may be that the life of these 

 centers is so short that even the isolation of centers would not permit 

 their being studied. Then again, light emission is usually accompanied 

 by many intricate phenomena such as ionization and chemical reactions 

 and this adds to the complexity of the problem. When we consider our 

 profound ignorance respecting even the nature of chemical forces, the 

 constitution of the molecule and atom, the nature of the electric and 

 magnetic fields and even the nature of light itself, it is not at all 

 remarkable that little definite and certain knowledge has been obtained 

 concerning the nature of light centers. 



Some Methods of Appeoachixg the Peoblem 

 There are several avenues of approaching the problem of the nature 

 and constitution of light centers that seem to be extremely inviting. 



1. At the present time a wonderful field is being opened concerning 

 the dynamics of chemical reactions. As chemical reactions are inti- 

 mately related to heat and light effects, the discoveries in this field are 

 bound to give a great deal of information concerning light centers. 



2. A study of the far infrared promises to break the gap between 

 electromagnetic waves and radiant heat and light centers will probably 

 be found to consist of molecular systems vibrating in a way similar to 

 that of the sources of electromagnetic waves. At the present time we 

 can compare light and heat centers with more or less well-known aggre- 

 gates of matter and make as many identifications as possible. 



3. The separation of complex line and band spectra into series of 

 related lines or bands promises to give us a great deal of information 

 ultimately as to the nature of the vibrating centers, although at present 

 the problem is so complex that no one has been able to devise any 

 mechanism or structure that is adequate to explain the known phe- 

 nomena. The theory of Eitz has been one of the most successful so 

 far advanced. 



4. The Zeeman effect obtained by placing the heat and light centers 

 in a magnetic field is important. This effect indicates that many of 

 the centers of spectral lines consists of negative electrons. 



5. The Humphreys-Mohler pressure shift of spectrum lines, the 

 Doppler shift of lines and bands emitted by moving centers as studied 

 by Stark and others, are also very important. 



Possible Structure of Light axd Heat Centers 

 One may picture light and heat centers as consisting in part as 

 follows : 



