90 THE ABSORPTION SPECTRA OF SOLUTIONS. 



Zeeman effect. The electrons that are indicated by the Zeeman effect of arc 

 and spark lines are but slightly affected by external forces, so that it has been 

 very generally assumed that these form parts of systems that make up the 

 fundamental structure of the ultimate units of matter. Indeed, a character- 

 istic spectrum has been used as defining an atom; and, in general, it has been 

 found that a characteristic arc and spark spectrum accompanies matter 

 which, according to other physical and chemical methods, is believed to be 

 composed of exactly the same atomic units. It is for this reason that the 

 assumption is generally made that the absorption and emission centers of 

 these arc and spark spectra are either the same as the atoms of the element, 

 or contain one or more atoms of the element. 



In making an assumption of this kind two of the greatest difficulties 

 encountered are the complexity of the spectra that are found to be character- 

 istic of an element and the fact that the light centers only seem to exist, or 

 at least are only active under certain conditions. The first problem is answered 

 by supposing that if the light center corresponds to the atomic unit of matter, 

 then there are subatomic systems that correspond to the various spectra. 

 Very little evidence of such subatomic systems is at hand, yet the present 

 theory of radioactivity gives very strong support to this view and indicates 

 that, whenever these subatomic systems are separated, entirely new atomic 

 units are formed, and that one atomic unit may consist of several smaller 

 atomic units. The second problem is illustrated by many examples, a good 

 one being that of sodium. The sodium atoms exist under conditions that 

 make them parts of entirely different light centers. The sodium atom may 

 form part of the absorbing or emitting centers of the arc or spark spectra, 

 the emitting centers of a certain vacuum discharge spectrum, the absorbing 

 centers of the fine-banded absorption spectrum, the emission centers of the 

 fluorescent spectrum, etc. The sodium atoms may also exist in various mole- 

 cules and be perfectly transparent in the visible part of the spectrum. Sug- 

 gestions have been made only as to what the constitution of the light centers 

 of these spectra may be, and these suggestions have usually been based on 

 the assumption that these light centers consisted of atomic or molecular 

 complexes. 



Most light centers seem to be formed only during very exceptional 

 physical and chemical conditions, so that they have been often considered 

 as being very unstable; and that an atom or molecule only forms a part of a 

 light center during a small part of the time. This view is strengthened by the 

 theory of absorption and dispersion, according to which it often happens 

 that the number of electrons taking part in the absorption or emission of 

 light is much less than the total number of atoms or molecules present in the 

 matter that is either emitting or absorbing the light. Exactly what conditions 

 are necessary for producing light centers ? Some evidence has been accumu- 

 lated which indicates that these conditions may be furnished by the dissocia- 

 tion or recombination of parts that form complex molecules. These are 

 apparently the conditions under which the light centers of the fine iodine, 

 bromine, sulphur, chlorine, etc., bands exist. 



The length of duration of the fluorescent and phosphorescent bands of 

 solids and liquids furnishes a method of analysis of spectra in that the duration 



