of Emission and Absorption Lines in a Gas-Spectrum. 273 



We now want to form some kind of idea as to what sort 

 of system this atom with its electrons is. Of course any 

 picture which may be imagined cannot in the least be said 

 to depict things as they actually exist. All we do is to 

 imagine some system that will coordinate and explain the 

 majority of the phenomena known to us. For this reason 

 we choose the hypothesis which has the advantage over all 

 others in attempting to explain all physical and chemical 

 phenomena by the properties of the one system, these 

 negative electrons, which appear in the present state of 

 knowledge to be the true " indivisible " atom. But since 

 the chemical atom is, as a whole, electrically neutral, it is 

 necessary to introduce somewhere a positive charge equal in 

 magnitude to the total charge on the electrons. Since there 

 is as yet no evidence as to the distribution of this charge, it 

 is best to adopt an hypothesis which renders most easy the 

 task of deducing the properties of the atom from its structure. 

 It will be supposed, therefore, that the positive electrification 

 is distributed rigidly throughout the volume of the atom, and 

 that the whole atom consists of this volume distribution of 

 electricity throughout which are distributed a number of 

 electrons. The model atom, as pictured by Kelvin and worked 

 with by J. J. Thomson, was assumed to be spherical in form 

 and the distribution of the positive charge to be uniform, but 

 we need not make these special assumptions for our present 

 theory. 



Now all optical phenomena are concerned with the oscil- 

 lation of this complicated electrical system, called an atom, 

 about a configuration of stable equilibrium. The oscillation 

 is that of the electrons, at least when we are dealing with 

 line spectra, and we discuss the subject in terms of electrons. 

 However, any method of procedure in which the motion of 

 the individual electrons is the object of our investigation, is 

 wholly useless when, as in gaseous bodies and liquids, the 

 distribution of the atoms is highly irregular. As Larmor 

 says *, " the development of the theory which is to be in line 

 with experience must instead concern itself with an effective 

 differential element of volume containing a crowd of mole- 

 cules numerous enough to be expressible continuously, as 

 regards their average relations, as a volume density. As 

 regards the actual distribution in the element of volume of 

 the really discrete electrons, all that we can usually take 

 cognizance of is an excess of one kind, positive or negative, 

 which constitutes a volume density of electrification, or else 



* ' JEther and Matter.' 

 Phil Mag. S. 6. Vol. 24. No. HO. Aug. 1912. T 



