Mechanism of Radiation. 441 



done at present. But enough has, perhaps, been said to show 

 that the general arrangement of lines in the spectrum which 

 is predicted by our theory is sufficiently general to include 

 that which is observed, while being at the same time sufficiently 

 definite to afford a test of capability of the atom as we are 

 now imagining it to reproduce the phenomena of the natural 

 atom. 



§ 26. A comparison between the theoretical and observed 

 spectrum suggests that the actual atom may be regarded as 

 approximately made up of: shells of ions of alternate signs, 

 or, still better, of shells of doublets. The same conclusion 

 might have been arrived at from a consideration of the con- 

 ditions of either equilibrium or stability. The evidence of 

 the Zeeman phenomenon — that the predominating vibrations 

 are those of negative ions, as well as the experimental result 

 that negative ions are dissociated from the atom more readily 

 than positive ions (there being no evidence that these latter 

 can be dissociated at all), leads to the conclusion that the 

 outermost layer of ions is of negative sign. 



§ 27. The class of tangential vibrations of frequency 

 pZ—0 which was found for the ideal atom (§15) will be 

 approximately reproduced in the real atom. The correspond- 

 ing values of p 1 will not now be actually zero, but will be 

 very small. 



Hence, in addition to the spectrum already discussed, 

 which has been shown to consist solely of a collection of 

 spectrum-series, there will be a second spectrum, far removed 

 from the former, and at the infra-red end of it. It is 

 not practicable to investigate the positions or arrangement 

 of the lines of this spectrum, since the whole value of p 2 con- 

 sists of a correction arising from the discrete structure of the 

 atom. At the same time the existence of this spectrum might 

 throw some light on the phenomenon of " electromagnetic 

 resonance " discovered by Drude. It is found that resonance 

 can be established between matter (in the case of certain 

 substances at least) and electromagnetic waves of which the 

 frequency is only about a millionth part of the frequencies of 

 the visible spectrum*. 



The Zeeman Phenomenon. 



§ 28. Let us return to the general equations of the ideal 

 atom, found in § 14, and suppose that u, r, w specify a 

 normal vibration under the influence of a magnetic field H, 



* Wied. Ann. lviii. p. 1 ; lix. p. 17; lxiv. p. 131. 



