14 A STUDY OF THE ABSORPTION SPECTRA. 



tral vibrations that can be produced by components that exist or may be 

 produced from the apparently simple U0 2 group: 



(1) We have the absorption spectrum described above. At low tem- 

 peratures most of these bands break up into much finer bands. 



(2) The uranyl salts under various methods of excitation emit a phos- 

 phorescent spectrum of a large number of rather fine bands throughout the 

 visible region of the spectrum. It is quite possible that this spectrum is 

 intimately connected with that of the absorption spectrum. 



(3) We have the absorption spectrum of the uranous salts which has 

 been described above. This spectrum has been probably caused by the 

 change of valency of the uranium atom. Uranium is known to form quite 

 a large number of oxides, and it is quite possible that for each valency of 

 the uranium we have a characteristic spectrum. (This also is being inves- 

 tigated.) It is also quite probable that at lower temperatures those spectra 

 would consist of quite fine bands. 



(4) We have the spark spectrum and the absorption spectrum of 

 oxygen, and 



(5) that of ozone, which bears no relation to that of oxygen. 



(6) There is the exceedingly complex spark spectrum of uranium, con- 

 sisting of thousands of fine lines, and also 



(7) the complex arc spectra. From radioactive experiments it is 

 known that uranium is continually breaking down into ionium. 



(8) Ionium possesses the properties of a chemical element and most 

 likely has a spectrum of its own. This would make eight spectra. 



(9) Ionium breaks down into radium and radium has a very charac- 

 teristic spark spectrum, as does also 



(10) the radium emanation. During the various radioactive trans- 

 formations several a-particles are emitted with a velocity almost as great 

 as that of light. It is probable that these particles are moving with very 

 great velocities in the uranium atom under ordinary conditions. 



(11) The a-particles are known to be charged helium atoms and there- 

 fore under proper excitation would give the helium spectrum. The radium 

 emanation breaks down into radium A, B, C, D, E, and F. These products 

 behave like chemical elements and probably have characteristic spectra. 



(12) The final product is lead, which has very complex spark and arc 

 spectra. During these transformations several electrons have been thrown 

 off from the various products with enormous velocities. In a very large 

 number of the above spectrum lines the Zeeman effect indicates the pres- 

 ence of negative electrons and charged doublets. 



We thus see what an extremely complex system the group U0 2 must 

 be, and it might seem almost hopeless to disentangle the mystery of its 

 various spectra. At present we know that the arc- and spark-spectra problem 

 is very complex and that we have very few methods of producing any 

 changes in them. Practically the only method of changing the frequency 

 of these vibrations is by applying a very powerful magnetic field or great 

 pressure and these changes in the frequency are very small. One very 

 important result, however, has been obtained by Kayser, Runge, Wood, 

 and others. This work consists in separating spectrum lines into various 



