56 THE ABSORPTION SPECTRA OF SOLUTIONS. 



absorption bands. Knowing something of the electromagnetic forces neces- 

 sary to alter the path of the a particles, it might be possible to find some of 

 the properties of the fields of force that exist in the neighborhood of the ura- 

 nium and thorium atoms, and the correlated effects upon the frequency of 

 the absorption bands. Similar effects might be obtained in the case of the 

 /3 and y rays. The particles of the active deposits might also be used as 

 nuclei about which aggregates would be formed. 



Another possible method of studying aggregates is by the phosphores- 

 cence produced by a and /3 rays. The effect of changes in temperature could 

 also be taken up in this connection. Several workers have concluded that 

 the phosphorescence produced by the bombardment of certain kinds of screens 

 by a rays is not due to the breaking up of crystals along lines or planes of 

 cleavage, but that it is due to the breaking up of aggregates that are of 

 approximately molecular dimensions; and that unless such aggregates are pres- 

 ent, no phosphorescence is produced. For example, pure zinc sulphide is not 

 phosphorescent, but when there are impurities present phosphorescent scin- 

 tillations can be produced. It is probable that in most phosphorescent screens 

 there are a large number of active centers present. When the centers on the 

 surface are destroyed, those farther in the screen will become sources of scin- 

 tillations. When the screen is new, each bombarding a particle probably 

 excites a considerable number of centers. As the bombardment is continued 

 the number of centers excited by each a particle decreases, although each par- 

 ticle will excite one or more centers, so that only the intensity and not the 

 number of scintillations is decreased. The calculated diameters of the active 

 centers of zinc sulphide and willemite are of molecular magnitude, whereas, 

 in the case of barium platinocyanide, the diameter is about a hundred times 

 this size. Marsden 1 has found that the number of scintillations is slightly 

 less at 100 than at 15. 



It would be interesting to prepare salts so that various solvates and aggre- 

 gates would be found, and then study the scintillations produced by them. 

 The above suggestions are made since they are along somewhat different 

 lines than those usually followed in the study of solutions. 



NEODYMIUM CHLORIDE IN ETHYL ACETATE AND ANTHRACENE. 



It was supposed by the older writers on the absorption spectra of solu- 

 tions, that if two colored salts were dissolved in the same solution the fre- 

 quencies of the absorbing centers of the one salt would be modified by the 

 presence of the other salt, if the frequencies of the absorbing centers of the 

 latter were almost the same as those of the former. Several attempts have 

 been made to test this theory by experiment, but thus far no case is known 

 where such an effect is shown, at least by the inorganic salts. 



According to our theory of aggregates we would hardly expect any two 

 absorbing centers to have any mutual effects on each other, unless both centers 

 were part of the same aggregate. It is, however, quite difficult to find any 

 solution containing such aggregates; indeed, it is doubtful if there are any 

 that contain absorbing centers that have bands in the same region of the 



1 Proc. Roy. Soc, 84, A, 548 (1910). 



