118 A STUDY OF THE ABSORPTION SPECTRA. 



He considers that the first effect of the exciting light is to produce dissoci- 

 ation. This dissociation may be either chemical or electrolytic. Electro- 

 lytic dissociation may consist of a dissociation similar to that taking place 

 in ordinary electrolysis, or in the expulsion of one or more electrons from 

 the molecule. Suppose that an electron is expelled from the atom; the two 

 resulting ions will then have very different mobilities. The electrons will 

 only temporarily attach themselves to molecules on account of their great 

 velocities. The positive ions, on the other hand, will much more likely 

 attach themselves either to the solvent molecules or to other molecules of 

 the active substance. For the reason that the mobility of the positive ion 

 is small, Merritt considers that the combinations which it forms are much 

 more stable than those formed by the electrons. 



As there are different kinds of combinations present there will be 

 different kinds of collisions, and it is in collisions and recombinations 

 of this kind that one is to find the source of the light emitted during 

 phosphorescence. The vibrations corresponding to the different modes 

 of recombination will probably differ in violence, frequency, and radiating 

 power. 



Very important work has been done by Urbain 1 and others upon the 

 phosphorescence of the rare earths. To each one of the elements of the rare 

 earths there corresponds a definite atomic weight, definite arc and spark 

 spectra, and definite absorption and phosphorescent spectra. The oxides of 

 europium, gadolinium, terbium, dysprosium, etc., are not phosphorescent. 

 Mixtures of these compounds are, however, extremely phosphorescent, 

 and in general there is a certain proportion at which the phosphorescence is 

 a maximum. For example, one part of the oxide of europium in two 

 hundred and fifty parts of gadolinium gives a maximum europium phos- 

 phorescence. The phosphorescence in this case is due to the europium 

 atom or molecule, and is not greatly affected by the diluent. However, 

 different diluents as lime or gadolinium do show a slight effect upon the 

 resulting spectra. The temperature of calcination and the acid radical of 

 the diluent also have an influence. For mixtures of europium and calcium 

 compounds there exist two different sets of bands possessing different 

 optima. Mixtures of europium and gadolinium, calcined at 1000 C. and 

 1600 C, give entirely different phosphorescent spectra. Gadolinium is 

 much more effective in exciting phosphorescence than calcium. The spec- 

 tra of different diluents do not change into each other but remain fixed, 

 the intensity being the only variable. 



A considerable amount of work on the phosphorescence of uranyl com- 

 pounds has been done by the Becquerels. E. Becquerel 2 found that the 

 phosphorescent spectra of pure uranyl chloride and of double salts of uranyl 

 and potassium or ammonium were quite different, and that apparently 

 the presence of potassium and ammonium caused the bands to shift towards 

 the longer wave-lengths. The wave-lengths of the bands as given by 

 Becquerel are: 



1 Compt. rend., 142,205; 1518; 143,229; 144,30; 1363; 147, 1472. Soc. Fran, de 



Phye., Feb. 6 (1906) ; July 6 (1906) ; Le Radium, June (1909). 



2 Ann. Chun. Phye. [4], 27, 539-579 (1872). 



