776 ANNUAL REPORT SMITHSONIAN INSTITUTION^ 1908. 



phosphorescing bodies than in gases, and the light emitted less simple; 

 yet Becqiierel was able to unravel definite relations. 



Edmond Becquerel had already shown that if the infra-red rays 

 strike an excited phosphorescing body the phosphorescence is de- 

 stroyed, just as it w^ould have been had the temperature of the body 

 been raised. The extinction is preceded by a temporary increase of 

 the phosphorescence, as if the stored-up energy was given out at a 

 greater intensity during a shorter time. Generally the two. phases 

 occur so rapidly that the final extinction alone is appreciable, Ed- 

 mond Becquerel had thus commenced the study of the infra-red of 

 the spectrum. Henri Becquerel resumed this study, making many 

 important advances. He studied the solar spectrum by means of 

 phosphorescence and described in this infra-red portion unknown or 

 little known bands between the wave lengths, 0.76/a and 1.9/*. Abney, 

 by direct photography, had gone as far as 0.98/x. Langley with the 

 bolometer had explored a much greater region and had recognized 

 the more interesting of these bands of Becquerel. But in the region, 

 relatively small to be sure, where the phosphorescent method is 

 applicable, it could then detect finer lines than the bolometer. Bec- 

 querel studied new absorption and emission spectra; he showed that 

 the liquid- w^ater absorption nearly coincides with that of atmospheric 

 water vapor; that the compounds of didymium and samarium have 

 characteristic lines in the infra-red which may serve as standard 

 marks; finally, he mapped the characteristic lines in the infra-red of 

 the incandescent vapors of potassium, aluminum, zinc, cadmium, 

 lead, bismuth, silver, and tin. 



While some substances lose their phosphorescence nearly uniformly 

 over the infra-red, with others the extinction is unequally rapid in 

 different regions. The extinction is produced under the influence of 

 definite radiations, often to the exclusion of the neighboring regions, 

 so that the spectrum consists of one or more bands where the extinc- 

 tion has been active, separated by regions where it has been either 

 much smaller or nil. Under the influence of the infra-red radia- 

 tions the phosphorescence varies in color with the time. This may 

 be noted in the phosphoroscope. So even in the various infra-red 

 bands in the same substance it w^as found that the color can not always 

 be the same. 



It is interesting to correlate these facts with the very similar be- 

 havior in the violet and ultra-violet. The ensemble of the bands of 

 excitation, of emission, and of extinction must be connected by anal- 

 ogous formulae with the various radiations emitted by incandescent 

 vapors, for both are intimately connected with the vibration periods 

 of the molecules. 



The most remarkable phenomena are shown by the compounds of 

 uranium, and it was the study of these which led to Becquerel's dis- 



