. Carnegie Institution of Washington publication. FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 27 THE ABSORPTION SPECTRUM AT ORDINARY TEMPERATURES. The resemblance of the absorption spectra of the uranyl salts to their fluorescence spectra, which is so striking as to have led both E. and H. Becquerel to regard the absorption series as a continuation of the series of fluorescence bands, can be fully investigated only by observations at low temperatures. Since the absorption extends into the ultra- violet, moreover, photographic methods are necessary. The study of the absorption at ordinary
. Carnegie Institution of Washington publication. FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 27 THE ABSORPTION SPECTRUM AT ORDINARY TEMPERATURES. The resemblance of the absorption spectra of the uranyl salts to their fluorescence spectra, which is so striking as to have led both E. and H. Becquerel to regard the absorption series as a continuation of the series of fluorescence bands, can be fully investigated only by observations at low temperatures. Since the absorption extends into the ultra- violet, moreover, photographic methods are necessary. The study of the absorption at ordinary temperatures is, however, not without sig- nificance, and the use of the spectrophotometer in this work brings out certain features not easily discernible in the photographic plates. The salts thus studied by us were in powdered form and the location, relative intensity,and character of the bands lying within the visible spectrum were determined by measur- ing the intensity of the light transmitted by an extremely thin layer between glass plates, or in some instances by ob- serving the spectrum of white light reflected from the surface of the powder. Recourse to the latter method is, indeed, frequently necessary because of the great and rapidly in- creasing opacit}^ of these sub- stances in the blue and violet. The nature of the results of such measurements is suffi- ciently shown in figure 20, which is plotted from deter- minations of the light transmitted by a thin layer of uranyl potassium sulphate. The source of light was an acetylene flame. The measurements cover not only a considerable portion of the absorbing region, but also a part of the region containing the fluores- cence bands. Three of these bands show very clearly, even when superposed upon the brilliant continuous spectrum of the acetylene flame. The absorption begins a little on the violet side of the brightest luminescence band and extends into the ultra-violet. It will be noticed that there are several definit
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