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The spectra of the uranyl sulphates, when resolved by excitation at the temperature of liquid air, exhibit fluorescence series whose frequency intervals range from 85.7 in caesium uranyl sulphate to 83.0 in potassium uranyl sulphate. The shift towards the violet with increasing molecular weight of the compound is unusually large. The absorption bands extend into group 7 of the fluorescence spectrum without break of interval and there are many reversals where fluorescence and absorption overlap. U RANYL sulphate (UO2SO4.3H2O) and the double uranyl sulphates of the alkaline metals are among the most brilliant of known fluorescent substances. Their spectra are characterized by an unusual complexity of narrow bands brought out by cooling to the temperature of liquid air. The group structure is by no means so obviously uniform as in the case of the compounds considered in previous papers nor is there the marked similarity between the spectra of the double sulphates which has been noted in the discussion of the fluorescence and absorption of the chlorides, nitrates and acetates.There are however certain characteristics common to all the sulphates thus far examined, i.e.,1. Fluorescence at -185 0 vanishes with the group 7 (frequency 2,000 to 2,070), which is the reversing region for this family of salts, and the eighth group lies entirely within the absorption region.2. Absorption of the type having the usual 70-frequency interval, extends without change of interval into group 7. What we have called the heads of the prominent absorption series lie in the region between 2,040 and 2,060 instead of at 2,170 as in the spectra of the acetates.3. The fluorescence groups are distinguished by a strong pair of bands fairly dominant in all the spectra excepting that of the sodium salt. The series formed by the members of shorter wave-length of these pairs was terminated towards the violet, where it meets the head of the corresponding absorption series mentioned above.4. The location of the fluorescence groups in the spectrum of the sulphates, is not approximately the same, for the different salts do not approximate coincidence as is the case with the corresponding acetates. On the contrary, there is, in general, a shift toward the violet with in-

III. ON FLUOREscENcE SPEcTRA. N a recent paper ' the authors have described spectrophotometric measurements of fluorescent solutions of fluorescein, eosin and naphthalin-roth, the results of which fully confirm the contention of Lommel ' that in the case of these substances Stokes' law is not fulfilled.In the opinion of Lommel all other fluorescent substances, with the probable exception of chlorophyll, conform to Stokes' law, the shortest wave-length of the fluorescence spectrum being always of less refrangibility than the longest wave-length capable of excitirig fluorescence.We -have since extended our measurements to various other fluorescent substances, determining in each case by means of the spectrophotometer the location and character of the absorption band with which fluorescence is associated and the form of the curve of relative intensities in the fluorescence spectrum.Rho/azjz& z an2 Resorci n-blau. Of the substances examined rhodamin and resorcin-blau belong to the same group as those previously examined, being organic dye stuffs which show fluorescence in solution.The fluorescence of these solutions was of sufficient intensity to permit of the use of the method described in our previous paper which, briefly, was as follows: the light of a Nernst filament was dispersed by means of a large spectrometer with a flint prism, and that portion of the spectrum which was to be used for excitation was isolated by means of a slit mounted in place of the eyepiece in the observing telescope. Two nearly monochromatic regions of the spectrum were selected in each case, one lying well within the absorption band of the solution and the other as far toward the red as it was possible to go without reducing fluorescence to an extent which rendered measurements inexact. The fluorescence spectrum was observed by means & Nichols and Merritt, PHYsicAL P~EvrEw, Vol. XVIII. , p, 403.Lommel, Poggendorff's Annalen, vols. ?43, zS9 and x60.

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