FLUORESCENCE OF THE URANYL SALTS BY EDWARD L. NICHOLS AND HORACE L. HOWES IN COLLABORATION WITH ERNEST MERRITT, D. T. WILBER, AND FRANCES G. WICK PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON WASHINGTON, 1919 CARNEGIE INSTITUTION OF WASHINGTON PUBLICATION No. 298 PRESS OF GIBSON BROTHERS, INC. WASHINGTON, D. C. CONTENTS. PAGE I. Historical Introduction 5 II. The Structure of Fluorescence Spectra 10 III. Preliminary Observations on Certain Uranyl Salts 15 IV. Phosphorescence of the Uranyl Salts 38 V. Intimate Structure on Cooling to —185° C 61 VI. Polarized Spectra of Double Chlorides 102 VII. The Nitrates and Phosphates; Water of Crystallization; Crystal Form 122 VIII. The Acetates 146 IX. The Sulphates 169 X. Frozen Solutions 180 Appendix 1. Chemistry and Crystallography of the Uranyl Salts 207 Appendix 2. On Phosphoroscopes 231 3 PREFACE. This volume, the completion of which has been much delayed by the participation of America in the World War, contains the results of an investigation covering a period of eight years. The discovery by Becquerel and Onnes, that the fluorescence of certain uranyl compounds is resolved into groups of narrow line-like bands when these substances are excited to luminescence at very low temperatures, suggested to the present authors the desirability of a thorough and systematic study of this subject. The spectra of numerous uranyl salts, many of which were espe- cially prepared for this purpose, have now been mapped. Owing to the extraordinarily complex character of the phenomena, no satis- factory theory has as yet been evolved, but the mass of facts here recorded and the general principles established will, it is hoped, afford a basis for the successful theoretical development of this important and little understood branch of the science of radiation. PHYSICAL LABOEATORY OF CORNELL UNIVERSITY, May 24, 1919. 4 FLUORESCENCE OF THE URANYL SALTS. BY EDWARD L. NICHOLS AND HORACE L. HOWES. I. HISTORICAL INTRODUCTION. The beginnings of precise knowledge concerning the luminescence of the compounds of uranium are to be found in the classical memoirs of George Gabriel Stokes and of Alexandre Edmond Becquerel. It is true that Brewster1, who observed the fluorescence of chlorophyl and other substances in 1833 and gave the phenomenon the name of internal dispersion, mentioned a yellow glass, doubtless the "canary glass" of commerce, which exhibited the same property, but it remained for Stokes,2 by means of the beautiful experiments described in his papers entitled "The Change in the Refrangibility of Light," to really eluci- date the phenomena and to lay the foundation for all subsequent work on fluorescence. Having observed, by the use of suitable light-filters and by his ingenious and elegant method of transverse dispersion, the unusual character of the fluorescence and absorption of this glass, Stokes pro- ceeded to the investigation of such compounds of uranium as he was able to procure. From the nitrate he made the acetate, oxalate, and phosphate; also uranates of potassium and calcium and the oxides. He also obtained specimens of autunite (uranyl calcium phosphate) and chalcolite (uranyl copper phosphate) . After observations of these minerals he writes (Sec. 145) : "The intervals between the absorption bands of green uranite were nearly equal to the intervals between the bright bands of which the derived spectrum (i. e., the fluorescence spectrum) consisted in the case of yellow uranite. After having seen both systems I could not fail to be impressed with the conviction of a most intimate connection between the causes of the two phenomena, unconnected as at first sight they might appear. The more I examined the compounds of uranium, the more this conviction was strengthened in my mind." Upon reading Stokes's memoir one can not but feel that had he had at his command a modern spectroscope he would infallibly have antici- pated by more than half a century much of the recent work on fluores- cence. He used light-filters to prevent the exciting beam from sub- merging the fluorescence on the one hand and to exclude the exciting 1 Sir David Brewster, Trans. Roy. Soc. Edin., vol. xn. 1833. 2 Stokes, Phil. Trans., 1852, p. 463; 1853, p. 385. 6 FLUORESCENCE OF THE URANYL SALTS. rays from entering the eye on the other, and thus by means of a prism held to the eye was able to observe the spectra of both fluorescence and absorption with surprising accuracy. Paragraph 148 of his paper describes his observations on uranyl nitrate. In the following quotation of that paragraph certain pas- sages forecast in an extraordinary manner some of the conclusions reached in subsequent chapters of the present monograph: "The sun's light was reflected horizontally by a mirror and condensed by passing through a large lens. It was then transmitted through a vessel with parallel sides containing a moderately strong ammoniacal solution of a salt of copper. The strength of the solution and the length of the path of the light within it were such as to allow of the transmission of a little green besides the blue and violet. "A crystal of nitrate of uranium was then attached to a narrow slit and placed in the blue beam which had been transmitted through the solution, the crystal being turned toward the incident light. The light coming from the crystal through the slit was then viewed from behind and analyzed by a prism. A most remarkable spectrum was then exhibited, consisting from end to end of nothing but bands arranged at regular intervals. The interval between consecutive bands appeared to increase gradually from the red to the violet, just as is the case with bands of interference. Although this interval appeared to alter continuously from one end of the spectrum to the other, the entire system of bands was made up of two distinct systems, different in appearance and very different in nature. The less refrangible part of the spectrum, where, only for the crystal, there would have been nothing but darkness, was filled with narrow bright bands due to the light that had changed its refrangibility. The more refrangible part of the spectrum was occupied by the system of bands of absorption. The interval between the most refrang- ible light band and the least refrangible dark band of absorption appeared to be a very little greater than one band interval, so that had there been one more band of either kind the least refrangible absorption band would have been situated immediately above the most refrangible bright band. With strong light I think I have seen an additional band of this nature." Becquerel, in the course of his work on phosphorescence, notes the fact that most of the compounds of uranium show a strong emission of light when examined with the phosphoroscope. He determined the duration as three to four thousandths of a second; and to test his empirical formulae made measurements of the rate of decay which, as will be seen, are in good agreement with the results described in Chapter IV of the present treatise. With a prism of carbon bisulphide he observed 8 bright bands in the spectrum of the phosphorescent light of uranyl nitrate; he determined the approximate range in the violet and ultra-violet of the exciting rays; noted that the bands in the spectra of various uranyl salts, such as the chloride, fluoride, and uranyl potassium sulphate, occupy different places. He also esti- mated their relative displacements when compared with the bands of the nitrate. By comparing the spectrum of the nitrate during excita- HISTORICAL. 7 tion with that of the afterglow, Becquerel reached the very important conclusion that the fluorescence and phosphorescence are identical. This point finds ample confirmation in the present work. In 1872, E. Becquerel returned to the study of the uranyl salts. The following are the conclusions reached in this investigation:1 (1) The salts of the protoxide of uranium are inactive. (2) Many, but not all, salts of the sesquioxide (uranyl salts) are active. (3) Five, six, and sometimes seven bright bands, or groups, are visible; lying between the Fraunhofer lines C and F. (4) The positions of the bands vary for different salts, but are always the same for a given salt. (5) The acid of composition determines the disposition of both bright and dark bands. (6) In double salts of the same acid the composition of individual groups is the same, but their position is not the same for the different salts. (7) In a given substance the distance between bands, as viewed in the spectroscope, increases from red to violet; but the differences of wave-length decrease. The ratio of the above distances to the square of the mean wave-length is nearly constant throughout the spectrum, and this ratio (d/2) is the same for the various salts. (8) No simple relation is apparent between the location of homologous bands in different compounds and the chemical properties of the compounds. (9) The absorption spectra also differ for the various compounds and the absorption bands seem to form a continuation of the fluorescence series. (10) The location and character of these spectra being fixed and definite for each compound, we have the basis for an analytical method similar to but less general than ordinary spectrum analysis. In 1873, Henry Morton and H. ,Carrington Bolton published an account of extended studies of the fluorescence and absorption of the uranyl salts.2 Their list contains 85 substances, chiefly of their own preparation, including 17 double acetates; but not all of these com- pounds were found to be fluorescent. Readings were made on the Bunsen scale in vogue at that time, and some of these, for comparison with our own determinations, will be found, reduced to approximate wave-lengths, in Chapter III. Figure 1, which is reproduced from the paper of Morton and Bolton, gives an excellent general view of some of the most interesting of their observations. The unshaded portions are fluorescence bands, the shaded regions are the bands of absorption. The partial resolution of the bands in several cases is clearly shown and the breaking-up into distinct groups of the uranyl ammonium chloride ; also the coincidence in certain cases of absorption and fluorescence in what in this mono- graph we shall term the reversing region. 1 E. Becquerel, Comptes Rendus, LXXV, p. 296. 1872. 2 Morton and Bolton, Chem. News, pp. 47, 113, 164, 273, 244, 257, 268. 1873. 8 FLUORESCENCE OF THE URANYL SALTS. The authors note specifically the following further important char- acteristics of the spectra of the uranyl salts: (1) The steeper gradation of light on the side toward the violet in the case of bands showing a single crest. (2) The weakness of the outer bands, both toward red and violet, com- pared with the central bands of the spectrum. (3) The overlap of fluorescence and absorption. (4) The systematic shift of bands when a salt is dissolved in water and other solvents. (5) The remarkable changes due to the dehydration of salts containing water of crystallization. (6) The effects of heating. 5678 1.0 , 1,1 . 1.2 . 1.3 . 14 , 1,5 1.6 1.0 , 1,1 1.2 , 13 . 14 , 15 , 16 12 , 1,3 14 , 1,5 , 16 10 , 1,1 , 12 , 1,3 . 14 , 1,5 1.6 1.0 , 11 , 1,2 , 1.3 . 14 , 15 1,6 10 1.1 12 , 1.3 . 14 , 15 1,6 1,0 , 1,1 , 1.2 . 13 , 14 , 1,5 , 16 1,0 , 1.1 , 12 . 13 14 , 15 1,6 456 10 1.1 12 . 13 , 14 1.5 1.6 Fio. 1. — 1. Uranic nitrate. 2. Uranic acetate. 3. Sodio-uranic acetate. 4. Uranic oxychlo- ride (acid), mixed hydrates. 5. Potassio-uranic oxychloride. 6. Uranic oxyfluoride. 7. Bario- uranic oxyfluoride. 8. Uranic phosphate, mixed hydrates. 9. Calcio-uranic phosphate. 10. Ammonio-uranic sulphate. HISTORICAL. 9 Morton and Bolton, like Becquerel, refer to the possibility of deter- mining the composition of uranyl compounds from the observation of their fluorescence spectra and state that even minute quantities, present as impurities, may be detected by means of their characteristic bands. Hagenbach1 likewise published a considerable list of fluorescence bands for the uranyl salts, but his paper adds little to the data of Becquerel and of Morton and Bolton. In 1903, J. Becquerel and Onne?, working in the cryogenic laboratory at Leyden, excited various uranyl salts to fluorescence at the tempera- tures of liquid air and of liquid hydrogen respectively. At —185° C. each band of the spectrum was found to be resolved into a group of much narrower bands. The spectra of a number of compounds were photographed, using a grating spectrograph, and the most prominent bands were mapped. It was shown in the course of this investigation that the resolved spectra are made up of series of bands, the frequency interval varying slightly for different compounds ; also that each group in a given spec- trum is similar to all the other groups as regards the arrangement and the relative intensities of its components. In the reversing region, where fluorescence goes over into absorption, the coincidence in posi- tion of bright and dark bands was pointed out. Further cooling to the temperature of liquid hydrogen rendered the individual bands sharper and more line-like, but there was no further resolution. This resolution of the fluorescence spectra by cooling constitutes the most important advance subsequent to the discoveries of Stokes and of E. Becquerel, since it affords a means of studying the more intimate structure of these remarkable spectra. It forms, indeed, the starting- point for the present investigation. 1 Hagenbach, Annalen der Physik., v. 146, p. 395. 1872. II. THE STRUCTURE OF FLUORESCENCE SPECTRA. L & K. No. 13. SR. Bl. NA»S04. -80 A fluorescence spectrum consists of one or more bright bands, and these may greatly vary in width, from the very broad bands, filling a great part of the visible spectrum, characteristic of the fluorescent dyestuffs and the phosphorescent sulphides, to the line-like bands of the ruby. Such a spectrum is either a homogeneous complex of systematically related components or a heterogeneous complex of unrelated compo- nents. In either case the components frequently overlap, giving the appearance of a single band, which may be described as a mixed band (an unresolved heterogeneous complex) or a homogeneous band, respec- tively. Where the components overlap less completely or not at all the appearance is that of a group of bands. It is probable that a heterogeneous complex is always the result of a mixture of two or more compounds the fluorescence of each of which by itself gives a homogeneous complex. The phosphorescent sul- phides afford spectra which may serve to illustrate the above classification. A stron- tium sulphide with bismuth as the active metal and a flux of sodium sulphate, for example, has a fluorescence spectrum which appears to the eye to consist of a single band with its crest at 0.480 /z. A recent spectrophotometric explora- tion by Dr. H. L. Howes,1 however, shows a group of closely over-lapping compo- nents (see fig. 2) . The crests of these are located as shown in table 1; and as they are systematically related, form- ing members of a series having a uniform interval of frequency ' 0)a difference, this is to be regarded as a homogeneous band or homogeneous complex. Similarly, the fluorescence of a barium sulphide with copper as the active metal and a flux of sodium borate, when viewed through an -60 -40 -20 1 Proceedings American Philosophical Society, LVI, p. 258. 1917. 10 GENERAL DISCUSSION OF FLUORESCENCE SPECTRA. 11 ordinary spectroscope, has a spectrum which seems to consist of a single very broad band. A spectrophotometric study reveals, how- ever, two neighboring and overlapping bands. These have then- crests in the red and green respectively and are complex. (See fig. 3.) TABLE 1. — Approximate wave-lengths of visible crests in the spectrum of a phosphorescent strontium sulphide (Sr; Bi; Na2SO), No. 13. M- Visible crests 1/MX103. Intervals. M- Visible crests 1/MX103. Intervals. 0.4430 .4547 2257 2199 58 58 0.5238 1909 2X58 4670 2141 58 5562 1793 .4801 2083 58 2X58 .4938 2025 .5921 1677 2X58 The components of the band in the green are members of a series having a constant frequency interval of 70 (see table 2), while the components of the band in the red form a series with an interval of L & K. No. 33. BA. CU. NA^ B4 07 -15 -10 FIG. 3. 26.6. The two series overlap, as may be seen from figure 3. In this example the spectrum as a whole forms a heterogeneous complex made up of two homogeneous complex bands which are partially super- imposed. The fluorescence spectrum of commercial anthracene affords an example of a heterogeneous complex easily resolved into a group of 12 FLUORESCENCE OF THE URANYL SALTS. bands. There are at least 7 such bands, 4 of which are seen in the spectroscope with a region in the violet not readily resolved by visual observations. This violet fluorescence has, however, been determined photographically by Miss McDowell.1 The approximate location of the bands in this spectrum is shown in figure 4. TABLE 2. — Approximate wave-lengths and frequencies of visible crests in the spectrum of a phosphorescent barium sulphide (Ba; Cu; Na2B4O7). Green complex. Red complex. Visible crests. M- Intervals. 1/MX103. Visible crests. M- Intervals. i/Vxio3. 0.4255 .4386 70 0.5000 26.6X2 70X2 70 70 .5136 .4673 .4831 .5000 26.6X2 .5283 26.6X7 70X2 .5861 .5376 26.6X2 .6049 26.6X2 .6250 26.6X2 26.6 .6465 .6578 .6695 26.6 While all of these bands are pressnt in the fluorescence of the impure commercial product, they are not all due to any one constituent. By solution and subsequent fractional sublimation, as is well known, it is possible to partially separate the substance into pure anthracene, which has a violet fluorescence and a residue containing chrysogen, the fluorescence of which is green. 1600 1800 2000 I I I I I 22.°° FIG. 4. Miss McDowell has shown that the bands 6, 7, and 8 belong to the anthracene thus obtained, while band 4 is also present in its spectrum. Bands 1, 2, 3, and 4 are characteristic of the green residue. 1 Miss L. S. McDowell, Physical Review (1), xxvi, p. 155. 1908. GENERAL DISCUSSION OF FLUORESCENCE SPECTRA. 13 The presence of band 4 in both spectra may be due to the imperfect separation of the two substances. That it is much stronger in spectra obtained from mixtures showing green fluorescence than from those the fluorescence of which is blue-violet would seem to warrant ascribing it to the chrysogen component, a conclusion strengthened by the con- sideration of the placing of the bands. The most probable positions of the crests are given in table 3. The positions of the three bands assigned to anthracene are from photographic measurements by Miss McDowell;1 those due to chrysogen are from spectrophotometric read- ings made in 1910,2 combined with more recent observations. TABLE 3. — Wave-lengths and frequencies of the bands of commercial anthracene. Band. M- 1/MX103. Difference. 1 0 . 6235 1603 130 2 .5790 1733 127 3. Chrysogen . . . 4 .5362 .5005 1860 1996 136 5 .4750 2105 122 6. Anthracene. . . 7 .4490 .4260 2227 2347 120 It will be noted that the three bands in the blue- violet are members of a series having a frequency interval of about 121; also that the 4 bands of greater wave-length form a series with a somewhat greater interval, i. e., about 131. Band 4 is too near to band 5 to belong to the anthracence series, but may, within the rather large errors due to the breadth and vagueness of these bands, be regarded as one of the chrysogen series. There are several criteria based on experimentally established facts by which the homogeneity or heterogeneity of a fluorescence band or complex may be determined. CRITERIA OF HOMOGENEITY. (1) The position and distribution of intensities in a homogeneous band is independent of the mode of excitation. This was established by various observations published several years ago,3 and subsequent experience strengthens our conviction that it is a general principle and that shifts in position and change of form are to be regarded as indica- tions of heterogeneity due to the presence of more than one lumines- cent substance. (2) The distribution of intensities in a homogeneous band is such that the curve has a single well-marked maximum. The slope toward the violet is steeper than that toward the red, like that in the corre- sponding curve of intensities of an incandescent black body. 1 Miss L. S. McDowell, 1. c. 2 Nichols, E. L., Proc. Am. Philos. Soc., XLIX, p. 277. 3 See Nichols and Merritt, Studies in Luminescence, Carnegie [Inst. Wash. Pub. No. 152, pp. 24, 38, 144. 14 FLUORESCENCE OF THE URANYL SALTS. In the case of a partially resolved or wholly resolved homogeneous complex the envelope obtained by drawing a curve through the crests of the group of bands has the above form, as has the curve of intensities of each of the component bands. A departure from this type indicates heterogeneity. Thus, for example, the curve in figure 2 suggests a partially resolved homogeneous complex, while that in figure 3 indicates a heterogeneous complex or mixed band. The best examples thus far are found among the uranyl salts. Figure 5 shows a typical case in which the envelope of the 7 bands of a uranyl salt is shown and with an enlarged scale of wave-lengths the distribu- tion of intensities of a single band of the same spectrum. Other illustrations will be found in subsequent chapters of this monograph. (3) In a homogeneous com- plex, the fluorescence spectrum is identical with that observed during phosphorescence as regards the position, relative intensity, and structure of its component bands. Nor is there any change in these FIG. 5. respects during the process of decay. Change of color in passing from the fluorescent to the phos- phorescent stage or during phosphorescence is therefore a criterion of heterogeneity, since such changes are due to the presence of bands having different rates of decay. Such subjective changes of color as are due to the loss of intensity during decay are excluded from the above statement. Most of the phosphorescent sulphides afford examples of heterogene- ity clearly indicated by the above criterion and confirmed in other ways, while the spectra of the uranyl salts, in spite of their great complexity, are found, from this criterion, too, strictly homogeneous. (4) .Persistence of color and of structure when excited to fluorescence at different temperatures, the different components of the spectrum suffering the same relative changes of intensity, may be regarded as a criterion of homogeneity, but the complex changes of structure revealed by the resolution of spectra in the process of cooling to the temperature of liquid air do not necessarily indicate heterogeneity. As will be shown in subsequent chapters, the fluorescent spectra of the uranyl salts, for example, are profoundly modified by the cooling of the substance, and yet these spectra conform to all other known criteria of homogeneity. III. PRELIMINARY OBSERVATIONS ON CERTAIN URANYL SALTS.1 Because of their brilliant luminescence and the interesting character of their spectra of fluorescence and absorption, the uranyl compounds have been the subject of extended study. A brief account of the work of previous observers in this field has been given in Chapter I. Our original purpose in taking up the study of these substances was to determine whether the different bands of the fluorescence spectrum are to be regarded as independent, each with its own region of excita- tion, or whether they form a homogeneous complex, such that the excitation of one necessarily involves the excitation of all. In this inquiry we have been led to the investigation of many other questions. Since, as was first shown by Becquerel and Onnes2 in the paper cited in Chapter I, the bands of the uranyl salts are resolved into groups of narrow components by cooling, it is at the temperature of liquid air and chiefly by photographic methods that the intimate structure of the fluo- rescence and absorption spectra is to be determined. The study of the spectra at ordinary tem- peratures, however, is not without significance. In this work, where the width of the bands is from 50 to 100 A. IT., the spectrophotometer is indispensable. Many of the measurements to be described were made with a special in- strument which combines the features of the constant deviation spectrometer and the Lummer- Brodhun spectrophotometer. It is essentially a spectrometer of the Hilger type, with two collimators C and C" (fig. 6), a Lummer-Brodhun. cube L. B., and a constant-deviation prism P with carefully calibrated drum. / 1 Certain of the observations contained in this chapter have been published in the Physical Review (1), xxxm, p. 355, but many of the data there given have been replaced by more complete investigations kindly done at our request by Dr. Frances G. Wick (Physical Review (2), v. 11, p. 121. Feb. 1918. 2 Becquerel and Onnes, Leiden Communications, 110. 1909. 15 L.B. FIG. 6. 16 FLUORESCENCE OF THE URANYL SALTS. For the determination of wave-lengths the eyepiece is provided with a pointer in the focal plane and also with the usual slides for isolating the region under observation. The collimator slits have micrometric adjustment, and to provide for convenient comparison through the very great range of intensities occurring in the study of fluorescence, the illumination of the com- parison slit can be varied by moving the comparison light along a photometer bar to any desired distance from the slit. The observing telescope can be replaced by a camera whenever photographs of the spectra are desired. With this instrument the wave-lengths of the bands could be determined by setting the pointer to the region of greatest brightness as estimated by the eye and the relative intensities could be measured spectrophometrically. Tables 4 to 10 contain the resulting data for several salts; also the frequencies corresponding to the wave-lengths and frequency intervals. The measurements and computations were kindly made by Miss Wick, who likewise determined the relative brightness of the bands in several of the spectra. From the data in these tables some of the salient features of the uranyl spectra may be deduced, viz: (1) The weakest bands are at the ends of the spectrum, i. e,, in the red and the blue. (2) The brightest band is not in the center, being third from the violet end and sixth from the red end when all 8 bands of the spectrum are visible. (3) Taking the frequency intervals, instead of the differences of wave-length, the bands, with the exception of the band of shortest wave-length (band 8), are equidistant, at least within the rather large TABLE 4. — Fluorescence bands of the nitrates. Uranyl nitrate (Anhydrous); (UO2 (NO3)2). Width of bands about 70 1. u. Uranyl nitrate (tri-hydrate) (UO2 (NO3)2+3H2O). Observations on a single large crystal ; width of bands about 100 A. u. Uranyl nitrate (hexahy- drate) (UO2(NO3)2+6 H20). Position of crest of band. 1/MX103. Inter- val. Inten- sity. Position of crest of band. 1/pXlO3. Inter- val. Inten- sity. Position of crest of band. i/Vxio3. Inter- val. 4720.0 2118.6 4871.0 2052.9 49.2 4720.8 2118.2 65.8 84.3 69.8 4871.2 2052.8 50.2 5079.7 1968.6 100.0 4881.8 2048.4 88.2 86.8 86.4 5090.0 1964.6 100.0 5314.0 1881 . 8 64.5 5096.3 1962.0 87.6 87.6 86.1 6327.6 1877.0 52.7 5573.4 1794 . 2 20.0 5330.6 1875.9 87.4 87.4 87.3 5587.6 1789.6 25.1 5859.0 1706.8 5591.2 1788.6 87.2 84.6 87.2 5874.0 1702.4 6164.3 1622.2 5877.5 1701.4 84.7 85.0 6181.2 1617.7 6186.5 1616.4 FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 17 TABLE 5. — Fluorescence bands of the double nitrates. Ammonium uranyl nitrate (/3), UC^CNOa^-NH* NO3. (Accuracy of setting not to be expected; the double crest (a and b) of each band is flat of nearly equal intensity, rather broad, and not well separated.) Potassium uranyl nitrate, UO2(N03)2-KN03. (Bands broad and fuzzy; over 100 units wide; crests distinctly double, with a third crest toward the violet.) Position of crest of band. l/M+103. Interval. Position of crest of band. 1/MX103. Interval. a-a 6-6 a-a. 6-6. a-4639 . 6 6-4705.3 a-4800.0 6-4841 . 6 a-4992 . 6 6-5048.3 a-5224.0 6-5280.6 a-5473.0 6-5535 . 6 a-5748.7 6-5825.6 a-6026 . 6 6-6130.8 2155.3 2125.0 2083.3 2065.4 2003 . 0 1980.8 1914.2 1893.6 1827.1 1806.5 1739.5 1716.5 1659.3 1631.0 a-4696.0 a-4860.0 6-4904.0 a-5077.5 6-5108.3 a-5303.5 6-5347.0 a-5556 . 0 6-5607.3 a-5836.5 6-5891.5 a-6146.3 6-6209.0 2129.5 2057.6 2039.0 1969.4 1957.5 1885.5 1870.2 1799.8 1783.5 1713.3 1697.3 1627.0 1610.6 71.9 72.0 59.6 88.2 81.5 80.3 84.6 83.7 87.3 88.8 87.2 85.7 86.7 87.1 87.1 86.5 86.2 87.6 90.0 86.3 86.7 80.2 85.5 TABLE 5. — continued. TABLE 6. — Fluorescence bands of the sulphate. Rubidium uranyl nitrate, UO2(NO3)2-RbNO3. (Bands very broad with two poorly defined crests. With less excitation the bands appear single. Settings are only roughly approximate.) Position of crest of band. 1/MX103. Internal. Intensity. a-a. 6-6. 6-6. 6-4828.0 a-4994.5 6-5045.5 a-5210.7 6-5265.3 a-5454.5 6-5525 . 0 a-5738.8 6-5820.5 a-6012.0 6-6119.0 2071.0 2002.2 1982.0 1919.2 1899.0 1833.3 1810.0 1742.6 1718.0 1663.0 1634.0 89.0 68.3 83.0 83.0 100.0 85.9 89.0 39.4 90.7 92.0 7.6 79.6 84.0 2.1 Uranyl sulphate (UO2SC>4 +3H2O) . Position of crest of band. i/Vxio3. Interval. Intensity. a-a. 6-6. a. 6. a— 6-0.4753 a- .4886 6- .4941 a- .5099 6- .5158 a- .5335 6- .5397 a- .5592 6- .5662 a- .5877 6- .5962 a- .6183 b- .6284 2103.9 2041.7 2023.9 1961.0 1938.7 1874.4 1852.9 1788.3 1766.2 1701.5 1677.3 1617.3 1590.2 80.0 85.7 85.2 27.97 38.72 86.6 85.8 91.79 100.00 86.1 86.7 50.04 53.79 86.8 88.9 19.27 20.14 84.2 87.1 18 FLUORESCENCE OF THE URANYL SALTS. uncertainties inevitable in the attempt to locate the crests of such broad bands. Of this band, which occupies the region lying, roughly, between 0.4650 /x and 0.4750 fj., only the less-refrangible edge is seen, the other side being more or less cut off by absorption. Its apparent distance from band 7 is thus reduced. (4) In some cases there is sufficient evidence of resolution to enable the location of two or more crests. Further evidences of complexity will be found in the spectrophotometric study of these spectra, to be considered in a subsequent paragraph. TABLE 7. — Fluorescence bands of the double sulphates. Sodium uranyl sulphate (UO2SO4. Na^SO^. Potassium uranyl sulphate (UO2SO4.K2SO4). Position o f crest of band. 1/MX103. Interval. Intensity. Position of crest of band. 1/MX103. Interval. Intensity. 4744.0 4910.0 5125.0 5354.2 5608.4 5890.5 6200.2 2107.9 2036.6 1951.0 1867.6 1783.0 1697.6 1612.8 71.3 85.6 83.4 84.6 85.4 84.8 4778.0 4935.5 5133.6 5365.6 5619.2 5902.9 6201.2 2093 . 0 2026.1 1948.0 1863 . 7 1779.5 1694.3 1612.6 66.9 78.1 84.3 84.2 85.2 81.7 27.29 100.00 46.77 15.13 5.88 35.33 100.00 41.22 12.85 Ammonium uranyl sulphate (UOsSC^. (NH^SOi). (The two crests of each band very close and narrow.) Rubidium uranyl sulphate (U02S04-Rb2S04). Position of crest of band. i/Vxio3. Interval. Intensity. Position of crest of band. 1/MX103. Interval. I ntensity. a-a. 6-6. a-4929.0 6-4950.0 a-5140.8 6-5164.8 a-5374.5 6-5399.5 a-5627 . 0 6-5657.8 a-5906 . 5 6-5935.5 a-6213.7 6-6251.4 2028.8 2020.2 1945.2 1936.2 1860.6 1852.0 . 1777.0 1767.5 1693.0 1684.8 1609.3 1599.6 83.6 84.6 83.6 84.0 83.7 84.0 84.2 84.5 82.7 85.2 25.5 4757.0 4930.0 5136.0 5368.7 5619.8 5894 . 0 6195.5 2102.2 2028.3 1947.4 1862.6 1779.4 1696.6 1614.1 73.9 80.9 84.8 83.2 82.8 82.5 38.52 100.00 49.35 19.77 100.00 21.43 11.86 FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 19 TABLE 7. — Fluorescence bands of the double sulphates — continued. TABLE 8. — Fluorescence bands of two acetates. Uranyl caesium sulphate (UOaSCU . CsSO4) . (Bands over 100 units wide with sharp maxima near the end toward red and secondary maxima (brack- eted) much less bright and sharp toward theviolet.) Wave-length of crest of band. 1/MX103. Interval. Intensity. 4702.0 4876.0 (4933.0) 5088.5 (5151.5) 5317.0 (5338.0) 5573.5 (5670.0) 5852.0 6165.0 2126.7 2050.9 1965.2 1880.8 1794.2 1708.8 1622.0 75.8 85.7 84.4 86.6 85.4 86.8 50.49 100.00 53.09 16.90 6.36 RELATIVE INTENSITIES OF THE BANDS. To indicate graphically the1 relative intensities of the bands, we may plot their strength, ex- pressed in terms of energy, as ordinates and wave-lengths of the crests as abscissae. The resulting curve (see figs. 7, 8, 9, and 10) is a sort of enve- lope for the entire spectrum cor- responding to the curve of distri- bution of energy. It resembles in type the curve of energy found in the case of the broad, single- banded fluorescence described in earlier communications / being single-crested and steeper toward the violet. As has been pointed out in Chapter II, these curves are very similar to the energy curve for temperature radiation. Figure 7 contains the envelopes thus plotted of 5 uranyl double sulphates. Of these, 4 have their crests at approximately the same wave-length (0.515ju). Curve E (caesium uranyl sulphate) is shifted slightly, an effect due to the presence of a strong component of each band on the violet side of the main crest which influences the estimates 1 Nichols and Merritt, Physical Review (1), xvm, p. 403; xix, p. 18. Uranyl acetate. Position of crest of band. 1/MX103. Interval. Intensity. 4710.0 4878.0 5094.0 5328.0 5586.0 5869.2 6182.3 2123.0 2050.0 1963 . 1 1876.9 1790.2 1703.8 1617.5 73.0 86.9 86.2 86.7 86.4 86.3 48.16 100.00 48.86 22.36 Ammonium uranyl acetate. Position of crest of band. 1/MX103. Interval. Intensity. a— a. 6-6. 6-4680 . 0 a-4804 . 5 6^884.3 a-5016.3 6-5094 . 6 a-5242.2 6-5330.8 a-5487.8 6-5581 . 0 a— 2136.0 2081.3 2047.4 1993.5 1962.9 1907.5 1875.9 1822.2 1791.7 88.6 84.5 87.0 84.2 88.3 92.5 87.8 86.0 85.3 61.42 100.00 46.12 18.40 6-5870.6 Or~ 1703.4 6-6207 . 5 1610.9 20 FLUORESCENCE OF THE URANYL SALTS. TABLE 9. — Fluorescence bands of two phosphates. Uranyl phosphate (H-UO2-PO4). (Bands narrow and distinct.) Ammonium uranyl phosphate (HZ(NH4)2UO2(PO4)2). (Bands very distinct with narrow crests.) Position of Position of crest of 1/MX103. Interval. crest of i/Vxio3. Interval. band. band. 4847.0 2063 . 0 4845.0 2063.9 71.7 69.7 5020.3 1991.9 5014.6 1994.2 83.8 82.7 5240.7 1908.1 5231.3 1911.5 83.7 83.2 5481 . 1 1824.4 5469 . 6 1828.3 84.9 83.7 5748.6 1739 . 5 5730.8 1744.6 82.9 83.8 6036.5 1656.6 6021.0 1660.8 82.5 6336.0 1578.3 TABLE 10. — Fluorescence bands of a nitrate, oxalate, and fluoride. Uranyl nitrate Uranyl oxalate Potassium uranvl fluoride (UO2(NO3)2-6H2O). UO2C2O4-3H2O. (UO2 F2-2KF). Position of Position of Position of crest of 1/MX103. Interval. crest of 1/MX103. Interval. crest of 1/MX103. Interval. band. band. band. 4700.0 2127.6 4715.0 2120.9 72.1 75.1 4865.0 2055.5 4888.0 2045.8 4803 . 2 2081.9 89.3 92.3 79.8 5085.8 1966.2 5119.0 1953.5 4994 . 8 2002 . 1 86.6 86.3 86.8 5320.0 1879.6 5355.0 1867.2 5219.5 1915.3 88.3 86.9 85.6 5582.3 1791.3 5617.0 1780.3 5465.2 1829.7 87.0 88.2 89.1 5867.0 1704.3 5910.0 1692.1 5745 . 0 1740.6 86.2 88.2 89.1 6179.8 1618.1 6235 . 0 1603.9 6055 . 0 1651.5 of the location of the latter. The envelopes of the 4 nitrates in figure 8 have the same characteristics. The crests of 3 agree (at 0.510^), while the envelope D of rubidium uranyl nitrate, in the spectrum of which the bands are vaguely double-crested, is displaced. In the two uranyl acetates (fig. 9) the same identity of type and position shows itself. To reduce these spectrophotometric measurements to relative energy units the distribution curve of the comparison light must be known. This curve for the acetylene flame, which was the source employed, has been carefully determined, and data published by Coblentz were used in the computation. FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 21 In certain cases the resolution of the bands is such that the brightness of two crests can be determined and two overlapping envelopes drawn, as in figure 10, which pertains to the spectrum of U02S04+3H2O. The relative brightness of the two crests is seen to vary slightly from band to band. .55/4 18|00 19100 20100 i slop I9JOO 20JOO FIG. 7. FIG. 8. The changes in the position of the crests in the case of the sulphates, nitrates, and acetates is illustrated in figure 11, in which a typical curve from each family of salts is given. While these measurements by Dr. Wick do not include all the spectra for which such determinations are possible, they suffice to demonstrate the essential uniformity of type of the envelopes and to show that within a given family, such as the double sulphates or the nitrates, the crests occupy the same region in the spectrum. It will be seen, when we come to the consideration of the detailed structure of these fluores- cence spectra, that there is a slight but definite shift of all the bands with molecular weight. Spectrophotometric measurements of single unresolved bands, prac- ticable with accuracy only in the case of some of the brightest, show the 22 FLUORESCENCE OF THE URANYL SALTS. curve of distribution to be of the same type as that obtained when the envelope is drawn for the entire spectrum, i. e., the type associated with what we have termed a simple band. (See A in fig. 5, Chapter II, which is the energy curve for the brightest band of uranyl potassium sul- .50 //- t sloo 20JOO FIG. 9. FIG. 10. I .55, 50 /* phate with the scale of wave-lengths, adjusted so as to make the width nearly the same as that of the envelope (B] for the same substance.) The most striking feature distinguishing these spectra from one another to the eye, excepting where partial resolu- tion occurs, is the vary ing width and sharpness of the bands. With the spectrophotometer it is possible to obtain a more definite expression of this fea- ture, as may be seen from fig- ures 12 and 13, in which are depicted, from such measure- ments, the three brightest bands of uranyl nitrate (crystallized) and uranyl potassium sulphate. It will be noted that the bands overlap at the base, but to a greater extent in the nitrate than in the potassium sulphate, where the bands are narrower and more sharply defined. 19100 20100 FIG. 11. FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 23 A more detailed use of the instrument, working with narrow slits and making settings at closer intervals, will often bring out the complexity of single bands, where the overlapping of the components is such as to conceal the structure Figures 14, 15, 16, and 17 give the results of such a study by Miss Wick. The existence of numerous partly sub- .48 .50 .5. 20JOO 19(30 — 20 FIG. 12. FIG. 13. Showing the relative intensities of the brightest fluorescence bands of uranyl nitrate (fig. 13) and uranyl potassium sulphate (fig. 14). merged crests is apparent in the curves, corresponding to the com- plexity of structure which these bands show when the substance is excited at the temperature of liquid air. The vertical lines indicate the position of the bands, as observed at low temperatures by methods to be described in subsequent chapters. That these lines in general do not co- incide with the positions of the crests might seem to indicate that there is no definite relation between the spectra at the two temperatures or that the accu- racy of the curves is in doubt; but the discrepancies are quite in accordance with the results obtained by the detailed study of the spectra of the double chlorides (Chapter V), where the spectra are sufficiently resolved at +20° C. to enable us to trace the changes on cool- ing, measure the definite shifts, and dis- cover the remarkable mechanism of the process of resolution. — 15 — 10 .49 J I 1 .50 .51 // FIG. 14. 24 FLUORESCENCE OF THE URANYL SALTS. EXCITATION BY LIGHT OF DIFFERENT WAVE-LENGTHS. If all the bands of the luminescence spectrum are due to the vibra- tions of a single connected system it would be natural to expect that an agency which excited one would also excite the rest, especially if luminescence is due to the recombination of ions dissociated by the exciting light, or to the return of an electron set free by the exciting I0|50 I9JOO — 20 — 15 — 10 ,50 I I .51 20 20JOO — 10 I I FIG. 15. .49 .50 .51 FIG. 16. 4: 20 20JOO — 15 — 10 agency. On the other hand, if each band is dne to some process going on in one particular compound or molecular aggre- gation, wave-lengths might be found which would excite one band and not the rest, or which would at any rate excite the bands in different degree. To test this matter we have measured the distribution of intensity in the bands for excitation by different lines in the ultra-violet spectrum of the quartz mercury lamp. The intensity of fluo- rescence with this excitation is not suffi- cient to permit the measurement of all the bands, so that the three brightest bands only have been measured. In table 11 the intensities for excitation by the different lines in the mer- cury spectrum are given for five different uranyl salts. Curves show- ing the variation of the relative intensity with the wave-length of the exciting light are shown for uranyl-nitrate crystals in figure 18, and for the double sulphate in figure 19. In each case the intensity of the most intense band has been put equal to 10. The variation was greater in the case of the double sulphate than in the case of any other salt studied. The observations were repeated in the case of this sub- stance on two different days and a comparison of the full and dotted .50 .51 M FIG. 17. FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 25 curves indicates the extent to which the results agree. In the case of the other salts studied, curves very similar to that of figure 18 were obtained. TABLE 11. — Relative intensity of excitation of the three brightest bands by five different lines in the spectrum of the mercury arc. Wave- length of exciting light. Intensity1 of luminescence at crest. Ratio a/6. Ratio c/b. Band a. Band b. Band c. Uranyl-potassium sulphate : Band a 4,920 0.436^ .407 .366 .313 .254 .436M .407 .366 .313 .254 .436M .407 .366 .313 .254 .436ju .407 .366 .313 .254 .436/z .407 .366 .313 .254 12.5 12.6 10.5 16.25 8.83 10.0 6.04 9.7 11.1 6.85 22.9 15.3 15.3 21.8 6.0 18.6 17.5 12.2 17.3 8.3 10.3 23.3 20.4 37.5 15.8 22.7 26.4 23.2 22.9 13.9 10.2 6.06 9.7 10.4 5.35 38.7 22 .7 20.7 31.6 7.7 27.2 24.5 18.3 25.6 10.5 11.2 25.0 25.7 43.4 17.6 7.45 8.45 6.65 8.27 4.25 3.07 1.90 2.47 3.04 0.55 .48 .45 .71 .64 .98 1.00 1.00 1.07 1.28 .59 .67 .47 .69 .78 .69 .73 .67 .67 .79 .92 .93 .79 .87 .90 0.33 .32 .29 .36 .31 .30 .31 .25 .29 b 5,130 c 5,360 Uranyl phosphate: Band a . 5,015 b 5,?39 c . 5,483 Uranyl nitrate (anhydrous) : Band a 4,849 15.5 9.30 8.87 11.7 3.2 10.0 8.6 7.5 9.6 4.0 3.7 7.7 7.6 11.5 5.7 .41 .41 .43 .37 .42 .37 .35 .41 .38 .38 .33 .37 .30 .27 .32 b... . 5,071 c 5,311 Uranyl nitrate (crystals) : Band a 4,869 b 5,086 c . 5,329 Uranyl fluorid fluor- ammonium: Band a 5,008 b 5,237 c 5,460 *The intensities given in table 11 are not corrected for energy distribution in the acetylene flame. It will be noticed that the lower curve in figures 18 and 19 indicates a very nearly constant ratio between the intensity of the brightest band and that of the band lying next in the direction of the red. But if we compare the brightest band with the band lying next to the violet side we find a considerable variation in the ratio of intensities, especially in the case of the double sulphate. It appears to us probable that this variation is the result of a partial absorption of the luminescence by the substance. The absorbing power of a given salt differs for the different mercury lines used, so that in some cases the exciting light may pene- trate much further into the substance than in others. It is clear that those bands for which the absorption is greatest will appear relatively weaker when the exciting light penetrates a considerable distance into the substance, even if the relative intensity of the excitation of the different bands is really the same for all wave-lengths of the exciting 26 FLUORESCENCE OF THE URANYL SALTS. light. The observed distribution of energy would correspond with the actual distribution only in case an excessively thin layer of the sub- stance is excited — so thin that the absorption of the light emitted is neg- ligible. As a matter of fact, the band lying to the violet side of the maximum is in a region where the absorption is considerable, while the brightest band and those lying to the red are in the region where the absorption is small. The constancy of the ratio in the case of the lower curves, and the small variation of the ratio shown by the upper curves, are therefore entirely consistent with the view that the observed variations are the result of absorption, and that the first effect of excitation, whatever may be the wave-length of the exciting light, is to produce all of the bands with a definite and constant intensity distribution. .30 .40^ .30 .40 FIG. 18. FIG. 19. Relative intensities of the brightest fluorescence bands of uranyl nitrate (fig. 18) and uranyl- potassium sulphate (fig. 19). The intensity of the brightest band is put equal to 10. The upper curve in each figure refers to the band lying next to the brightest toward the violet. The lower curve refers to the band toward the red. Abscissae give the wave-length of the exciting light. (See table 11.) The observations recorded in the foregoing paragraphs all tend to indicate that the fluorescence spectrum of a uranyl salt is a homo- geneous complex. The envelope is single-crested and has the form typical of a simple band. Neither its position nor form is modified by changing the mode of excitation. To test this conclusion we have made many experiments under widely varying conditions, especially in the way of selective and monochromatic excitation of the resolved spectra, where it should be possible to observe critically the disappearance or enhancement of single narrow components of groups of series. The remarkable effects of selective excitation recorded by Wood in the case of fluorescent vapors might lead to the expectation of similar or analogous changes in the uranyl spectra. All these attempts have thus far been without result, and we are inclined, therefore, to regard the spectrum as a unit and to consider it as a broad, simple band, which unlike the other bands of this type as yet discovered, consists of resolved instead of completely overlapping components. Studies to be described in Chapter IV are in confirmation of this view in that the criterion for a simple band, based upon the phenomena of phosphorescence, is fulfilled. 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 definite and narrow absorption bands, which appear to be superposed upon a broad band, or region, of general absorption. This appearance of a broad band might result from the overlapping of the group of narrow absorption bands, only the crests of which can be observed. In estimating the relative intensity of the FIG. 20. — Transmission of a thin layer of uranyl- potassium sulphate, showing absorption bands and three of the fluorescence bands. Curves F and A show the relative intensities of the bands of fluorescence and absorption respectively. 28 FLUORESCENCE OF THE URANYL SALTS. absorption bands we have adopted the first view and have assumed a general absorption1 such as is indicated by the dotted line of figure 20. The deviations from this dotted curve have been ascribed to the effect of the narrow bands. The intensity of each band is determined by taking the ratio of the diminution of the transmission which it produces to the transmission which would be expected if the general absorption only were present. Both the absorption bands and the fluorescence bands have been indicated in figure 20 by lines whose lengths are proportional to the intensities of the bands. If a line is drawn through the ends of the lines that give the intensity of the absorption bands a curve (A) is. obtained which is very similar in form to the absorption curve* for a substance having a single broad band. This curve also has the same position with reference to the envelope of the luminescence bands (F) that the absorption curve in such cases has to the luminescence curve. It appears highly probable that just as a broad luminescence band may result from the overlapping of a group of bands, so the absorption of the same substance may result from the overlapping of a similar group of absorption bands. The transmission curve for a thin layer of powdered uranyl sulphate is shown in figure 21, the source of light being an acetylene flame. In its general features this curve is similar to that for the double sulphate of uranyl and potassium. The fluorescence of the sulphate is not so brilliant and the fluorescence bands therefore show less prominently. The sulphate, as has been shown in a preceding paragraph, has the peculiarity of possessing two series of fluorescence bands lying close together, one set of bands being much more intense than the other. It will be noticed that the absorbed bands are also double. If we think of the more intense luminescence bands as constituting the principal series and the less intense bands forming a secondary series, a curious reversal is noticeable as we pass from the region of fluorescence to the region of absorption. Each band of the principal series in the lumines- cence region lies a little to the right of the corresponding band of the secondary series. The positions of the bands are indicated by short vertical lines in the lower part of figure 21, the bands of the secondary series being represented by dotted lines. When we pass to the absorp- tion series, however, the more intense band lies to the left in each case. For example, the absorption band at 4,925 corresponds in position with a fluorescence band of the principal series ; but the absorption band at 4,880, which probably corresponds to the band 4,890 of the secondary fluorescence series, is by far the more intense of the two. 1 The fact that all the uranyl salts, so far as known, increase rapidly in opacity as the wave- length of the transmitted light decreases, even when the bands are greatly reduced in width by cooling, seems conclusive as to this assumption. FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 29 It will be observed that the absorption bands of uranyl potassium sulphate occurring at 4,760 and 4,920 (fig. 20) appear to coincide in position with two of the luminescence bands of the same substance. In other words, these two bands are " reversible" and may appear either as absorption bands or as luminescence bands, according to the con- ditions under which they are observed. The double sulphate thus shows the same phenomenon that was first described by H. Becquerel1 in 1885 in the case of uranyl nitrate. 10 I .45 .50 .55^ FIG. 21. — Transmission of a thin layer of uranyl sulphate. It was, however, of interest to study these relations in the case of the uranyl spectra at ordinary temperatures also. Special precautions were necessary, for when a luminescence band occurs in a region where there is appreciable absorption it is clear that the apparent position of the crest of the band may be influenced by absorption in case the latter is not uniform. Where measurements of absorption are made with light containing rays that are capable of exciting fluorescence there may also be a displacement of the crest of the absorption band, owing to the presence of luminescence. There could be no displacement of 1 Comptes Rendus, vol. 101, p. 1252. 1885. 30 FLUORESCENCE OF THE URANYL SALTS. this sort in case the light emitted were strictly proportional to the coefficient of absorption ; but if the fluorescence band and the absorp- tion band do not exactly coincide in position or in form, such a dis- placement is to be expected. In order to avoid the necessity of changing the adjustment of the spectrophotometer, or the position of the substance, between measure- ments a thin layer of the uranyl potassium sulphate was in some cases mounted permanently in front of the slit. To locate the absorption bands the slit was illuminated, through the specimen, with light from an acetylene flame. To observe the luminescence bands a piece of blue glass was placed in front of the flame, so as to cut off the rays having the same wave-length as the bands, while permitting the exciting rays to pass ; or in some cases the acetylene flame was replaced by a mercury arc. To guard against the presence of fluorescence in measurements of absorption a green glass was sometimes used. With the relatively thick specimen first used the absorption was so great that the band at 4,760 could not be observed. The band at 4,920 was well defined, however, and could be accurately located. If the eyepiece pointer was set at the crest of the absorption band and the source of light then changed so as to bring out the fluorescence band, the latter was seen to be very obviously displaced toward the red. Photographs of the absorption and fluorescence spectra taken on the same plate also showed the relative displacement of the two bands very clearly. The wave-length of the fluorescence band as measured under these conditions was not the same, however, as that previously deter- mined, and the whole appearance of the band was different from what had been observed when looking at the front surface of the luminescent substance. More definite conditions for observing the absorption band were obtained by using nearly monochromatic light for transmission meas- urements. The spectrum of a Nernst glower was formed by a large spectrometer and a small region of the spectrum was isolated by means of a suitable screen containing a slit. The light coming through this slit, after passing through the specimen to be studied, fell upon the slit of the spectrophotometer. By suitable adjustment the center of the band of transmitted light could be made practically coincident with the center of the absorption band and the latter could be located with considerable accuracy. Under these circumstances the transmitted light contained no rays capable of exciting any observable fluorescence, so that we may look upon the determinations of absorption by this method as uninfluenced by errors due to the presence of luminescence. Using a relatively thick layer, the absorption band was located at 4,919, while the crest of the fluorescence band (observed by transmis- sion) lay at 4,974. An excessively thin layer, formed by depositing the salt from a solution, or suspension, in alcohol, gave a fluorescence FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 31 band whose crest was at 4,925, while the wave-length of the very faint absorption band was 4,922. Our previous determination of the wave- length of the luminescence band, when looking at the surface exposed to the exciting rays, was 4,920. These results appear to us to warrant the conclusion that if disturbances due to absorption could be entirely eliminated the two bands would be found to have exactly the same wave-length. It must not be forgotten, however, that it is nearly impossible to observe the fluorescence spectrum under conditions which entirely eliminate effects due to absorption. The exciting light always pene- trates to some extent beneath the surface, so that some of the emitted light must pass through the fluorescent material before it can reach the eye. It is natural, therefore, to expect a slight displacement in all cases. Although our most reliable measurement of the wave-length of the absorption band, 4,919, and our best determination of the crest of the luminescence band, 4,920, differ by less than the probable errors of measurement, we feel that it is not unlikely that the difference is a real one, due to the cause just cited. The absorption band at 4,760 in the double sulphate differs in posi- tion by 5 units from the fluorescence band at 4,765. A portion of this difference may also be explained by absorption. But it is probably chiefly due to the difficulty in accurately locating the crests of these bands. The fluorescence band is extremely faint, while the absorption band is not very sharp, because of the large general absorption ir^ this region. Using a thick layer, formed by grinding down a translucent mass of adhering crystals until a piece about 0.5 mm. thick was obtained, a faint absorption band was observed at 5,127. This corresponds to the brilliant fluorescence band at 5,130. In all likelihood the coincidence here is complete, since measurements of the fluorescence band made at the same time and with the same specimen as that used for absorption measurements gave the same wave-length, 5,127, for both bands. EXCITATION BY LIGHT CORRESPONDING TO DIFFERENT PARTS OF THE ABSORPTION REGION. It seemed a matter of some interest to determine the relative effec- tiveness of light of different wave-lengths in producing fluorescence, and experiments having this end in view have been made in the case of the double sulphate. We were particularly interested in determining whether wave-lengths falling within the sharp absorption bands at 4,918, 4,760, 4,615, etc., were especially effective in exciting lumines- cence. The source of the exciting light used in these experiments was a Nernst glower which was mounted in place of the slit of a spectrometer. The spectrum was focussed upon an opaque screen containing a narrow slit, and the light passing through this slit was used in exciting the speci- 32 FLUORESCENCE OF THE URANYL SALTS. men tested. The fluorescence spectrum was observed in a spectro- photometer, the specimen being set up at an angle of approximately 45° with the path of the exciting light, so that the collimator of the spectrophotometer could be pointed at the illuminated surface without interfering with the exciting light. Enough of the exciting rays were reflected into the spectrophotometer to enable the range of wave-lengths used in each case to be determined. The spectrophotometer was then set at the crest of the principal fluorescence band and the intensity measured by com- parison with an acetylene standard. Observations of this sort were repeated throughout the absorbing region. The results are shown in figure 22. It will be noticed that the regions of strong excita- tion at 4,910 and 4,775 correspond very closely to the two absorption bands at 4,920 and 4,766. Some slight indication is also present of the other absorption bands. It is clear, however, that the ability to excite luminescence is not con- fined to rays falling within the narrow absorption bands, but extends to the region of general absorption lying be- tween. It is not possible to determine the specific exciting power of different rays, as has been done in the case of eosin and resorufin,1 because of our ignorance of the absorbing power of the salt for different wave-lengths.2 The results indicate, however, that the specific exciting power varies only slightly with the wave-length, as in the case of resorufin and eosin. THE RELATION BETWEEN ABSORPTION AND FLUORESCENCE AS IT APPEARS AT ORDINARY TEMPERATURES. In 1885 H. Becquerel3 made measurements of the spectrum of uranyl nitrate from which it would appear that the frequency interval remains constant in passing from the fluorescence to the absorption spectrum and that the suggestion of E. Becquerel in his classical memoir of 1872, that the emission bands and absorption bands belong to the same series, is in accordance with the facts. H. Becquerel also showed that two of the bands are reversible, ap- pearing as emission bands when suitably excited, whereas if light free 1 Physical Review, xxxi, p. 381. 2 The distribution of energy in the spectrum of the Nernst glower also has not been determined. 3 H. Becquerel, Comptes Rendus, 101, p. 1252. .42 .46 FIG. 22. — Intensity of fluorescence (ordinates) produced by exciting light of different wave-length (ab- scissa?) . FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 33 from exciting rays be passed through the substance, absorption bands in the same location are observed. In our own work upon uranyl nitrate and potassium uranyl sulphate we have confirmed the results of H. Becquerel so far as the existence of reversible bands is concerned and have found for these substances 3 such bands instead of 2. The frequency interval between absorption bands, like the fluores- cence interval, is approximately constant, but, as may be seen from tables 12 and 13, it is much smaller. Additional evidence on this point will be found in the chapters dealing with the double chlorides and with the spectra at low tempera- tures, where it will be established as a relation common to all uranyl spectra. The study of the absorption spectra at +20° C. is uncertain and unsatisfactory, because we have to do with unresolved groups of bands, and these two examples will suffice to illustrate the remarkable way in which the two frequencies interlock where fluorescence goes over into absorption. TABLE 12. — Absorption and fluorescence bands of potassium uranyl sulphate at -\-20° C. Absorption. Fluorescence. M- 1/MX103. Interval. M- 1/juXlO2. Interval. 0.4350 2298.9 62.8 .4472 2236.1 68.8 .4614 2167.3 66.5 .4760 2100.8 0.4765 2098.6 68.3 66.1 .4920 2032.5 .4920 2032.5 82.0 83.2 .5127 1950.5 .5130 1949.3 83.6 .5360 1865.7 81.9 .5606 1783.8 83.4 .5881 1700.4 84.9 .6190 1615.5 It will be seen from tables 12 and 13 that the last 3 fluorescence bands, counting from the red, are nearly or quite coincident with the first three absorption bands. Whether or not these coincidences are to be regarded as exact can not be determined from observations on unresolved spectra. It will be demonstrated later that reversals are exact between the ultimate components of bands, but not, in general, between unresolved aggregates. That the fluorescence interval changes to conform to the absorption interval at the last step appears not only from the data in tables 12 34 FLUORESCENCE OF THE URANYL SALTS. and 13, but also in the determinations for other salts (tables 4 to 10) wherever the final fluorescence band (8) has been observed. The corresponding change in the absorption interval to conform with the fluorescence interval is much more difficult to establish, because the last absorption band toward the red is entirely invisible under ordinary conditions. TABLE 13. — Absorption and fluorescence bands of uranyl nitrate at +20° C. Absorption bands.1 Fluorescence bands. M- 1/MX103. Interval. JU. 1/MX103. Interval. 0.3830 2610.9 69.6 .3935 2541.3 72.2 .4050 2469.1 71.0 .4170 2398.1 58.9 .4275 2339.2 69.1 .4405 2270.1 72.3 .4550 2197.8 71.9 .4705 2125.4 0.4708 2124.0 72.0 70.2 .4870 2053.4 .4869 2053 . 8 87.2 86.6 .5086 1966.2 .5086 1966.2 89.7 • .5329 1876.5 86.0 .5585 1790.5 85.8 .5866 1704.7 88.7 1 .6188 1616.0 1 Absorption bands, excepting that at 0.5086 are from measurements by Jones and Strong (Am. Chem. Journal, 1910). EFFECT OF WATER OF CRYSTALLIZATION— BEHAVIOR OF SOLUTIONS. The effects of water of crystallization and the comparison of the spectra of the solid uranyl compounds with those of their solutions are to be treated at some length in subsequent chapters. A few points which have been brought out in the course of our work on the spectra at +20° C. are, however, recorded here. The effect of water of crystallization in the case of uranyl nitrate is to shift the luminescence bands slightly in the direction of the longer waves. (Compare the hexahydrate with the anhydrous form in table 1.) This is the effect which it would seem most natural to expect, since the mass of the vibrating system is increased by the addition of water of crystallization without any increase, so far as we know, in the elastic FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 35 forces of the system. In fact, the presence of water so intimately associated with the salt molecule would probably increase the effective dielectric constant of the region in which the vibrations occur, and would thus cause a decrease in frequency quite independent of any effect due to increase in mass. It has been shown by Deusen1 and by Jones and Strong2 that the absorption spectrum of the crystallized nitrate is nearly coincident with the absorption spectrum of the aqueous solution. In many cases no difference can be detected in the wave-length of the band in solu- tion and in the solid crystal. In the case of other bands, however, the difference appears to be too great to be accidental. It seems not unlikely that the absorption spectrum contains several series of bands, some of which occupy almost identically the same po- sitions for the solution as for the solid salt. We must assume, therefore, that at least a part of the dissolved salt has the same molecular structure as the solid crystals. In the case of the uranyl sulphate studied by us the phenomena are more complicated. As has already been shown, the luminescence spectrum of this salt, even at ordinary temperatures, contains two series of bands, which for convenience we shall designate the a and (3 series respectively. The a bands are by far the stronger and 6 of these could be observed. Of the relatively weak 0 bands only 3 could be seen. In the absorption spectrum of the solid salt 2 series of bands were also found (see fig. 21) which we shall call the a' and /3' bands. Two of the a' bands corresponded in position with two of the a bands of luminescence, while one band of the /3' series corresponded with one of the |8 bands. The wave-lengths are given in table 14 and are shown graphically in figure 23. It is a remarkable fact that while the a bands .54 .58 .46 .50 FIG. 23. — Position of fluorescence and absorption bands of uranyl sulphate. TABLE 14. — Uranyl sulphate fluorescence and absorption bands. Fluorescence: Crystals— Principal series (a) 4763 4929 5148 Crystals — Secondary series OS) 4894 5098 Dehydrated salt (7) 4843 5049 Concentrated solution 4928 5145 Absorption : Crystals— a' series 4595 4755 4925 Crystals— 0' series 4555 4720 4880 Concentrated solution.. 4718 4887 5659 5395 5340 5285 5538 5387 5095 5925 1 Annalen der Physik, 43, p. 1128. 1898. 2 American Chemical Journal, vol. XLIII, p. 37, 1910. See also Vogel, Spectralanalyse, p. 270, 1889. 36 FLUORESCENCE OF THE URANYL SALTS. are much the brighter in the luminescence spectrum, the of bands in the absorption spectrum are much weaker than the (3f bands. The sulphate used in this experiment was in the form of small crystals, When the salt was dehydrated by being kept for about an hour in a stream of warm, dry air its luminescence spectrum was found to be absolutely different, each band being shifted toward the violet by about 100 A. u. Brief exposure to the air apparently permitted a portion of the salt to return to the original condition, so that the original a and/3 bands could be seen as well as the 7 bands charac- teristic of the dehydrated salt. In the case of a thin layer of the sulphate which had been dehydrated and then exposed for a short time to the air, each of the lumi- nescence bands was found to consist of three overlapping bands, the components corresponding in position to the a, /3, and 7 bands respectively. Spectrophotometric measurements (with a rather wide slit) of the brightest luminescence band and of a portion of the absorption spectrum of the same layer are shown in figure 24. In the luminescence spectrum the (3 bands are by far the most prominent,1 while in the ab- sorption spectrum the of bands are strong- est and no 7' bands can be detected. The results point to the existence of two dif- ferent hydrated salts corresponding to the a and /3 bands respectively, but further study would be necessary to make possible an entirely satisfactory explanation of the observed phenomena. The concentrated aqueous solution of the sulphate showed weak fluor- escence, and the three brightest bands, which could be located with reasonable accuracy, were found to agree in position with three of the a bands of the solid crystallized salt. In the absorption spectrum of the concentrated solution it was possible to locate three well-defined bands, two of which corresponded with two of the /? bands of the solid salt (see fig. 25). The solution showed no trace of any fluorescence corresponding to the /3 series, nor did it show any trace of absorption corresponding to the a' series. 1 The a' band appears in fig. 24 to be shifted by about 15 Angstrom units toward the violet; whether this is a real shift, or whether it is due to disturbances caused by simultaneous absorp- tion and luminescence we are unable to say. .48 .so FIG. 24. — Uranyl sulphate (solid), showing the brightest fluo- rescence band at about 0.51 n and a group of absorption bands at about 0.49 /i. FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 37 In a concentrated solution of potassium uranyl sulphate (see fig. 26) three absorption bands were found at 4,910, 4,730, and 4,570. These FIG. 25. — Uranyl sulphate (solution), show- ing at the left a portion of the trans- mission spectrum for a thin layer and at the right for a thick layer. .50 FIG. 26. — Transmission of a concentrated solution of uranyl potassium sulphate. do not agree in position with the corresponding bands of the solid salt, which occur at 4,920, 4,760, and 4,472. The solution of the double sulphate shows no trace of fluorescence. IV. PHOSPHORESCENCE OF THE URANYL SALTS. Concerning the phosphorescence of the uranyl compounds, we find little on record beyond the early observations of E. Becquerel,1 who, in his classic paper of 1861, noted the brilliant and very short-lived after-glow and made some observations on the law of decay. For the study of the phenomena of phosphorescence in these sub- stances and in other cases having a duration of glow of a few thou- sandths of a second, we devised a new instrument, the synchrono- phosphoroscope. Indeed, for the experiments to be described in this chapter, and which involved the use of surfaces of considerable size, the cooling of the substance during excitation, simultaneous observa- tions during fluorescence and phosphorescence, etc., none of the exist- ing forms are easily adapted. The original phosphoroscope of Bec- querel,2 later modified by E. Wiedemann,3 and also the revolving drum type used successively in various forms by Becquerel,4 Tyndall,5Kester,6 and Waggoner,7 afford sufficient speed, as does Merritt's8 phosphoro- scope of 1908; but none of these could be used without modification. The new apparatus9 consists of a small synchronous, alternating- current motor A. (7., figure 27, and a small direct-current motor D. C. upon a common shaft. To one end of the shaft is attached a sectored disk, WW, figures 27 and 28, with four equal open and four closed sec- tors, corresponding to the four poles of the A. C. motor. On the cir- cuit of 60 cycles this machine, when brought to speed by the D. C. motor and released, runs steadily at 30 revolutions per second. A "step-up" transformer TT, in the same alternating-current circuit, produces discharges at the spark-gap, or series of gaps (E), at each alternation, i. e., 120 times a second. This discharge may be reduced to a single spark by proper adjustment of the resistance and capacity of the circuit, or more conveniently for many purposes the discharge may be confined to the peak of the wave by means of the four-pointed star- wheel SS (figs. 27 and 28), which is mounted on the shaft and carefully adjusted as to phase. The direct-current motor may also be used to drive the sectored disk at other speeds, in which case the circuit of the motor A. C. is broken and the discharge is derived from any convenient source capable of producing a proper spark at each quarter revolution. 1 E. Becquerel. Annales de Chimie et de Physique (3), LXII, p. 1. 1861. 2 Ibid., LV, p. 5. 1859. 3 E. Wiedemann, Wiedmann a Annalen, xxxix, p. 446, 1888. 4 E. Becquerel, 1. c. 8 Tyndall. See Lewis Wright's volume on light, p. 152. London, 1882. 9 Kester, Physical Review (1), ix, p. 164. 7 Waggoner, Carnegie Inst. Wash. Pub. No. 152. 8 Nichols and Merritt, Carnegie Inst. Wash. Pub. No. 152. 9 E. L. Nichols: Proc. Nat. Acad. of Sciences, v. 2, p. 328. 1916. Also Science, XLIII, p. 937. 1916. 38 PHOSPHORESCENCE SPECTRA. 39 When the sectored disk WW is so adjusted on the shaft that the closed sectors conceal the phosphorescent surface during excitation by the spark, an observer, looking through the open sectors as they pass, sees the phosphorescence as it appears a few ten thousandths of a second after. The apparatus is thus suitable for the study of phosphorescence of very short duration or of the earliest stages in cases of slow decay. By shifting the sector on the shaft it is possible without variation in o , FIG. 27. FIG. 28. the rate of rotation to make observations at the very beginnings of phosphorescence and to compare, by simultaneous vision, the appear- ances just before and immediately after the close of excitation, or, on the other hand, the earlier with the later stages, up to about 0.004 second. The photometer, spectroscope, spectrophotometer, camera, etc., may all readily be used with this form of phosphoroscope and studies of the most varied character become possible. Phosphorescence is commonly regarded simply as the after-effect of fluorescence, the emission spectrum immediately after the close of exci- tation being identical with that immediately before excitation ceases. This has hitherto been only an assumption, since it is thinkable that the process which prepares a substance for phosphorescence might pro- duce emission during excitation differing from that which consti- tutes phosphorescence and which together with the latter would be present during fluorescence. It is also thinkable, although unlikely, that the phosphorescence might contain some components requiring a measurable time for development and observable only after an appre- ciable interval. This is a matter which it would be very difficult to settle in the cases of phosphorescence hitherto studied, because the spectrum of fluorescence and phosphorescence consists of broad bands or complexes of overlap- ping bands, and almost the only criterion of identity is that of color. 40 FLUORESCENCE OF THE URANYL SALTS. The uranyl salts, because of their remarkable spectra, afford an unusual opportunity to establish the exact relation between the emis- sion of light during excitation and at various times after excitation has ceased, and it was for this purpose that the first experiments with the new phosphoroscope were undertaken. The method, briefly outlined, is as follows : The substance, inclosed in a flat tube of glass BA about 8 cm. long and 2 cm. wide, is viewed through the rapidly revolving sectored disk of the synchrono-phos- phoroscope. It is mounted vertically, with its axis at right angles to the radius of the disk, as shown in figure 29. D 1 I I <— <-_-,— H I B FIQ. 29. FIG. 30. It is uniformly excited by zinc sparks 120 times a second while hidden by the closed sectors and is visible for 1/240 of a second during the passage of each of the intervening open sectors. A phosphorescent substance of slow decay appears under these cir- cumstances to be equally bright from top to bottom, but if one of the uranyl salts, such as the double uranyl-ammonio sulphate, which was the substance selected for detailed study, be used, it appears a very bright green at the bottom of the tube, shading off to bare visibility at the top. The rate of decay of this substance and of the other uranyl salts is so rapid that the upper end of the tube, which is seen at the intensity which corresponds approximately to the instant 0.003 second after excitation, has only a small fraction of the brightness of the lower end, which is viewed about 0.0005 second after excitation. The particular salt mentioned above was selected because at low temperatures its spectrum is unusually well resolved in groups of com- plexes of narrow, line-like bands, making it possible to detect changes in the individual components. To obtain simultaneous observations a pair of right-angled prisms was mounted before the slit of a Hilger spectroscope, as shown in figure 30. Light from the lower end of the tube A enters the lower half of the slit. That from the upper end B, after two total reflections, enters the upper half of the slit, and we have two spectra one above the other, PHOSPHORESCENCE SPECTRA. 41 FIQ. 3 la. Fio. 316. coinciding throughout as to wave-length, but separated by a dark line formed by the lower edge of the second prism (#')• To compare fluorescence with phosphorescence, the sectored disk was shifted upon its shaft until the lower end of the tube was viewed during excitation, the upper end immediately after (fig. 31a). To compare the phosphorescence spectrum at an earlier and later stage, the disk was so set that its position at the moment of excitation was as shown in figure 316. By means of the reflecting prisms at the slit of the spec- troscope, already described, the spectrum of the light emitted from region A was compared with that at B in each case. At +20° C. the banded spectra were found to be identical in every respect, except in brightness ; and the same was true at low temperatures, . where it was possible to in- spect each of the numerous line-like bands individually. Of the seven homologous series distinguishable in the fluorescence spectrum, all were present in phosphorescent light, unshif ted as to posi- tion and not perceptibly enhanced or diminished in relative brightness. The comparison was less satisfactory as regards minor details in the case of the early and late stages of phosphorescence, some of the fainter bands being invisible, but changes such as might be looked for, i. e., those due to the greater persistence of certain series, could scarcely have escaped notice. The significance of these observations is two-fold: On the one hand we find that for the only examples of luminescence which admit of such detailed inspection the spectrum of phosphorescence is identical with that of fluorescence, and since there are no indications to the contrary in the case of other classes of substances thus far studied, it is probable that the above statement will apply to all phosphorescent materials. On the other hand, we find that, in spite of its great complexity, the lumi- nescence spectrum of a uranyl salt is to be regarded as a unit, all its components decaying at the same rate after the cessation of excitation. Thus this class of substances (i. e., the uranyl salts) not only conform to the first three criteria of homogeneity discussed in Chapter II but likewise to that based upon the phenomena of phosphorescence. CURVES OF DECAY. To determine the change of intensity of phosphorescence with the time a simple form of photometer previously used in a study of the phosphorescence of kunzite1 was mounted in front of the sectored disk. 1 Nichols and Howes, Physical Review (2), iv, p. 19. 1914. 42 FLUORESCENCE OF THE URANYL SALTS. A lateral strip of the phosphorescent salt 1 cm. wide was excited by sparks from a single spark-gap between zinc terminals and measure- ments of the brightness were made at various times after the close of excitation. The necessary conditions were attained by shifting the disk successively through small angles, so as to vary the interval between excitation and observation. The time could be estimated with sufficient accuracy by noting the instantaneous positions of the disk for each adjustment, as given by the strictly synchronous illumina- tion due to the spark. zx 0 — 1 Y -i L.a. x _ \J c s FIG. 32. The arrangement of the apparatus is shown in figure 32, in which P is the phosphorescent surface, DD the sectored disk, L. B. the Lummer- Brodhun cube of the photometer, E the eyepiece, S a color-screen and matte translucent plate, C the comparison lamp which traveled along the track of an optical bench. The cross at Z indicates the position of the spark-gap. In table 15 relative intensities 7, the reciprocals l/A/zT and times T after excitation are given. Figure 33 shows the relations between I and T, and 1/VFand T respectively in the usual manner. TABLE 15. T. /. i/Vi. T. /. i/Vi. 0.000479 59.49 0.130 0.00170 2.03 0.702 . 000637 27.78 .190 .00193 .971 1.014 .000856 16.02 .250 .00212 .610 1.280 .000949 12.62 .281 .00247 .296 1.836 .00110 9.80 .319 . 00287 .159 2.524 .00146 5.03 .446 As appears from the table and curve ABC, figure 33, this substance exhibits a remarkably rapid decay, falling in the interval between 0.0005 second after close of excitation and 0.003 second to less than three-thousandths of its intensity at the beginning of that interval. To show the degree of accuracy with which the lower intensities were observed, the portion of the curve BC is reproduced with ordinates magnified ten times B'C'. _The results are likewise plotted in the cus- tomary manner with 1/Vl as ordinates (curve DEF), and this brings PHOSPHORESCENCE SPECTRA. 43 out an unusual characteristic. It is usual to find two processes of phosphorescence succeeding one another and represented by the two straight arms of the curve DE and FG, but in all the numerous cases hitherto described, excepting two to be discussed in a subsequent para- graph, the later process (FG) is indicated by a curve of lesser slope. In the case of this uranyl salt, however, FG trends very sharply up- ward, showing a greatly accelerated decay. I. URANYL AMMONIUM SULPHATE. 2.URAMYL POTASSIUM SULPHATE. 3. URANYL NITRATE. 4. URANYL SULPHATE. 5. URANYL AMMONIUM CHLORIDE. URANYL AMMONIUM SULPHATE 0.0 .002 SEC 001 .002 .003 SEC. FIG. 33. FIG. 34. By means of these preliminary observations certain facts may be regarded as established.1 These may be summarized as follows: (1) There is no appreciable change of color during decay. (2) The decay of phosphorescence is exceedingly rapid, the intensity falling to one-thousandth of its initial value within 0.0035 second. (3) The very complex fluorescence spectrum at — 180° C. is identical in structure and relative distribution of intensities with that observed during the earlier and later stages of phosphorescence. 1 Nichols, Proceedings National Academy of Sciences, vol. n, p. 328. 1916. 44 FLUORESCENCE OF THE URANYL SALTS. (4) The curve of decay of phosphorescence differs from the prevailing type in that although as usual two successive processes are distinguish- able, the second process is more rapid instead of being slower than the first. The study of these phenomena has since been extended to several other typical uranyl salts, the curves of decay of which were deter- mined by the method just described and under conditions of excita- tion, etc., as nearly constant as possible.1 These curves of decay are of the same new type originally found in the uranyl ammonium sul- phate. The two processes, as determined by the customary method of plotting 7-1/2 as a function of the time are indicated by straight lines differing from one another in slope and the second process has in all cases the steeper gradient. Later experiments, in which the intensity of excitation was increased, revealed the presence of a third process not included within the interval of time covered by our earlier experiments. STUDIES INVOLVING THE FIRST AND SECOND PROCESSES. The curves shown in figures 34 and 35 are typical of the results obtained with all the salts under observation. They represent the decay of the phosphorescence of the compounds shown in table 16. The initial intensity, under like TABLE 16 excitation, varies greatly in the different salts, as also, to some ex- tent, does the rate of decay. It will be noted that the initial intensities of the ammonium and potassium sulphates, for example, are several times greater than those of the nitrate, the sulphate, and the am- monium chloride. This is, however, a question of previous history as well as of chemical and physical constitution, as was determined in the following manner: Uranyl potassium sulphate was dissolved in hot water and a mass of the minute crystals which were thrown down on cooling the solution were immediately sealed up in a glass tube. Care was taken through- out these manipulations to protect the precipitate from the action of light. This sample, still in darkness, was mounted in the synchrono-phos- phoroscope and a curve of decay was taken, the first exposure to excit- ing light being that at the beginning of the run. The substance then showed, temporarily, a brilliancy of phosphorescence much above that to be obtained under ordinary circumstances, but was soon reduced to its normal and semi-permanent condition, after which the usual curve of decay was obtained. 1 Nichols and Howes, Physical Review (2), ix, p. 292. 1917. Curve. 1 2 3 4 5 Substance. Uranyl ammonium sulphate. Uranyl potassium sulphate. Uranyl nitrate + GHaO. Uranyl sulphate. Uranyl ammonium chloride. PHOSPHORESCENCE SPECTRA. 45 I" 80 EXCITATION IN THE PRESENCE OF RED AND INFRA-RED RAYS. To determine whether red or infra-red rays have an effect on these substances similar to that observed in the case of the phosphorescent sulphides, a modification of the apparatus was made such that the surface under examination could be subjected to the intense illumina- tion obtained by focusing the crater of an electric arc upon it. A screen of excellent ruby glass was interposed to cut off all but the longer waves and observations were made through a screen quite impervious to red. Exposure to this source was found to affect measurably neither the brightness of fluorescence nor of phosphorescence. Curves taken after exposure to this source, those taken with the substance subjected to it interruptedly throughout the run, and curves in the determination of which readings were taken alternately with and without red light were all identical with those taken in entire absence from such expos- ures. The striking contrast between this negative result and the well-known effects of infra-red radiation upon the phosphorescence of the sul- phides is notable. The observations already cited, showing the complete identity of the spectrum of fluorescence with that of phos- phorescence seemed to indicate that the intensity would go over from that of fluorescence to that of phosphorescence without discontinuity. This conclusion was confirmed, within the errors of observa- tion, by measurements just before and after the close of excitation. The only previous instances where this relation has been experimentally established, so far as we know, are to be found in Waggoner's1 studies of phosphorescence of short duration and in recent observations on the luminescence of kunzite.2 In view of the unexpected character of the decay curves for the phos- phorescence of the uranyl compounds, the question arises whether the rather unusual mode of excitation employed, i. e., periodically repeated exposures, 120 times a second, to groups of sparks of high frequency, might produce such a result, or whether the decay curves are character- 1 Waggoner, Physical Review, xxvu, p. 209. 2 Nichols and Howes, Physical Review (2), iv, p. 26. eo 40 20 FIG. 35. 46 FLUORESCENCE OF THE URANYL SALTS. istic of this class of compounds, whatever the mode of excitation. It is true that both Waggoner1 and Zeller,2 using a Merritt phosphoro- scope, found in their studies of phosphorescence of short duration that excitation by means of a spark discharge very similar to our own gave decay curves of the usual type. It is also obvious from the measurements already described that the interval between excitation, i. e., 1/120 second, is sufficient for the com- plete discharge of the phosphorescent glow, and since the absence of any effect of red and infra-red indicates that there is no storage of undeveloped energy to be carried over, such as occurs in the phosphor- escent sulphides, it seems probable that the decay curves do not vary greatly from that which might be obtained, were it possible to make the experiment, from a single exposure. To test this a run was made upon the sample of uranyl ammonium sulphate previously used, but with the Merritt phosphoroscope. By driving the disk of this instrument 3,000 revolutions a minute, much the same range of time intervals was available as with the synchrono-phosphoroscope. To further vary the conditions, a quartz mercury arc was substituted for the spark-gap of Waggoner and Zeller. The arrangement of appar- atus was as shown in figure 36, in which DD is the revolving disk, H the mercury lamp, P the phosphorescent substance, LB the Lummer-Brodhun cube of the photometer, SS a color- filter and milk-glass screen. The device for shifting the oblique mirror M with reference to the aperture A in the disk is not shown. Although the decay was some- what more rapid in this determination on account of the less intense excitation, the curve was of precisely the type obtained by the prev- ious method. Measurements upon some of the bands of brief duration in the spec- trum of the phosphorescent sulphides, recently made with the syn- chrono-phosphoroscope under experimental conditions identical with those here described,3 yield curves of the usual type associated with these sulphides, so that the question of the change of form being due to the phosphoroscope employed is effectually eliminated. 1 ) S.&£ 3 P / i t O" 1 | L.B. FIG. 36. 1 Waggoner, Physical Review, xxvn, p. 209. 2 Zeller, Physical Review, xxxi, p. 367. 3 Nichols, Proc. Am. Philosophical Society, LV, p. 494. 1916. PHOSPHORESCENCE SPECTRA. SOLID SOLUTIONS AND SEMI-FLUIDS. 47 The uranyl salts differ from nearly all if not all phosphorescent sub- stances hitherto studied. We do not have, as in the phosphorescent sulphides, the preparations of Waggoner, the ruby, etc., to deal with a trace of active material in solid solution, but with compounds that are in themselves brilliantly phosphorescent. If the peculiar character of the curve of decay is due to that fact it might be expected that uranium URANYL SULPHATE URANYL POTASSIUM SULPHATE ' URANYL AMMONIUM SULPHATE FIG. 37 FIG. 38. glass, in which the active material is considered to be in a state of solid solution, would have a law of decay corresponding to the prevailing type for such solutions, i. e., with the first process as indicated by the curve for /-1/2, and time, represented by a line of steeper slope than the line for the second process. A piece of uranium glass gave, however, a decay curve similar to those of the uranyl salts (see fig. 37). Another 48 FLUORESCENCE OF THE URANYL SALTS. preparation which differs from most of the uranyl salts is the uranyl sodium phosphate, a sample of which was made by D. T. Wilber for certain studies in fluorescence recently published.1 This substance is a very viscous liquid with the characteristic green fluorescence of the uranyl compounds. One might expect, in accordance with the findings of Becquerel for liquids in general,2 that there would be no observable after-glow. It EFFECT OF TEMPERATURE ON URANYL AMMONIUM NITRATE 20" 30 -180° 20 10 jQ07 SEC. FIG. 39. is true that Becquerel expressed the belief that with a phosphoroscope of sufficient speed, phosphorescence would probably be detected in fluorescent liquids, but no one, so far as we know, save Dewar in an unconfirmed statement concerning a supposed phosphorescence of liquid air, has since that time (1859) recorded an instance of phos- phorescence excepting in solids and gases. When a tube containing the phosphate was tested with the syn- chrono-phosphoroscope no phosphorescence was found of duration 1 Howes and Wilber, Physical Review (2), vol. 7, p. 394. Mar. 1916. 2 See E. Becquerel, La Lumiere, vol. i, chapter on Phosphorescence. PHOSPHORESCENCE SPECTRA. 49 sufficient to be detected. Another sample so prepared as to reduce the amount of water to a minimum did, however, exhibit phosphorescence of measurable duration. This preparation, so slow was its rate of flow, might be regarded as a plastic solid rather than a viscous liquid. A bead of microcosmic salt, colored in the usual manner with uranium oxide, was comparable in its phosphorescence with canary glass. URAMYL AMMONIUM SULPHATE I'* 50 40 30 20 10 .001 .002 .003 SEC. FIG. 40. It appears that the persistence of luminescence is due to the con- sistency of the substance and disappears as the fluidity increases ; also that the peculiar type of decay here described is common, not only to the crystalline uranyl salts in general, but also to the gelatinous forms, as in this double salt, and to substances in which uranium appears in solid solution, as in the case of the canary glass. THE THIRD PROCESS. E. Becquerel,1 in the course of his great pioneer work on phosphores- cence, made a number of observations on the uranyl salts and on 1 E. Becquerel, Annales de Chimie et de Physique (3), LXII, p. 1. 1861. 50 FLUORESCENCE OF THE URANYL SALTS. uranium glass. He noted the brilliant initial intensity and very rapid decay, and to test the independence of the constant in his equation of decay when the illumination varied he made many measurements. If from his data we compute I~1/2, as a function of the time, we obtain curves of the same general form as those in figure 32. Becquerel's observations are not numerous enough, taken by them- selves, to determine completely the type of curve. His measurements, however, cover a larger time interval than ours and the values for the longest times indicate an even more rapid decay following the second process. We had, indeed, found some indications of a similar tendency which had been omitted from our curves as lying almost beyond the range of definite determination. To investigate the further trend of the curves of decay, the intensity of excitation was increased by readustment of the sparking circuit, by which means it was found possible to extend the time interval for more than 0.006 second beyond the cessation of excitation. Careful, often repeated measurements, of the various salts showed in fact a third linear process beginning where our previous determina- tions had ceased and having a steeper slope, indicative of still more rapid decay. Typical results are indicated in figures 38, 39, 40, etc. These processes may be numbered for convenience 1, 2, and 3 in the order in which they occur. Processes 1 and 2 are in general of about equal duration for a given salt. The abruptness of transition, however, varies greatly, and in some instances the change of slope is so gradual as to encroach seriously on process 2 at both ends. THE INFLUENCE OF TEMPERATURE. The only previous instances of decay of phosphorescence in which the later stages are more rapid than those preceding are noted by Ives and Luckiesh1 in their study of the influence of temperature on phosphores- cence, and by E. H. Kennard2 in a more recent paper. Ives and Luckiesh measured the phosphorescence of one of Lenard and Klatt's sulphides (BaBiK from Leppin and Masche). This sub- stance was found to be very sensitive to change of temperature and the results at 0°, 22°, and 35°, C. when plotted for 7-1/2 and time in the usual manner, gave curves varying greatly in slope. The curve for 0° is concave toward the time axis, that for 22° linear, and that for 35° strongly convex. They show that a linear relation may be obtained for each of these curves by varying the exponent of 7. The effect of temperature in the case of the phosphorescent sul- phides, where one has to do with a composite of many overlapping bands of varying duration, is undoubtedly different from that to be 1 Ives and Luckiesh, Astrophysical Journal,. xxxvi, p. 330 (1912). 2 Kennard, Physical Review (2), iv, p. 278 (1914). PHOSPHORESCENCE SPECTRA. 51 expected with the uranyl salts, where the spectrum, in spite of its complexity of structure, is a unit. It was deemed of interest, however, to determine the effect of temperature upon the latter. For this purpose a specimen of the uranyl ammonium nitrate was mounted within a cylindrical Dewar flask with unsilvered walls and its decay of phosphorescence was determined with a synchrono-phos- phoroscope at a temperature a few degrees above that of liquid air (about —180°) at +20° and at 4-60°. The last-named temperature was maintained during the run by means of an electrical heating-coil. 2.8 5.2 ~ URANYL RUBIDIUM NITRATE VARYING EXCITATION 11.9 50 40 30 20 sec. FIG. 41. The principal change consists in a marked retardation of decay with lowering temperature (see fig. 39), but this is not a universal charac- teristic of the uranyl compounds. Uranyl ammonium sulphate, for example (fig. 40), is but slightly influenced in its rate of decay by change of temperature and the curve for — 180° is intermediate between those for +20° and +60°. • 52 FLUORESCENCE OF THE URANYL SALTS. THE EFFECT OF VARYING THE INTENSITY OF EXCITATION. To determine the effect of the intensity of excitation, a series of measurements were made with the spark-gap at various distances from the phosphorescent surface. The substance observed in these experi- ments was uranyl rubidium nitrate. It was found possible to make observations of the decay of phosphorescence with the excitation reduced to a two-hundredth of that usually employed. From the curves obtained, of which four are given in figure 41, it will be noted that all three processes are present, whatever be the intensity of the exciting light; also that, taken roughly, processes 1 and 2 are of nearly equal duration, and that with decreasing intensity of excitation the duration of each of these processes diminishes. DURATION OF PROCESSES WITH EXCITATION E. .40 Ifc2 .30 .20 .10 .001 .002 FIG. 42. .003 SEC. These relations are better shown in figure 42, in which the duration of process 1 and the sum of the duration of processes 1 and 2, counting from the close of excitation, are plotted, with the intensity of the ex- citing light as ordinates. Approximately in both cases the duration is proportional to the natural logarithm of the excitation. (See table 17.) This decrease in the duration of the two processes with falling excitation affords an obvious explanation of the varying character of PHOSPHORESCENCE SPECTRA. 53 the curves of decay of phosphorescent substances. Where the excita- tion is chiefly superficial, as in the case of some powders, the excitation may be nearly of one intensity and the curve made up of well-defined linear processes with sharp inflection-points. We have found this to be the case in many instances. Where, on the other hand, fluorescence is excited within the crystalline mass by rays that have suffered con- siderable loss by absorption, etc., there will be a wide range of intensi- ties of excitation and a curve results with distributed knees and linear processes shortened and sometimes almost obliterated. We observed this particularly where a clear crystal was mounted with faces per- pendicular to the photometer and was excited from behind so that the light emitted by the surface nearest the exciting source passed through TABLE 17. — Variation of length of processes with excitation (phosphorescence of uranyl rubidium nitrate). Duration. Intensity of excitation / T~I\ Nat. log. E. Process Process Process (E). 1. 2. 1+2. sec. sec. sec. 41.70 6.033 0 . 0022 0.0018 0.00400 13.70 4.919 .00170 .0014 .00310 2.52 3.220 .00147 . 000993 .00240 1.35 2.590 .00118 .00080 .00198 .900 2.190 .00080 .00090 .00170 .476 1.560 .00070 .00076 .00146 .201 .698 . 00050 . 00060 .00110 the crystal and was partially absorbed. Excitation occurred within the crystal in diminishing amount with increasing depth and the com- posite phosphorescence reaching the eye under such conditions showed this blending effect to a marked degree. The same crystal when excited from in front gave a curve in which the angles between pro- cesses were made more sharply defined. The effect in question is probably a general one and may well account for the perplexing differ- ences in the curves of decay obtained under slightly varying circum- stances. Thus, one observer will obtain an angular curve, where another, studying the same material, can detect no linear processes. The same observer, indeed, in attempting to repeat his measurements, will often find the above-mentioned change of type under conditions which seem to be identical but in which the same relations as regards superficial and internal excitation are not preserved. We found in the study of this effect a crystal one smooth face of which gave the blended curve, while the opposite face, which was rough, gave the angular curve, a change produced and reproducible by merely rotating the crystal through 180°. 54 FLUORESCENCE OF THE URANYL SALTS. THE PHOSPHORESCENCE OF VARIOUS NITRATES. Observations were made on a series of nitrates previously prepared for the detailed comparison of the fluorescence spectra of that salt.1 These consist of crystals with 6 H20 (rhombic), 3 H2O (triclinic), and 2 H2O (system undetermined) as water of crystallization and a speci- men sealed in glass which had been rendered as nearly anhydrous as was possible without decomposing the nitrate. The curves of decay indicate a much slower rate of decay for the crystalline forms than for the anhydrous nitrate. Whatever effect the amount of water of crystallization may have is doubtless masked by the far greater influence of the crystalline form. This is perhaps to be expected, since, as will be shown in Chapter VII, these specimens exhibit as great differences in the structure and appearance of their fluores- cence and absorption spectra as commonly exist between entirely distinct uranyl salts. Similar differences in the case of salts similar in composition but differing in crystalline form will likewise be described in a subsequent chapter. A — " E3 A . U1^ 1 FIG. 43. OBSERVATIONS ON POLARIZED PHOSPHORESCENCE. Certain crystals of the double chlorides of uranyl exhibit fluorescence spectra consisting of sets of bands polarized at right angles to one another. To determine whether these components after the close of excitation decay independently or without change in their relative intensities, the following experiment was made : A crystal of the rubidium uranyl chloride that exhibited the phe- nomenon of polarized fluorescence was mounted behind the disk of the synchrono-phosphoroscope and was observed with a spectroscope. 'Nichols and Howes; Physical Review (2), ix, p. 292. 1917. PHOSPHORESCENCE SPECTRA. 55 The slit of the latter instrument was divided into two parts by means of an opaque strip across the middle (S, fig. 43) . Within the collimator a doubly refracting rhomb R and Nicol prism N were mounted. The rhomb gave two slit-images vertically displaced and the adjustment was such that the lower part (A) of one image was contiguous with the upper part (B) of the other. Thus two spectra of the phosphorescent field were obtained corre- sponding to the two polarized components. These presented the usual distinctive structures at whatever stage of the phosphorescent decay they were observed. By rotation of the Nicol prism the two fields could be brought to equal brightness for any given part of the spec- trum, and this balance, if made with the sector of the phosphoroscope set so as to give observations at 0.0005 second after extinction, was found equally correct up to 0.005 second or as long as phosphorescence was observable. The two components therefore decay at the same rate. SUMMARY OF PHOSPHORESCENCE OF SHORT DURATION. (1) All uranyl salts thus far examined possess the same type of phosphorescence; i. e., with increasing instead of diminishing rates of decay. (2) This is true not only of the crystalline forms, but also of uranyl compounds in solid solution or in the plastic state characteristic of the double phosphates. (3) The initial brightness of phosphorescence under like excitation varies greatly with the different salts, as does also to some extent the rate of decay. (4) The brightness of a salt newly prepared in darkness is greater when first excited than subsequently, but it soon reaches a nearly stable condition. (5) Exposure to red and infra-red rays is without effect as regards the rate of decay. (6) The phosphorescence, like the fluorescence, of the uranyl salts appears to be independent of the mode of excitation, and the structure of the intricate spectrum is the same during excitation and throughout the observable phosphorescent interval. (7) Changes in the rate of decay are not continuous, but occur in definite steps, there being at least three successive processes within the interval covered by observations, i. e., about 0.006 second. These processes follow a law such that 7-1/2 is jn linear relation to the time. (8) The first and second processes, counting from the close of excitation, are of nearly equal duration, increasing in duration with the intensity of excitation in such a manner that the duration of the process is approximately proportional to the natural logarithm of the excitation. 56 FLUORESCENCE OF THE URANYL SALTS. (9) In certain salts, such as uranyl ammonium nitrate, decay is retarded by cooling; in other cases the temperature effect is slight. (10) Uranyl nitrates with 2, 3, and 6 molecules of water of crystal- lization vary greatly in the rate of decay, but the changes in crystalline form appear to be more important in this respect than the amount of water. (11) In the case of the polarized spectra of the double chlorides, both components decay at the same rate and no change in relative bright- ness can be detected throughout the range covered by observation. PHOSPHORESCENCE OF LONG DURATION. While comparing the spectra of uranyl salts under excitation by kathode rays and under photo-excitation, in 1917, Misses Wick and McDowell discovered that certain salts continued to glow for several minutes after bombardment in the vacuum tube, at the temperature of liquid air. Many uranyl compounds are unstable in vacuo, and of those which are not decomposed rapidly, some, notably the chlorides, are prac- tically inactive under the kathode rays. The following salts, which were prepared by Mr. Wilber in the form of fairly large, well-formed crystals, gave bright fluorescence and were fairly stable : Uranyl potassium nitrate, K2UOz(NOs)4 (crystallized from 10 to 30 per cent nitric acid). Uranyl potassium nitrate, K2U02(NO3)4 (long crystals from 2 to 3 per cent nitric acid). Uranyl potassium nitrate, KUO2(NO3)3 (water form). Uranyl potassium nitrate, KUO2(NOs)3 (anhydrous). Uranyl potassium sulphate. Uranyl potassium sulphate (with 2 molecules of water). An examination was made of all of this group. They were found to exhibit phosphorescence in varying degree. Some showed no phos- phorescence of noticeable duration. The following, which were among the brightest, were selected for study: (1 and 2) K^UO^NOsV The first form, A, was crystallized from a 10 to 30 per cent solution of nitric acid, and the second form, B, from a 2 to 3 per cent solution. Although the crystallographic form is identical, form A crystallizes in short, thick crystals and form B in long, slender crystals. There appeared to be a slight difference in the phosphorescence of the two forms. It is possible, however, that the difference observed might have been due to some variation in the conditions under which the phosphorescence was produced. (3) K2U02(S04)2. To ascertain whether, as the result of exposure to the kathode rays, the surface layer of the crystals had undergone some change which rendered them capable of persistent phosphores- cence under photo-excitation, they were alternately illuminated by the light of a carbon arc and bombarded by the kathode rays. To accom- PHOSPHORESCENCE SPECTRA. 57 plish this without changing any conditions except the mode of excita- tion the tube containing the crystal under observation was mounted within an unsilvered cylindrical Dewar flask and cooled to the tempera- ture of liquid air. Light from a carbon arc was focussed upon the crystal through the walls of the Dewar flask and of the vacuum-tube, producing intense fluorescence, but there was no after-glow of duration sufficient to be detected. The kathode discharge, however, caused the persistent phosphorescence already described and the effect appeared to be distinctly cumulative, requiring excitation for several seconds. After the phosphorescence had died away, photo-excitation was re- sumed, and this process was repeated many times without observable change in the effect of the light. IDENTITY OF THE SPECTRA DURING FLUORESCENCE AND KATHODE- PHOSPHORESCENCE. To determine whether the spectrum, during this persistent phos- phorescence, corresponded with the fluorescence spectrum, settings on several of the brightest bands were made with the Hilger spec- troscope. The result was the same as the observations upon the brief phosphorescence following photo-excitation, described in an earlier paragraph of this chapter; i. e., the spectra were found to be identical during and after excitation and remained unchanged in character as long as they were visible. CURVES OF DECAY FOR THE KATHODE-PHOSPHORESCENCE. Misses Wick and McDowell also determined the law of decay for the three salts (1, 2, and 3) selected for investigation. Since the effect lasted for several minutes, it was possible to use the method commonly employed in such measurements. The arrangement of the apparatus is shown in figure 44. FIG. 44. A Lummer-Brodhun cube A was placed at one end of a track XY, about 3.5 meters long. The crystal B was placed opposite one face of the cube. The comparison source L was a 5-volt tungsten lamp placed in parallel with a suitable rheostat upon a 55-volt circuit. The lamp was mounted in a carriage C, running on the track XY, on which, at intervals of about 25 cm., stops were placed. Green, blue, and ground glass absorption plates P and P' were inserted to obtain a comparison source of the proper color and intensity. A chronograph was used to record the time. The zero of time was in every instance recorded when the primary circuit of the induction coil was broken. When the 58 FLUORESCENCE OF THE URANYL SALTS. intensity of phosphorescence matched that of the source in the first possible position, the time was again recorded and the carriage moved to the next stop, and allowed to remain until a match was made as before. This procedure was continued until the phosphorescence was too faint to observe or until the end of the track was reached. The interpretation of the results was somewhat difficult, since the instability of the crystals rendered uncertain both the control of the vacuum and the maintenance of the crystal surface unchanged during prolonged bombardment. The general shape of the decay curve after long excitation is shown in figure 45. The curves are of the type usual with phosphorescence of long duration, consisting of two linear processes, of which the first is the more rapid, whereas, as has been shown in the previous portions of this chapter, the decay following photo-excitation is of a new and entirely different type. too 200 400 SECONDS Fio. 45. 100 200 SCCONDS FIG. 46. 100 20* SECONDS FIG. 47. Under different conditions, phosphorescence was observed to last from less than a minute to 10 or 15 minutes. The exact form of the curve varied with the time of excitation. The time of decay was found to increase with the time of excitation, as shown in figure 46, but the initial brightness changed relatively little. There was some evidence to indicate that under similar conditions of vacuum the rate of the first process remained practically unchanged for varying times of excita- tion, but that the second process began sooner for longer excitation, as shown in figures 47 and 48. In figure 48, curves 1 and 2, obtained by a short-time excitation, show only the first process, whereas curves 45 and 46, obtained by excitations of 40 and 80 seconds respectively, indicate that a state of saturation had been reached such that added excitation produced no change in the phosphorescence. As has been stated, the initial brightness and rate of decay were found also to depend upon the strength of the bombardment, as varied by the pressure in the tube and by the voltage applied to the induction PHOSPHORESCENCE SPECTRA. 59 coil. The curves of figure 45, for example, were obtained with a rela- tively high vacuum, whereas those of figure 49 were obtained with a very low vacuum, so that the decay was comparatively rapid and there was only a suggestion of the beginning of the second process in the position of the last point observed. Slight changes in temperature, such as were produced when the liquid air fell below the line of the crystal, were found also to produce changes in the initial brightness and rate of decay. too 25 50 75 SECONDS FIG. 48. too To determine whether the excitation produced any secondary change in the crystal, which persisted after the phosphorescence had dis- appeared, so that there would be a progressive building up of the phosphorescence, excitations were made of equal length, repeated at as nearly equal intervals as decay observations permitted. Figure 49 shows that, at a fairly low cathode vacuum, an excitation of 20 seconds, repeated at approximately 1 -minute intervals, produced identical n aoo too 10 soo ri 100 to SECONDS FIG. 49. 100 SECONDS FIG. 50. 250~ decay curves. The same effect is shown in figure 50 for a much longer period of decay. When the time between excitations was short as compared to the time and strength of excitation, there appeared to be a progressive change, as indicated in figure 51. 60 FLUORESCENCE OF THE TJRANYL SALTS. From this investigation by Misses Wick and McDowell, two definite conclusions may be drawn: (1) The spectrum of the long-time phosphorescence produced by cathode-ray excitation at liquid-air temperatures is identical with the fluorescence spectrum. 20 10 SO too SECONDS FIG. 51. (2) The decay curve of the cathode phosphorescence differs in the most striking manner from that of the brief photo-phosphorescence. It corresponds in type with that usually found in cases of phosphores- cence of long duration. V. THE MORE INTIMATE STRUCTURE OF URANYL SPECTRA AS REVEALED BY COOLING. It was first shown by J. and H. Becquerel and Onnes,1 who studied the spectra of several of the uranyl salts when excited to fluorescence at the temperature of liquid air and ultimately at that of liquid hydro- gen, that each band of the spectrum as we know it at +20° is resolved into a group of much narrower bands. It was further shown by these investigators that all of the various groups of bands in a given spectrum were resolved in precisely the same manner, the homologous com- ponents forming series. This more intimate structure, which is revealed by cooling, may be studied to great advantage in the case of the double chlorides, which salts, as has been noted in Chapter III, have spectra sufficiently resolved at +20° so that the origin of the components observed at — 185° can be traced and the relation of the two spectra to one another much more definitely determined than is the case where the spectrum at +20° consists of unresolved bands. Four of these chlorides have the following composition : U02C12. 2KC1+2H2O. UO2Cl2.2RbCl+2H2O. U02C12. 2NH4C1+2H20. UO«Cl2.2CsCl. They crystallize in triclinic plates which are strongly fluorescent and their spectra, which are almost identical in structure, are resolved at room temperature into 8 groups of narrow bands. Each group, which corresponds to a single band of the ordinary uranyl fluorescence spectrum, consists of 5 nearly equidistant bands. The symmetry of these spectra, as they appear to the eye when viewed with a spectro- scope of moderate dispersion, is most striking. The bands are well separated from their neighbors and are about one-tenth as wide as the bands of the ordinary type of uranyl spectra. The distribution of intensities within the group has been determined for the visually brightest group in the spectrum of the ammonium uranyl chloride by means of the spectrophotometer. The results of such a determination are given in table 18, and are shown graphically in figure 52. The curve (fig. 52) which forms an envelope of the group of bands is of the same type as that for the distribution of intensities in a single band of the ordinary uranyl fluorescence spectrum and of the envelope of the set of bands in such a spectrum and is also similar to curves of distribution of the fluorescent spectra having a single broad band.2 The effect of cooling is likewise analogous, the envelope for —185° being narrower on account of the great relative reduction in brightness of the outlying members of the group. All the bands are shifted in 1 Becquerel and Onnes, Leiden Communications, 110. 1909. 2 See Chapters II and III. 61 62 FLUORESCENCE OF THE URANYL SALTS. TABLE 18. — Intensities of bands in group 6 (excited at +20° C). Band. Intensity. 5306M 18 5259 33 5208 24 5159 11 5119 0) 1 Visible, but too dim for spectrophotometric measurement. position as well as changed in intensity in a manner to be described in a subsequent paragraph. To determine as closely as possible the wave-lengths of the bands, photographs of the spectra of the four double chlorides were taken and many visual settings were made. Fluorescence was excited by means of the carbon arc, the light from which passed through a screen opaque to rays of wave-length greater than about 0.45 /* and was focussed upon the crystal. Various exposures were employed on account of the great d fferenees in the intensity of the bands and special plates were used for the red end of the spectrum. The exciting light was excluded from the camera by the use of suitable screens opaque to the blue and violet except where the absorption spectrum was to be recorded. The various negatives were measured by mount- ing them on a micrometer stage in the field of the lantern. The micrometer-screw was carefully cali- brated, so that wave-lengths could be determined by measuring the distance of the crests of the bands from certain reference lines of the mercury spectrum, which was photo- graphed on each negative so as to overlap the fluorescence spectrum. This method of projection was found better than the use of the comparator commonly employed in the measurement of line spectra, because of the hazy character of the bands and because bands that are so weak and vague as to be invis- ible even under a low-power micro- scope could be seen and ocated by means of the lantern. Many measurements of the stronger bands were made with the comparator as a check on the determinations with the lantern. These measurements confirmed to a remarkable degree the apparent symmetry of the spectrum. When all the bands are plotted on a large scale, in a diagram with the recip- rocal of wave-lengths as abscissae, the spectrum is seen to consist of 8 groups of 5 bands each, as already described. The nearly uniform arrangement of the bands of each group repeats itself precisely from group to group, so that corresponding members of the groups form an .52 FIG. 52. INTIMATE STRUCTURE ON COOLING. 63 homologous series of equidistant bands. This interval, moreover, is very nearly the same for all five of these homologous series; but although the departures from equality are of the same order as the errors of measurement, there is reason, as will be seen later, to regard them as real. The general arrangement of the bands in these spectra is roughly depicted in figure 53, which is based upon measurements of the fluores- cence spectrum of the ammonium uranyl chloride. Horizontal dis- tances are plotted on the scale of frequencies, the corresponding wave- .AA7 . .6'4 .6'0 l\ .5T6 FIG. 53. A /Ui lengths being indicated for convenience. Vertical heights indicate relative intensities, but with some pretence of accuracy. The first and eighth groups at the extreme left and right, for example, if drawn to scale, would be scarcely visible. They are, in fact, so feeble that they can be observed only with the greatest difficulty. The location of the various bands of the 4 double chlorides, in wave-lengths and in frequencies (l//j X 103) is given in table 29 at the end of this chapter. The values given are the averages of several readings from the photo- graphs and from visual settings. The bands in each group from the red toward the violet are designated by the letters B, C, D, E, and A, and bands having the same letter thus form homologous series. To determine the intervals between groups, the position of what may be called the center of each group was found by averaging the frequencies of all 5 bands. The intervals between these centers for groups 2, 3, 4, 5, 6, and 7 are given in table 19. Groups 1 and 8, for which insufficient data were available, were omitted, except in the case of the ammonium chloride. The only indication of a systematic departure from uniformity of interval for a single salt appears in the case of the caesium chloride, the average group-interval for which is smaller than that of the other salts by nearly 0.5 per cent. 64 FLUORESCENCE OF THE URANYL SALTS. As will appear in the course of the subsequent consideration of individual series, the tendency of the group intervals of the csesium salt to diminish toward the violet is not, as might seem at first sight, an indication that the groups are made up of series having a diminish- ing interval. As regards the other salts, it will be noted that the dis- tance between groups is essentially constant. TABLE 19. — Distances between fluorescence groups. Group. Potassium uranyl chloride. Ammonium uranyl chloride. Rubidium uranyl chloride. Csesium uranyl chloride. Center of group. Inter- val. Center of group. Inter- val. Center of group. Inter- val. Center of group. Inter- val. 1 1505.3 1588.6 1671.9 1755.0 1838.4 1922.3 2005.6 2 1587.2 1670.8 1754.0 1836.6 1919.6 2003.3 83.3 83.3 83.1 83.4 83.9 83.3 1591.5 1074.9 1758.3 1 S40 . 7 1924.2 2007.8 1592.6 1675 . 9 1759.3 1841.8 1924.7 2006.6 3 83.6 83.2 82.6 83.0 83.7 83.4 83.4 82.4 83.5 83.6 83.3 83.4 82.5 82.9 81.9 4 5 6 ... 7. . Average distj incea. . . . 83.22 83.38 83.26 82.80 DISTRIBUTION OF BANDS WITHIN THE GROUPS. While to the eye the fluorescence spectra under consideration present the appearance of evenly spaced bands varying periodically in intensity so as to form the groups, this is not strictly the case, as may readily be shown by subtracting neighboring values in table 29. The average distances thus obtained are given, for convenience, in table 20. The greatest departures from uniformity of distribution occur in the spectra of the rubidium chloride and the cesium chloride. TABLE 20. — Average distances between neighboring bands in the fluorescence spectrum at +20° C. Fluorescing substances. Distances between bands. C to B. D to C. E to D. A to E. B to A. UO2 C12 2K Cl 15.97 17.56 16.20 18.25 18.66 17.74 18.43 12.85 17.96 17.86 18.50 18.63 14.70 15.67 12.75 14 . 52 15.58 15.67 17.12 18.10 UO2Cl2.2NH4Cl.... UO2 Cl2.2Rb Cl UO2 C12 . 2Cs Cl Averages 16.99 16.92 18.24 14.41 16.62 In the rubidium spectrum, bands A and E are crowded together, the average interval being 12.75, and in the csesium spectrum D and C are similarly crowded. It will be noted that the average distance between A and E is less for the four chlorides than any of the corre- sponding distances between other bands. The arrangement of the bands within the group in the four salts is conveniently compared by means of the diagram in figure 54, in which INTIMATE STRUCTURE ON COOLING. 65 the geometrical centers of the groups are in the same vertical line. It will be seen from the diagram: (1) That the group center is in all four cases almost coincident with the crest of the D band. (2) That the distance between D and E is approximately the same in all. (3) That the arrangement of bands within the group is essentially the same in all, except for the displacement of band A in the spectrum of the rubidium and of B and C in that of the caesium salt, as mentioned above. Further discussion of these discrepancies will be found in a later paragraph of this chapter. INTERVALS OF THE INDIVIDUAL SERIES. For the consideration of the frequency intervals of the individual series, the values from table 29 have been arranged by series in table 30. Distances between the observed positions of neighboring members of each series are given in the column marked "Intervals." To facilitate the detection of systematic departures from uniformity of interval, a column of values calculated by the following method is likewise given: A " center" for each group was found in the manner already employed for determining the group centers. Around this the calculated positions were arranged under the assumption of a con- stant frequency interval equal to the average of the observed intervals for each series sepa- rately. The column marked 1 ' Differences ' ' indicates the de- parture of the observed values from those thus calculated. The departures from uniform- ity of interval are unsyste- matic, indicating, for all the salts, that the series may be regarded as having a constant frequency interval. This interval has been computed for each series by subtracting the observed frequency of each band from the frequencies of all the other bands of the series and dividing the sum by the total number of inter- vals in question. The results are presented in table 21. These data indicate no progressive change of interval with the molecular weight, except that the interval is definitely smaller for the caesium uranyl chloride. The other three salts, so far as this deter- mination goes, must be regarded as having the same average interval. It is likewise difficult to distinguish with certainty differences in the intervals of different bands in a given salt, except that the C band has in general a smaller interval than the other series, or of a given band in the different salts, excepting in the case of the caesium chloride. 3 C V C ) t I i \ K 1 i NhU Rb Cs 40 ?0 C ; '0 40 FIG. 54. 66 FLUORESCENCE OF THE URANYL SALTS. At the same time, while not obviously systematic, the variations in these values are considerably larger than those resulting from the measurement of the interval of any given series, taken by itself, which should not exceed, at most, 0.2. TABLE 21. — Average intervals at -{-20° C. for the four double chlorides. Series. K. NH4. Rb. Cs. Av. by series. B 83.42 83 34 83 49 82 32 83 14 C .. 83 11 82 99 82 97 82 50 82 89 D 83.30 83.21 83 17 82 85 83 13 E 83.00 83.81 82 97 83 45 83 31 A 83.23 83.65 83 77 82 85 83 37 Av... 83.21 83.40 83.27 82.80 83.17 These seeming discrepancies are not to be considered as wholly accidental, but as being due to the fact that the bands are complex, and variously so, as will appear from the study of these spectra at low temperatures. While he determinations thus far described may be regarded as indecisive as to small differences of interval between the various series and salts, excepting as noted above, the influence of molecular weight upon the posi- tion of bands in the spectrum is unmistakable. In tables 29 and 30 (at end of chapter) the almost universal and fairly regular increase in the frequency of each band as we pass from potassium to csesium is sufficiently evident. In figure 55 this general shift, which is present in all the groups and affects all series, can be seen at a glance. Almost the only re- versed shifts occur in the case of those bands of the csesium spec- trum which show anomolous plac- ing in the spectral groups. In table 19, where the groups are units, the accidental errors pertaining to individual bands are submerged in the processes of aver- aging and the shift with molecular weight appears as a still more 1 1 1 1 1 1 1 1 ~\ 15 16 17 18 19 20 21 SERIES B x 1 1 i 1 1 1 1 NH*| I ! I I I I ft 1 1 1 1 1 1 1 1 1 1 I 1 1 1 C K 1 1 1 1 1 1 1 NH4 | | | | | | | - 1 1 1 1 1 1 1 Cs | | | | | I | D K | | I I I I NH< | | | | | | Rb | | I I I I Cs | | | | | | E * 1 1 1 1 1 1 NH* | | | | | | Rb I I I I 1 1 Cs | | | | | | A K 1 1 1 1 1 1 NH4 | | | | | | Rb 1 1 1 1 1 1 Cs 1 1 1 1 1 1 jfc a84Al -5i6/i -4i8At FIG. 55. INTIMATE STRUCTURE ON COOLING. 67 systematic phenomenon. Ignoring group 7, in which the bands are displaced by absorption in a manner to be discussed later, we find the following shifts to exist. TABLE 22. — Shift of the groups. Group 2 3 4 5 6 Shift 5.4 5.1 5.3 5.2 5.1 Average shift from K to Cs, 5.2 Ths shift is therefore to be regarded as approximately uniform throughout the spectrum. The shift is much greater between NHi and lib than in the other cases, the averages being as shown in table 23. TABLE 23. — Average shift of groups. K-NH4 1.6 NH4-Rb 2.7 Rb-Cs 9 It will be noticed that in this discussion the order of molecular weights used is K, NH4, Rb, Cs — NH4 being placed between K and Rb instead of in its proper position. This is in accordance with the results of Tutton,1 who has shown that in various optical properties of crystals which depend on the molecular weights, NH4 always lies between K and Rb, as though its effective molecular weight were larger instead of being smaller than K. THE EFFECTS OF TEMPERATURE. The narrow, line-like bands into which the ordinary uranyl spec- trum is resolved at low temperatures2 form a rather complex aggre- gation separable into a series of identically arranged groups corre- sponding to the unresolved bands at +20°, but related to the over- lapping components of the latter in a manner not easily capable of direct determination. It was deemed of especial interest, therefore, to observe the effect of cooling on the double chlorides, where the relation, owing to the partial resolution at +20°, should be more obvious. For this purpose a crystal, C, of the salt to be examined was mounted within a long cylindrical Dewar flask, D, with unsilvered walls (fig. 56) . The carbon arc A was focussed on the crystal by the lens L. A water- cell W was inserted between the arc and the condenser. The light- filter F was opaque to all but the violet and ultra-violet rays used for excitation. Observations with the Hilger spectroscope H, a portion of the collimator of which is shown, were made through a second filter E opaque to the exciting light but transmitting the fluorescence. The arc and specimen were well screened by an opaque box BB. When it was desired to photograph the spectrum a camera was substituted for the observing telescope of the spectrometer. The control and adjustment of temperature were effected by attach- ing the crystal at the upper end of a vertical copper rod which could be 1 Tutton, A. E., Crystalline Structure and Chemical Constitution. (London, 1916.) 1 See Becquerel and Onnes, 1. c. 68 FLUORESCENCE OF THE URANYL SALTS. immersed more or less deeply in the liquid air by raising or lowering the Dewar flask. To preclude the gathering of frost or moisture on the surface of the crystal, it was kept during the entire experiment at a sufficient distance below the lip of the flask, where it was surrounded with the dry atmosphere above the slowly evaporating mass of liquid air. Measurements of the temperature were by means of a small coil of fine copper wire mounted at the same level as the crystal, so as to have always, as nearly as possible, the temperature of the latter. Changes in the resistance of the coil were indicated on the sheet of a Callender recorder, carefully calibrated to read directly in degrees centigrade and adjusted for a range from +20° to —200°. A X B C D E A -185 iA FIG. 56. B? C Cf D D EE A A FIG. 57. The crystal was mounted so as to cover a transverse slot in the copper rod. It could thus be illuminated either from the front, as shown above, or from behind by light transmitted through the slot. The latter arrangement was employed especially in the study of the absorption spectrum. When the substance, excited to fluorescence in the manner already described, was gradually cooled to the temperature of liquid air and the spectrum was observed through the Hilger spectrometer, the fol- lowing changes were noted: (1) The bands become narrower and better defined until at the temperature of liquid air they correspond in appearance to the usual line-like bands characteristic of the fluorescence spectra of the uranyl salts at low temperatures. (2) As the temperature falls the bands are gradually resolved into doublets. One component of each doublet becomes rapidly brighter, while the other frequently becomes more indistinct and sometimes disappears. The general effect is that of a shift toward the violet INTIMATE STRUCTURE ON COOLING. 69 amounting to about a third of the distance between the original bands. The nature of this apparent shift is as follows : Each band at +20° may be regarded as an unresolved doublet, of which in general the member of longer wave-length is relatively so much the stronger that its position determines approximately the location of the crest of the composite band (see fig. 57). The effect of cooling is to resolve this doublet into separately distinguishable bands and at the same time to cause a subsidence of the stronger and an increase of the weaker member. The member of the shorter wave- length usually becomes dominant at low temperatures, and in so far as this occurs the arrangement of the spectrum appears to be undis- turbed but shifted toward the violet by an amount representing the width of the doublet. There are, however, certain exceptions to this rule, so that the relation of the resolved spectrum to that at +20° is not so simple as the above description would imply. The appearance of the group, if this be its real structure (i. e., a set of nearly equi- distant doublets, the distance between the members of all the doublets being nearly the same), would then be as shown in figure 57. At +20°, B', C', Df, E', and A' are entirely concealed by the over- lapping of the bands. At —185°, B, (7, D', E, and A may or may not be visible, according to their intensity or the completeness of the resolu- tion, which in fact varies greatly in different parts of the spectrum. It will be noticed that in the lower diagram in figure 57, D and not D' is the dominant component. This is a condition which obtains in the ammonium chloride, with the result that C' and D, which appear to have replaced the strong C and D bands of the spectrum of +20°, are near together, D and E' far apart, and the symmetry of the group is impaired. Similar complications occur likewise in the spectra of the other double chlorides. To illustrate the application of this assumption, the spectrum of the ammonium uranyl chloride has been mapped in the manner shown in figure 58, in which the fluorescence bands of the 8 groups as they occur at —185° are shown in their relation to a hypothetical grouping given at the head of the diagram. This grouping consists of the set of imagined doublets of which, as in a previous paragraph, the spectrum at +20° is supposed to be made up. The spacing for each doublet is that determined from the observed average shift on cooling and the relative divergence from this arrangement is shown for all the bands of each group. A scrutiny of the fluorescence spectrum at —185°, group by group, by means of this diagram, affords very satisfactory confirmation of this hypothesis concerning the apparent shift. It is obvious : (1) That not all the components B, C, D, E, and A will necessarily be visible in every group of the resolved spectrum. 70 FLUORESCENCE OF THE URANYL SALTS. (2) That lack of resolution in any region may give the appearance of a single band with intermediate crest in place of the doublet. (3) That the position of crests of the unresolved doublets at +20° will not necessarily coincide exactly with that of either component. Bearing these points in mind, it will be seen that were resolution complete all the observed bands of the spectrum at —185° would probably fall into the system proposed above. We may imagine that the difference between the resolution of the bands C and D, for example, as seen in figure 54, is produced by changes in the unresolved doublets at +20° when the temperature is reduced to —185°, of the kind indicated in figure 59. The doublet CC" forms a single band with crest nearly coincident with C at +20°, and this owing to the sub- sidence of C and growth of C' takes the resolved form shown at — 185°. In the case of D, however, the unresolved ( B 1 •' I 1 1 | f if lili 1 1 1 1 1 1 , 1 1 , II L_i 1 II 1 , 1 II |_j 1 1 1 1 1 i III! -20 FIG. 58. -185" Fia. 59. -20 -185 band has an intermediate crest at D, but is really composed of over- lapping components D' and D" which are separately visible at — 185°. The wave-lengths and frequencies of the bands in the resolved spectra of the four double chlorides, as observed when excited at the temperature of liquid air, are given in table 31 at the end of the chapter. The nomenclature used in this and subsequent tables is chosen to indicate as far as possible the relation of the bands at — 185° to those at +20°. Thus BI, B2, etc., denote components of B, etc., which have been rendered visible by the resolution effected by cooling. The explanation offered above to account for the relation between the spectra at +20° and at —185°, and which was illustrated in the case of the ammonium uranyl chloride (see fig. 58), was confirmed by observations upon the spectrum of that salt at intermediate tempera- tures. It was thus possible to watch the gradual appearance of the INTIMATE STRUCTURE ON COOLING. 71 components characteristic of the spectrum at low temperatures and the simultaneous fading away of those dominant at +20°. The same explanation applies equally well to the potassium and rubidium double chlorides. In the case of csesium uranyl chloride the relations are complicated by the further resolution of these components, so that the connection with the original complexes is less easily traced. To indicate the general character of these resolutions and the apparent temperature shift which results therefrom, the positions of the bands of group 6 at —185° are plotted for all four chlorides (see fig. 60). Intensities of the —185° bands are indicated roughly by the height of the lines. The corresponding crests of the bands at +20° are represented by dotted lines. Group 6 was selected because it offers better examples of the further breaking-up of the components and of other phases of the process of resolution than do groups toward the red in which resolution is progressively less complete. Two questions which were left undetermined in the study of the spectra at +20° may be regarded as settled by these measurements of the bands at —185°. (1) That the intervals are not the same for all series in a given spectrum is clearly established. For example, the components Ci, Cz, which take the place of the C bands in all four spectra, have distinctly different in- tervals, i. e., 84.00 for d and 82.75 for C2. It is noteworthy that C2, which becomes the crest of the group in place of C also, has the small interval. It might be questioned whether these so-called components are not merely accidental neighbors rather than products of the same vibrating system, but for the fact that they are present in all the spectra and have very nearly if not precisely the same relative positions to each other in all. (2) The average interval of all series in the spectrum of the csesium chloride (82.80) at +20°, which causes the notable displacement of the bands of that substance, becomes 83.44 when we take the average of the intervals of the bands at — 185°. That is to say, it is, within the errors of observation, the same as the general average for the other salts. On the basis of the measurements at low temperatures (see table 24), we must conclude that the four double chlorides have approx- imately the same average frequency interval. The averages given in table 24 are obtained from the data of table 32, which contains the frequencies of all the fluorescence bands observed in the spectra of the four double chlorides when excited at the tempera- 1900 FIG. 60. 1950 72 FLUORESCENCE OF THE URANYL SALTS. ture of liquid air. As in the corresponding table for +20° (table 30) the arrangement is by series. TABLE 24. — Average intervals of the fluorescence series at -185° C. Series. K. NH4. Rh. Cs. Average. BL. . B2.. . B3.. . 83.9 83.1 83.5 83.0 83.2 84.2 83.6 83.0 83.4 83.53 83.33 Ci.. . C2.. . Di.. . D,'. . . 84.9 82.7 83.1 84.1 82.7 83.8 84.0 82.9 83.6 83.7 82.8 83.1 84.5 84.18 82.78 83.40 D2.... D2' . . . 84.1 84.2 84.0 83.6 83.6 83.98 E2'. . . . 83.6 82.5 83.2 83.10 E2" . . . 83.3 83 5 83.40 At . 83.3 83.1 82 1 82 83 A2 . 83 6 83 4 83 50 83.58 83.32 83.50 83.44 THE ABSORPTION SPECTRA. A glance at the absorption spectra of the double chlorides, obtained by viewing through a spectroscope the light transmitted by the crystals at room temperature, shows the same higher degree of resolu- tion that characterizes the fluorescence spectra of these salts. The salient feature is a series of strong, rather narrow bands, equally spaced as to frequency, like the broader bands of the other uranyl compounds. The interval, as in all uranyl absorption spectra, is distinctly smaller than the fluorescence interval. Between these are several series of weaker bands. The complete mapping of the absorption spectra is difficult. It can not be done visually, since the bands extend out into the darkness of the ultra-violet. Photography adds considerable detail, but does not greatly extend the range toward the shorter wave-lengths on account of the rapidly increasing opacity. In the brighter regions of the spectrum, on the other hand, more can be seen with the eye than can be found on the photographic plate. The data which we have obtained and which are presented in the tables at the end of this chapter have been procured by supplementing the photographic method, wherever desirable, by visual observations. A great variety of light-filters and combinations of light-filters have been employed in different parts of the spectrum, with widely different exposures for the strong and weak bands. The thickness of the trans- mitting layer has likewise been varied as far as the available material would permit. We are convinced, however, that the extreme limits of the absorption, in both directions, have not as yet been reached. By using crystals of unusual thickness, especially prepared for this work and sometimes by mounting several crystals one behind the INTIMATE STRUCTURE ON COOLING. 73 other, so as to greatly increase the depth of the transmitting substance, it has been found possible1 to greatly extend the absorption spectrum toward the red. Since the crystals are of a greenish-yellow color, they become rapidly transparent as the light admitted is changed from blue to yellow; hence the use of increasingly thicker layers to bring out the absorption bands. To a certain extent the crystal acts as a^screen to absorb the blue light which would cause fluorescence; nevertheless it was found necessary to interpose orange or yellow screens of different densities to eliminate fluorescence in a region where ordinarily it is at a maximum. At first colored glasses obtained from Dr. H. P. Gage, of the Corning Glass Company, were used as filters ; later, solutions of potassium bichromate of varying concentration. It is evident that the screening must be constantly changed when light from the arc is used as a background for bands of increasingly longer wave-length. It was thought that a beam of monochromatic light could be used as a background and thus obviate exciting the crystal to fluorescence, but a preliminary study indicated that such a beam of dispersed light could not be made of sufficient intensity to bring out the dimmer bands. 1 1 1 1. *•// *,« •*» M WAVE-LEKaTN. 1 1 . , 1 , 1 J L 1 I'll U II 11 1 1 : 1 i ! ii : j i i i a i ! i i I | I 1 , . I I 1 I 1 i I . . i i i il 1 1 1 1 I ! i fi i i i i! i ii l. 1 1 1 * ' I 1 1 L i , I 1 1 I 1 , i 1 1 1 1 i , 1 1 1 MI ! i ' i i i : i 4. ! i i i ; | 1 1 . „ 1 l i i 1 III! 1 1 n i 1 ' ' i j >l ' ' ' III i 1 l 1 || ! ! I ! te|c 0 i»|o 0 zo|o D 21)00 FREQUENCY. FIG. 61. — Fluorescence bands are indicated by lines above the horizontal. Old absorption bands are indicated by dotted bands below the line; new absorption bands by solid bands below the horizontal. The plot shows only a portion of the complete spectra of the following salts at +30° C: (1) potassium uranyl chloride; (2) ammonium uranyl chloride; (3) rubidium uranyl chloride; (4) csesium uranyl chloride. In figure 61 is pictured a portion of the fluorescence and absorption spectrum of each of the double chlorides studied. Fluorescence bands are designated by lines above the horizontal line. The older, well- established absorption bands are designated by dotted lines below the horizontal and the new bands by solid lines below the horizontal. The relative positions of the fluorescence and absorption bands are readily seen. These bands appear to be of two distinct classes: 1 Howes, H. L., Physical Review (2), xi, p. 66. 1918. 74 FLUORESCENCE OF THE URANYL SALTS. (1) Most of them at +20°, as may be seen from the diagram and from table 25, in which they are listed together with the corresponding fluorescence, are reversals of fluorescence. These do not form a con- tinuation of the absorption series lying farther toward the violet, nor can they be grouped in series having the absorption interval of 71 ^ In all four species every fluorescence band of groups 5 and 6 has its TABLE 25. — New absorption bands at +#0° C. Potassium uranyl chloride. Ammonium uranyl chloride. Absorp- tion. Fluores- cence. Fluores- cence, series. Absorp- tion series. Absorp- tion. Fluores- cence. Fluores- cence series. Absorp- tion series. 1802.1 1820.2 1836.5 1846.0 1855.3 1865.0 1869.4 1879.0 1885.1 1902.2 1920.9 1937.6 1954.7 1801.4 1819.3 1837.6 B C D 1802.5 1820.8 1838.9 1848.8 1857.8 1869.2 1871.8 1886.5 1906.2 1924.2 1942.3 1957.4 1803.1 1820.7 1839.7 B C D c c 1855.3 E 1856.9 E d d" 1869.6 A 1871.8 1886.8 1904.6 1923.2 1940.5 1956.3 A B C D E A e 1884.7 1901.5 1920.1 1938.3 1953.5 B C D E A Rubidium uranyl chloride. Caesium uranyl chloride. Absorp- tion. Fluores- cence. Fluores- cence series. Absorp- tion series. Absorp- tion. Fluores- cence. Fluores- cence series. Absorp- tion series. 1740.0 1778.7 1789.5 1806.1 1823.2 1834.9 1841.6 1859.8 1872.0 1889.0 1907.2 1926.7 1941.7? 1944.0 1952.0? 1958.7 1741.6 1777.8 1789.4 1806.1 1822.8 C E A B C 1791.5 1808.0 1829.2 1843.0 1846.4 1861.2 1873.0 1890.7 1911.1 1923.8 1944.4 1957.8 1789.7 1808.6 1827.5 1840.5 A B C D 0 b 1859.1 1873.1 1891.1 1910.4 1923.6 1942.7 1955.7 E A B C D E A 1841.5 1859.8 1873.1 1890.0 1905.5 1925.0 D E A B C D d 1943.5 E el 1957.1 A corresponding absorption band, and this relation extends to some of the bands of group 4. Indeed, the suspicion would seem warranted that were the proper experimental conditions attainable throughout the spectrum, every fluorescence band would be found to have its related absorption band and to be reversible in the sense in which that term is defined in a subsequent paragraph. INTIMATE STRUCTURE ON COOLING. 75 (2) The remaining bands listed in table 25 are not reversals of fluorescence. They belong to existent absorption series, of which they are the members of greatest wave-length as yet observed. It should be noted that special precautions were taken to avoid bias. They were not sought for by locating the fluorescence bands and looking for reversals, but found under conditions of illumination which TABLE 26. — New absorption bands at —185° C. Potassium uranyl chloride. Ammonium uranyl chloride. Absorp- tion. Fluores- cence. Fluores- cence series. Absorp- tion series. Absorp- tion. Fluores- cence. Fluores- cence series. Absorp- tion series. 1941.7 1947.6 1954.7 1960.4 1965.8 1972.4 1977.5 1984.9 1989.3 1998.0 2008.8 1940.0 Ea' d2 ei e*f V Oi &2 b3 C2' C2" d! d2 1945.9 1953.5 1956.6 19,63.5 1967.7 1973 . 6 1977.1 1981.0 1984.9 1992.0 1996.8 2002.8 2006.8 2014.1 1945.0 1953.7 E2" Ai d2" ei to" bi V 62 b2" 63 C2 1963.9 1972.3 1977.8 Bj B2 B3 1968.7 Bi 1977.9 B2 1997.2 2007.4 D! D2 1992.7 C2 *' d," d," Rubidium uranyl chloride. Caesium uranyl chloride. Absorp- tion. Fluores- cence. Fluores- cence series. Absorp- tion series. Absorp- tion. Fluores- cence. Fluores- cence series. Absorp- tion series. 1944.4 1952.4 1954.7 1958.1 1963.9 1973.9 1981.0 1985.7 1995.6 2005.2 2010.1 2016.1 d2" ei' 1953.9 1956.6 1958.9 1967.0 '1970.8 1974.3 1978.2 1982.9 1987.7 1991.3 1997.6 2005.6 2009.6 2016.1 2022.2 d," ei' d' 1957.9 Ai e>' 01' bi" b* 5i' bi" W 62" &3 Cl C2 di & dt" C2 di d*' d2" 2003.7 D! 1997.6 Ci 2008.5 2014.9 Dj D2' — i rigorously excluded fluorescence, and in many instances their existence and place was checked by two observers working independently. The fact that practically the entire group was in approximate coin- cidence with fluorescence was an unlooked for result of which we became aware only after the measurements had subsequently been plotted. The expectation was that these bands would prove to be members of the absorption series lying farther toward the violet. 76 FLUORESCENCE OF THE URANYL SALTS. A search by similar methods failed to reveal any bands of class (1), mentioned above, in the spectra of the crystals when cooled to the temperature of liquid air. No selective absorption could be detected beyond the violet end of group 6, 1/X 1940, and while a considerable number of new absorption bands were detected, nearly all of these (see table 26) were found to be members of series already recognized. The exceptions, two each in the spectra of the ammonium, rubidium, and caesium double chlorides, do not appear to be related to the fluores- cence. Coincidences between fluorescence and absorption are of the sort already established as characteristic of the reversing region. TABLE 27. — Average intervals of absorption series at -\-20° C. Series. K. NH4. Rb. Cs. Av. b 70 5 71 6 70 4 70 9 70 9 /3 70.6 c 70.8 68.8 70.3 70.0 •y. 70 4 70 7 70 4 70.5 d' 70.9 d 71.1 71.0 71.2 70.3 70.9 d". 70 6 70.3 70.5 e 70.0 69.7 70.8 70.6 70.3 e" 70.4 70.8 69.6 70.3 a, . ... 70.8 a" 70.0 69.3 69.7 Av. . . . 70.5 70.4 70.3 70.6 The failure to find the bands in groups 5 and 6 is not surprising. They are sufficiently difficult objects at +20°, where two or more components are blended into a broader band. The existence of these components at —185° may be regarded as probable, but they were invisible under the conditions which we have thus far been able to obtain. The absorption spectra of the double chlorides do not exhibit the same remarkable approach to identity of structure and regularity of arrangement manifested in the fluorescence spectra. Upon analysis, however, they are all found to consist of series having intervals of approximately 70 frequency units. As may be seen from table 27, this interval for a given series is very nearly the same for all four salts. The average interval for all the series of a given salt is constant within the errors of observation. These averages are based on the values in table 33 at the end of this chapter. The absorption bands, unlike those of the fluorescence spectrum, do not appear to fall into a succession of strictly homologous groups, but this is because some series disappear, while others increase in strength INTIMATE STRUCTURE ON COOLING. 77 toward the violet. A group near the fluorescence region, therefore, differs notably in aspect from one in the extreme violet, and it is difficult to base conclusions on the location of the centers of the groups, as was done in the study of the fluorescence spectra. As may be observed in figure 62, where the ninth group for the four spectra at +20° is plotted, the distances between the consecutive bands are less nearly equal than the distances between fluorescence bands. It is also evident from this figure that with increasing molecular weight there is a general shift toward the violet. The shift is apparently less systematic than with the fluorescence bands and several reverse shifts seem to occur. In general, however, the total displacement is approximately the same as that observed for fluorescence, i. e., 5 frequency units from potassium to caesium. EFFECT OF TEMPERATURE ON THE ABSORPTION SPECTRA. In the study of the absorption of the double chlorides at —185°, a modification of the method described in a previous paragraph in the investigation of the fluorescence at low temperatures was made. (See fig. 56.) The crystal under observation was mounted within a Dewar flask and submerged in liquid air. Light was transmitted through the crystal instead of being reflected from its surface and a nitrogen-filled tungsten lamp was, in general, substituted for the carbon arc. Both photographic and visual methods were tried, and in the reversing region, especially, where fluorescence and absorption overlap, much attention was given to the selection of color-screens to exclude fluorescence from the portion under consideration. A complete list of the absorption bands observed at — 185° will be found in table 34. The three most obvious results of cool- ing to the temperature of liquid air are : (1) a general shift toward the violet; (2) a great increase in the number of bands ; (3) a very decided narrowing and sharpening of the bands. These changes are readily accounted for by the assumption already made, in this chapter, that the bands at + 20° C. are con- cealed doublets anol that the effect of cool- ing is to resolve them while simultaneously reducing the strength of the stronger and increasing the strength of the weaker component. The apparent shift thus produced will vary from zero to 5 or more units, according to the distance between the com- ponents. 21|00 21130 «'l , 1 „ 1 III IV " -185° 1 1 ill., +20° 1111 1 -185° 1 +20' n i 1 1 1 Ho -.85° 1 II 1 1 1 +20° f» 1 l 1 Us 1 1 i ii i 1 It 78 FLUORESCENCE OF THE URANYL SALTS. A few bands at — 185° are so located with regard to the +20° bands that to explain them by this theory we must suppose them to be too feeble at +20° for detection and greatly increased in intensity by cooling. There is also evidence in places of further resolution into closer narrow doublets and as the degree of resolution is not always the same with fluorescence and the corresponding absorption, this is a source of trouble in the attempt to find the fluorescence series which belongs to each series in the absorption spectrum. Every low-temperature band, however, falls into a series of constant frequency, whatever its position or degree of resolution. The effect of temperature on the average intervals can be studied by comparing tables 27 and 28. Although the intervals range from 69 to 71, there is little that can be termed systematic in the variations. At liquid air temperature, where two or more components are present, we have used subscripts. Thus dif corresponds to DI, d? to D2, etc. Where the reversal is doubled in the manner shown in figure 63, we have designated this doublet as di and di", etc. The average interval of each salt is approximately the same at both temperatures. It will be noticed in table 27 that 70.28, the average of the c components is smaller than the b, d, e, or a averages. This is of interest because the strong C series, which join these series, are also the shortest of the fluorescence series. Since the —185° bands are very sharp and easy to locate, no doubt the differences found in table 28 are indicative of real variations in the constant-frequency intervals. It does not follow that the smaller intervals are con- fined to one salt or one set of bands, however, since, as has been noted in the case of series Ci and C% of the fluorescence series, the maximum difference in interval may be associated with two series which are nearly co- incident. The comparison of table 27 with table 28 shows that the effect of changing temperature on the average interval of a salt is almost negligible, but that the two com- ponents of one series of the +20° spectrum may vary by as much as 1.9 units in frequency interval. The character of the change in the absorption spectra when we pass from +20° to —185° can best be seen in detail by plotting a single group in the spectrum of each salt, as has been done for group 9 in figure 62. A better idea of the phenomena of cooling, as a whole, is obtained by means of maps like those in figures 64, 65, 66, and 67, in which all the bands of fluorescence and absorption are given at both temperatures, first in a single line as they occur in the spectrum of each salt. Fluorescence is indicated by vertical lines above the horizontal and absorption below. Length of line indicates roughly the strength d, \ INTIMATE STRUCTURE ON COOLING. 79 of the bands. No attempt has been made to denote the width of the bands. Below each spectrum the absorption bands are sorted out into their respective series. The figure is necessarily on a greatly reduced scale. Our working maps of these spectra are about 2 meters in width. From these maps some of the statements already made can be verified at a glance; e. g., the increased number of bands and series at —185°; the greater extent of absorption toward the red at +20° than at —185°, and that there is in general a greater degree of resolu- tion of absorption than of fluorescence. It may also be noted that the known absorption spectrum is of greater extent than the fluorescence spectrum and that the absorption, considered as a unit, suffers a nar- rowing on cooling which is more marked on the side toward the red. TABLE 28. — Average intervals of absorption series at —185° C. Series. K. NH4. Rb. Cs. Average. Series. K. NH4. Rb. Cs. Average. 61" 70.6 70.4 70 50 ei' 70 7 70 70 61 ... 71.4 71 40 d 70 6 70 7 70 9 70 73 61" . 71 3 71 30 e-i 70 5 70 0 70 25 b2' 71.0 70.6 70.80 62 ... . 70 8 70 3 70 . 55 bi 70.5 70.7 70.2 70 47 e2" 71 4 70 8 71 10 b2" 71 4 71 40 b3 71.2 70 7 70 95 c av. 70 67 b av 70 83 a,' 71 0 71 0 71 00 7O O 71 "i 7n fi^ d' 70 3 70 30 QI 0,2. 70 4 70 4 70 6 70 47 Ci . . 70 7 70 5 70 60 a,". 71 9 71 90 C-,' 69 3 70 9 70 10 C2 70.0 70.9 70.4 70.43 a av. . 71 00 f'l fiQ 0 70 Q fiQ QS Av 70 y> 71 Ofi 70 S4 70 "vl c av 70 28 di 69.8 70.9 70 35 di" 70 8 70 2 70 50 ck' 71 2 70 5 70 85 d* ... 70 0 71 0 70 50 d2". 70 5 71 1 70 7 70 68 dz 70.2 70 20 d av. . 70 51 REVERSALS AND THE REVERSING REGION. The phenomena of the reversing region, where fluorescence and absorption overlap, are complicated. Some points applicable particu- larly to the double chlorides are, however, discussed here. The early observers of uranyl spectra were of the opinion that some connection or relation must exist between the system of bands of fluorescence and absorption. Becquerel and Onnes, who first studied these spectra at low temperatures, were able to confirm the impression of Stokes that the two systems overlapped and that there was actual 80 FLUORESCENCE OF THE URANYL SALTS. coincidence of position between certain fluorescence bands and absorp- tion bands. In the case of the double chlorides at +20°, each series of bands of the fluorescence system comes into coincidence, or near coincidence, with an absorption band in what we have termed the reversing region, which is approximately that region occupied by group 7 of the fluores- cence spectrum. The fact that the reversal sometimes appears to be exact, within the errors of observation, while sometimes there is a displacement of several units of frequency, might seem to render such a general relation doubt- ful, but the discrepancy can be shown to be a necessary consequence of the fact that both fluorescence and absorption bands at this tempera- ture are unresolved complexes. The true nature of the case may be 1 1 1 I 1 ii.i LL, 1 U 1 1 Ill lull K 1 1 1 t It 1 ' Ml 1 1 | 1 ' | 1 ,,. II '' 1 II1 1 1 ' 1 1 II II 1 1 i i 1 1 I i i i 1 1 1 1 1 1 i i l i -185° 1, 1 1 . l.lll Mllll ll ,, IJI l[|'l'' ITT II |l " "T TTlT "T T III T~ III l i 1 1 1 II i 1 1 III n i II ii III II II II n i • II 1 i 1 1 II i i i 1 1 1 i n i i +20° l i , i I.,,. 1 1 , 1 1 III 1 1 1 [l i , i, NK U 1 ml) 1 1 i 1 m T 'I '1 1 T1 i ' pop 1 l M|' 1 1 l l i i i 1 i 1 1 i i 1 II i i 1 i i i i i i \ i II ~\ 1 1 i i i i i H85° | i ,l ll, i L, , II Ii 1 hull lu. 1 IHIIB rF 1 i 1 MM '1 \\ \ 1 ii ' jiipnin in "I1" 1— " Illl II III ii ii i ii ii M i n •I i 1 |l i i i i T i n i 1 n n n n i 1 l 16100 18100 i 1 1 20100 n l 1 I 2 [ii i n ii 00 , 24100 26100 s seen from figure 68, which is from a sketch of such a reversal at — 185°, where the resolution is more nearly complete. Here the fluorescence and absorption are complementary, the strong components of fluores- cence coinciding with the weak absorption component and vice versa. When the resolution is less complete, the weaker components will dis- appear, and although the reversal for each component is exact, there will be an apparent failure to reverse, or, in other words, we see the strong components displaced. An actual instance in which this relation between fluorescence appears is given in plate 1, a, which is from a photograph of a small portion of the reversing region. The upper half contains the com- ponents of a resolved fluorescence band, the lower half the correspond- INTIMATE STRUCTURE ON COOLING. 81 ing components of the absorption band with fluorescence eliminated. In this photograph each component of the fluorescence has its exact reversal in absorption, with reciprocal relations as to intensity indi- cated in figure 68. The weaker component of fluorescence is coin- cident with the stronger component of the absorption doublet, and vice versa. In the reversing region fluorescence and absorption are mutually destructive. Consequently one or both are sometimes invisible; but knowing the intervals, we can locate the reversal. By proper screening the fluorescence may be prevented and the absorption band brought out; and by taking extra precautions to secure a dark background and to increase the excitation the fluorescence may be seen. Thus the com- putation may be confirmed. +20°, I I | h i Mil 1 h 1 1 III ,, 1 Nil to to o £ to 6 £ III! Rjb Illll lm|!ii|l' 11 1 n 1 1 " 1 "1"" 1' INI 1 1 1 1 I i 1 1 1 1 II i i i 1 i i i i 1 1 1 i 1 1 i i 1 i i i -185° 1 ll , ,il ll !,l il ml l,,(. I1 |T 1 '"1 T T • r1 riMi|iii in II II u u i i n 1 1 ll i n 1 i 1 | 1 II 1 II II II n i u n 1 II II II Hi in n n i i 1 1 1 i i i 1 +20° 1 I i i Nil ' ' \ ] 1 1, 1 1 111 1 1 1 < U • ill Mil] Illl 1 " 1 r p ']'! ' 1 " 1 HI 1 i i 1 1 i i i 1 1 i i ii i i i i 1 1 i 1 i i i 1 -185° i ll 1 Jh ll 1 111 MM 1, i Ik i •wnr ilf'i ¥ n T '|"n i "if ll HI 'Piii|."i II H |iiy V\" \ ii| Illll II 1 n n II 1 II n I H II n n n II II II 1 1 1 i in nn III! in III III II 1 II II n 1 N n 1 1 16100 |8 100 1 20 1 K>o 1 i 22 00 1 1 i 24 00 i i 26 00 1 In the study of the double chlorides the matter is further confused because the difference between the fluorescence interval (83+) and that of the absorption interval (70+) is approximately equal to the distance between neighboring bands in the fluorescence groups. An absorption series which comes into coincidence with band C, group 7, will therefore nearly coincide with band B, group 8, etc. Furthermore, the degree of resolution in the absorption spectrum, as has already been mentioned, is often greater than in the fluorescence spectrum, and certain series are observable of which the corresponding fluorescence bands can not be identified. 82 FLUORESCENCE OF THE URANYL SALTS. FL. So far as the spectra at +20° are concerned, we find that: (1) All absorption bands toward the violet from the reversing region occur in series with constant-frequency intervals. (2) For every fluorescence series there is a corresponding absorption series. Whether the relation between absorption and fluorescence outlined above is significant can best be determined by the study of the spectra for --185°. If, for example, the explanation of the numerous instances of inexact coincidence is valid, we should expect exact reversals of the components; also that the components of the resolved absorption spectra form series definitely related to the components of the fluorescence spectra in a manner consistent with the system indicated for the spectra at +20°. From a study of the exactness of the reversals in the resolved spectra at low temperatures it appears that 25 out of 38 fluorescence series are certainly reversed and that 36 fluorescence series join absorption series in the seventh group. The experimental error in this group does not exceed 1.5 units. The difference in position between fluorescence and absorption is sometimes greater than 1.5, but this may be ascribed to the dissymmetry in the form of the bands. Fluorescence bands have their crest toward the violet, absorption bands toward the red. In the case of reversals, these regions tend to annul each other, leaving a rem- nant of fluorescence on the red side and a remnant of absorption on the violet. The result is that in regions where fluorescence and absorption exist together, fluo- rescence bands are apt to be given too great a wave- length, and vice versa. In the C2 series of the rubidium chloride, for example, there is a displacement of 2.6 units between the observed positions of fluorescence and absorption. If, however, we compute the proper positions of these bands, using the average intervals for the C2 and c2 series respectively, thus elimi- nating the displacements in the reversal region, the fluorescence band and absorption thus established agree in position within 0.3 unit. The impossibility of excluding all absorption when fluorescence is present, and the impossibility of preventing a tendency toward fluorescence when absorption alone is sought for may well account for the resulting displacement. The case of the C2 series is not an isolated one — probably every reversal is affected somewhat and the stronger bands the most; there being always an apparent shift of the absorption band toward the violet and of the fluorescence band toward the red. This phenomenon has long been recognized by the authors in connection with broad fluorescence bands, and it must now be recognized in the reversing of the narrow, line-like bands at the temperature of liquid air. ABS. FIG. 68. INTIMATE STRUCTURE ON COOLING. 83 In the above, the reversals which connect fluorescence to absorption series have been sought for in the seventh group. There are, however, other possible connections, for coincidences occur in the sixth and eighth groups as well. Since, as has already been pointed out, the difference in spacing between a fluorescence and absorption interval is nearly the same as the spacing between fluorescence bands, it is often I9IQO 20100 POTASSIUM URANYL CHLORIDE 21100 2JOO 1 r 1 2J AMMONIUM URANYL CHLORIDE TT T T JLJ I 2J I RUBIDIUM URANYL CHLORIDE \ _B_L J TI „ r _ TT L CAESIUM URANYL CHLORIDE ! 1 FIG. 69. possible to join equally well two fluorescence series to one absorption series, a fact which makes it difficult to determine the true relation in the case of this class of salts. The actual manner in which the reversals between fluorescence and absorption occur is shown in figure 69, which is a diagram of the revers- ing region. Here the plotting is quite accurate, the fluorescence bands above and the absorption bands below the horizontal. Dotted lines ndicate computed positions. This figure is approximately 10 times as 84 FLUORESCENCE OF THE URANYL SALTS. large as the original negatives. To avoid confusion, the various series occurring in each salt are vertically displaced instead of being drawn on a single line, as they appear in the actual spectra. An inspection of this diagram will suffice to indicate the approach to complete coin- cidence in the reversals and the type of departure from coincidence. TABLE 29. — General list of fluorescence bands in spectra of the double uranyl chlorides at +20° C. Potassium Ammonium Rubidium Caesium Group uranyl chloride. uranyl chloride. uranyl chloride. uranyl chloride. and series. X X X -xio3 X X -XIO3 X X X B 0 . 6809 1469.7 c .6716 1489.9 1- D .6635 1507.1 E .6571 1521.8 A .6501 1538.2 B 0.6436 1553.7 .6430 1555.3 0.6420 1557.6 0.6401 1562.3 f C .6375 1568.6 .6358 1572.6 .6354 1573.8 .6336 1578.3 2- D .6303 1586.6 .6291 1589.6 .6281 1592.2 .6289 1590.1 E .6225 1606.4 .6231 1604.9 .6206 1611.3 .6219 1608.0 LA .6171 1620.5 .6172 1620.2 .6162 1622.8 .6156 1624.4 'B .6111 1636.5 .6103 1638.6 .6098 1640.0 .6090 1642.0 C .6051 1652.5 .6041 1655.3 .6030 1658.3 .6015 1662.5 3- D .5983 1671.5 .5978 1672.7 .5967 1675.9 .5970 1675.0 E .5919 1689.5 .5923 1688.2 .5903 1694.1 .5911 1691.9 ,A .5869 1704.0 .5866 1704.8 .5860 1706.4 .5854 1708.2 'B .5816 1719.4 .5813 1720.3 .5800 1724.0 .5789 1727.4 C .5759 1736.4 .5752 1738.6 .5742 1741.6 .5729 1745.4 4- D .5698 1754.9 .5696 1755.7 .5686 1758.7 .5689 1757.9 E .5642 1772.3 .5642 1772.3 .5625 1777.8 .5631 1775.9 A .5595 1787.2 .5593 1787.9 .5588 1789.4 .5587 1789.7 B .5551 1801.4 .5546 1803.1 .5537 1806.1 .5529 1808.6 C .5497 1819.3 .5492 1820.7 .5486 1822.8 .5472 1827.5 5, D .5442 1837.6 .5436 1839.7 .5430 1841.5 .5433 1840.5 E .5390 1855.3 .5385 1856.9 .5377 1859.8 .5379 1859.1 A .5349 1869.6 .5342 1871.8 .5339 1873.1 .5339 1873.1 B .5306 1884.7 .5300 1886.8 .5291 1890.0 .5288 1891.1 C .5259 1901.5 .5250 1904.6 .5248 1905.5 .5234 1910.4 6- D .5208 1920.1 .5200 1923.2 .5195 1925.0 .5198 1923.6 E .5159 1938.3 .5153 1940.5 .5145 1943.5 .5147 1942.7 A .5119 1953.5 .5112 1956.3 .5110 1957.1 .5113 1955.7 B .5078 1969.4 .5072 1971.5 .5066 1973.8 .5067 1973.5 C .5039 1984.4 .5031 1987.6 .5027 1989.1 .5024 1990.3 7 D .4990 2004 . 0 .4986 2005.7 .4979 2008.4 .4989 2004.4 E .4946 2021.7 .4940 2024 . 1 .4935 2026.2 .4937 2025 . 6 A .4909 2036.9 .4904 2039.2 .4899 2041.4 .4904 2039.2 B .4869 2053.8 .4867 2054.6 .4857 2059.0 .4863 2056.3 e C .4836 2068.0 .4829 2071.0 .4824 2072.8 .4819 2075.0 O E A INTIMATE STRUCTURE ON COOLING. 85 With regard to the reversing region at — 185°, it can be stated that — (1) The majority of the fluorescence series reverse in the seventh group. (2) 36 out of 38 fluorescence series are joined in the seventh group to absorption series. (3) The exactness of reversal depends not only on the structure of the band, but on the simultaneous presence of fluorescence and absorp- tion in this region. (4) Other reversals and connections are present in the groups adja- cent to group 7. TABLE 30. — Frequencies and intervals of fluorescence series at -\-20° C. Group 1. Group 2. Group 3. Group 4. Group 5. Group 6. Group 7. Group 8. [Frequencies (observed) 1636.5 1719.4 1801.4 1884.7 1969.4 2053 . 8 K J Frequencies (calculated) .... 1635.5 1718.9 1802.3 1885.7 1969.1 2052.6 ] Differences -1.0 -0.5 +0.9 +1.0 -0.3 -1.2 [Intervals (observed) 82 .9 82 .0 83 .3 84 .7 84 .4 [Frequencies (observed) 1469.7 1555.3 1638.6 1720.3 1803.1 1886.8 1971.5 2054.6 NH«I Frequencies (calculated) .... | Differences 1470.9 + 1.2 1554.3 -1.0 1637.6 -1.0 1720.9 +0.6 1804.3 + 1.2 1887.6 +0.8 1970.9 -0.6 2054 . 3 -0.3 [intervals (observed) 85. 6 83 .3 81 .7 82 .8 83 .7 84 .7 83 .1 fFrequencies (observed) 1557.6 1640.0 1724.0 1806.1 1890.0 1973.8 2059.0 Rb 1 Frequencies (calculated . 1556.2 1639.7 1723.2 1806.7 1890.1 1973.6 2057.1 1 Differences — 1.4 -0.3 -0.8 +0.6 +0.1 -0.2 — 1.9 [intervals (observed) 83 .4 84 .0 82 .1 83 .9 83 .8 85 .2 fFrequencies (observed) 1562.3 1643.9 1727.4 1808.6 1891.1 1973.5 2056.3 Cs I Frequencies (calculated) .... 1562.0 1644.3 1726.6 1809.0 1891.3 1973.6 2055.9 1 Differences -0.3 +0.4 -0.8 +0.4 +0.2 +0.1 -0.4 [intervals (observed) 81 .6 83 .5 81 .2 82 .5 82 .4 82 .8 Series ( -T [Frequencies (observed) . . 1568.6 1652.5 1736.4 1819.3 1901.5 1984.4 2068 . 0 K I Frequencies (calculated) 1569.3 1652.4 1735.6 1818.7 1901.8 1984.9 2068.0 ] Differences +0.7 -0.1 -0.8 -0.6 +0.3 +0.5 0.0 [intervals 83 .9 83 .9 82 .9 82 .2 82 .9 83 .6 fFrequencies (observed) 1489.9 1572.9 1655.3 1738.6 1820.7 1904 . 6 1987.6 2071.0 NH4 1 Frequencies (calculated) .... 1 Differences 1489.4 -0.5 1572.4 -0.5 1655.4 +0.1 1738.4 -0.2 1821.4 +0.7 1904.4 -0.2 1987.4 -0.2 2070.4 -0.6 [intervals 83 .0 82 .4 83 .3 82 . 1 83 .9 83 .0 83 .4 [Frequencies (observed) 1573 8 1658.3 1741.6 1822.8 1905.5 1989.1 2072 . 8 Rb J Frequencies (calculated) . . . 1574.5 1657.5 1740.4 1823.4 1906.4 1989.3 2072.3 ) Differences +0.7 -0.8 —1.2 +0.6 +0.9 +0.2 -0.5 [Intervals 84 .5 83 .3 81 .2 82 .7 83 .6 83 .7 fFrequencies (observed) ..... 1578.5 1662.5 1745.4 1827.5 1910.4 1990.3 2075.0 Cs I Frequencies (calculated) . 1579.6 1662.1 1744 . 6 1827.1 1909.6 1992.0 2074 . 6 I Differences + 1.1 -0.4 -0.8 -0.4 -0.8 +1.7 -0.4 [intervals 84 .0 82 .9 82 .1 82 .9 7£ .9 84 .7 86 FLUORESCENCE OF THE URANYL SALTS. TABLE 30. — Frequencies and intervals of fluorescence series at +20° C — continued. Series D. Group 1. Group 2. Group 3. Group 4. Group 5. Group 6. Group 7. Group 8. f Frequencies (observed) 1586.6 1587.6 + 1.0 84 1589.6 1589.8 +0.2 5 83 1592.2 1592.4 +0.2 83 1590.1 1592.2 + 1.1 84 1671.5 1670.9 -0.6 9 83 1672.7 1673.0 +0.3 1 83 1675.9 1675.6 -0.3 7 82 1675.0 1675.0 0.0 9 82 1754.9 1754.2 -0.7 4 82 1755.7 1756.2 +0.5 0 84 1758.7 1758.7 0.0 8 82 1757.9 1757.9 0.0 9 82 1837.6 1837.5 -0.1 7 82 1839.7 1839.4 -0.3 0 83 1841.5 1841.9 +0.4 8 83 1840.5 1840.7 +0.2 6 83 1920.1 1920.8 +0.7 5 83. 1923.2 1922.6 -0.6 5 82 1925.0 1925.1 +0.1 5 83 1923.6 1923.6 0.0 .1 80 2004 . 0 2004 . 1 +0.1 9 2005.7 2005.8 +0.1 5 2008.4 2008.2 -0.2 4 2004.4 2006.4 +2.0 8 K I Frequencies (calculated) 1 Differences [Intervals [Frequencies (observed) 1507.1 1506.6 -0.5 82 NHJ Frequencies (calculated) .... ) Differences [Intervals f Frequencies (observed) Rb 1 Frequencies (calculated) .... 1 Differences. . . . . . [Intervals f Frequencies (observed) Cs I Frequencies (calculated) I Differences [intervals Series E. (Frequencies (observed) 1606.4 1606.4 0.0 83 1604.9 1605.0 +0.1 .1 83 1611.3 1611.4 +0.1 82 1608.0 1608.4 +0.4 83 1689.5 1689.4 -0.1 .1 82 1688.2 1688.8 +0.6 .3 84 1694 . 1 1694.1 0.0 .8 83 1691.9 1691.9 0.0 .9 84 1772.3 1772.4 +0.1 .8 83 1772.3 1772.6 +0.3 .1 84 1777.8 1777.3 -0.5 .7 82 1775.9 1775.4 -0.5 .0 83 1855.3 1855.4 +0.1 .0 83 1856.9 1856.4 -0.5 .6 83 1859.8 1860.3 +0.5 .0 83 1859.1 1858.9 -0.2 .2 83 1938.3 1938.4 +0.1 .0 83 1940.5 1940.2 -0.3 .6 83 1943.5 1943.3 -0.2 .7 82 1942.7 1942.3 -0.4 .6 82 2021.7 2021.4 -0.3 .4 2024.1 2024.0 -0.1 .6 2026 . 2 2026 . 2 0.0 .7 2025 . 1 2025 . 8 +0.7 .4 K 1 Frequencies (calculated) ] Differences [intervals [Frequencies (observed) . 1521.8 1521.2 -0.6 83 NH4 1 Frequencies (calculated) .... j Differences [Intervals [Frequencies (observed) Rb 1 Frequencies (calculated) I Differences [Intervals ^Frequencies (observed) Cs I Frequencies (calf ulated) 1 Differences [Intervals Series A. f Frequencies (observed 1620.5 1620.5 0.0 83 1620.2 1620.9 +0.7 .0 84 1704.0 1703.8 -0.2 .5 83 1704.8 1704.5 -0.3 .6 83 1706 . 4 1706. ( -0.4 83 1708.2 1707.4 +0.8 .8 81 1787.2 1787.0 -0.2 .2 82 1787.9 1788.2 +0.3 .1 83 1789.4 1789.7 +0.3 .0 83 1789.7 1790.3 +0.6 .5 83 1869.6 1870.2 +0.6 .4 83 1871.8 1871.8 0.0 .9 84 1873.1 1873. 5 +0.4 .7 84 1873.1 1873.1 O.C .4 82 1953.5 1953.5 0.0 .9 83 1956.3 1955.5 -0.8 .5 82 1957.1 1957.3 +0.2 .0 84 1955.7 1956.0 +0.3 '.6 83 2036 . 9 2036 . 7 -0.2 .4 2039 . 2 2039.1 -0.1 .9 2041.4 2041.0 -0.4 .3 2039 . 2 2038.8 -0.4 .5 K I Frequencies (calculated) I Differences [intervals f Frequencies (observed) 1538.2 1537.2 -1.0 82 NH4J Frequencies (calculated) .... | Differences ... [Intervals . . . [Frequencies (observed) . . . Rb 1 Frequencies (calculated) I Differences [Intervals f Frequencies (observed) . . 1624.4 1624. € +0.2 8S Cs J Frequencies (calculated) . ) Differences [Intervals . INTIMATE STRUCTURE ON COOLING. 87 TABLE 31. — General list of fluorescence bands in spectra of the double uranyl chlorides at -185° C. Group and series. Potassium uranyl chloride. Ammonium uranyl chloride. Rubidium uranyl chloride. Caesium uranyl chloride. X -X 103 X X -X103 A X -X103 X X -X103 X 1 3< 4- 5< 6< 7 8 B2 C2 D2 E2" Bi B2 C2 Di D2 E2" B2 Ci C2 D! D2 E/ E2" A! A2 B, B2 Ci C2 Di' D! D2' D2 E2' E2" A, LA2 'Bi B2 B3 Ci Co D/ Dt D2' D2 E2' E2" A! A2 Bi B2 B3 d C2 D! D./ D2 E! E2' En 2 Ai' Ai As B, 0.6398 .6330 .6283 .6207 .6110 .6079 .6016 1563.0 1579.8 1591.5 1611.0 1636.7 1645.0 1662.1 0.6056 .5991 .5964 1651.3 1669.2 1676.7 0.6035 .6006 1657.0 1665.0 0.6018 .5990 1661.7 1669.4 . 5968 .5899 .5791 1675.6 1695.0 1726.9 .5803 1723.2 .5764 .5721 .5705 .5684 .5652 1734.9 1747.9 1752.7 1759.4 1769.3 .5752 .5724 1738.4 1747.0 .5733 .5704 .5677 1744.4 1753.1 1761.4 .5731 .5703 1745.0 1753.4 .5641 1772.7 .5624 .5595 1778.1 1787.3 .5603 1784.6 .5573 .5546 .5520 .5489 .5471 1794.5 1803.1 1811.5 1821.9 1827.8 .5564 .5526 .5500 .5464 .5452 .5440 .5427 .5412 .5395 1797.2 1809.5 1818.1 1830.2 1834.1 1838.2 1842.5 1847.7 1853.7 .5569 .5542 .5508 .5489 1795.8 1804.4 1815.5 1821.7 .5524 . 5493 .5471 1810.4 1820.5 1827.7 .5461 1831.0 .5445 1836.7 .5444 1836.9 .5437 .5389 1839.4 1855.7 .5420 .5379 .5370 .5345 1845.1 1859.0 1862.0 1870.8 .5419 1845.2 .5358 1866.4 .5354 1867.6 .5326 .5300 .5277 1877.7 1886.8 1895.0 .5318 .5286 .5260 1880.4 1891.8 1901.1 .5321 .5297 .5279 .5262 .5250 1879.5 1888.0 1894.3 1900.4 1904.6 .5279 1894.4 .5250 .5234 1904.8 1910.6 .5247 .5231 1905.9 1911.7 .5223 .5214 .5201 .5191 .5179 .5163 .5137 .5127 1914.6 1918.0 1922.7 1926.3 1930.9 1937.9 1946.8 1950.4 .5226 1913.5 .5206 1921.0 .5207 1920.3 .5200 .5155 1922.9 1939.9 .5184 .5149 .5141 .5118 1929.2 1941.9 1945.0 1953.7 .5182 1929.9 .5124 1951.6 .5107 .5098 .5073 .5054 1957.9 1961.6 1971.4 1978.6 .5092 .5059 .5038 1963.9 1976.5 1984.9 .5092 .5070 .5056 1963.9 1972.3 1977.8 .5080 .5056 1968.7 1977.9 .5028 .5018 .4989 1988.7 1992.7 2004.5 .5006 .4979 .4963 .4950 1997.6 2008.5 2014.9 2020.2 .5031 .5007 1987.6 1997.2 .5018 .4991 1993.0 2003 . 7 .4982 .4956 2007.4 2017.8 .4967 2013.4 .4967 .4938 2013.2 2025 . 1 .4940 2024 . 1 .4926 .4918 2030.0 2033 . 3 .4930 .4916 .4904 2028.4 2034.2 2039 . 2 .4917 .4902 2033.8 2040.0 .4857 2058.9 88 FLUORESCENCE OF THE URANYL SALTS. TABLE 32. — Frequencies and intervals of fluorescence bands in the spectra of the four double chlorides at -185° C. Series and group Potassium uranyl chloride. Ammonium uranyl chloride. Rubidium uranyl chloride. Caesium uranyl chloride. -X103 M Inter- val. -X103 M Inter- val. -X103 M Inter- val. -X103 M Inter- val. B! '3 5 6 7 8 1636.7 1795.8 1803.1 1809 . 5 83.7 4X83.0 83.7 82.3 1879.5 1886.8 1891.8 84.4 84.6 84.7 1963.9 1968.7 1971.4 1976.5 82.4 2058.9 B2 2 3 4 5 6 7 1563.0 82.0 1645.0 1651.3 81.9 83.6 1723.2 1726.9 1734.9 81.2 83.5 83.2 1804.4 1810.4 1811.5 1818.1 83.6 84.0 83.5 83.0 1888.0 1894.4 1895.0 1901 . 1 84.3 83.5 83.6 83.8 1972.3 1977.9 1978.6 1984.9 f6 H 1894.3 83.5 1977.8 Ci • 4 5 6 7 1747.9 82.3 1815.5 1820.5 1821.9 1830.2 84.9 84.3 84.0 84.4 1900.4 1904.8 83.9 1988.7 1905.9 1914.6 83.0 1997.6 C2 • '2 3 4 5 6 7 1579.8 82.3 1657.0 1662.1 1661.7 1669.2 81.4 82.3 83.3 83.5 1738.4 1744.4 1745.0 1752.7 83.3 82.3 82.8 81.4 1821.7 1827.7 1827.8 1834.1 82.9 82.9 83.9 83.9 1904.6 1910.6 1911.7 1918.0 83.0 82.1 81.3 1987.6 1992.7 1993.0 D,'- 5 6 1838.2 84.5 1922.7 D! 3 4 5 6 7 1665.0 1669.4 1676.7 82.0 84.0 82.7 1747.0 1753.1 1753.4 1759.4 84.0 83.6 83.5 83.1 1831.0 1836.7 1836.9 1842.5 82.5 84.3 83.4 83.8 1913.5 1921.0 1920.3 1926.3 83.7 83.5 83.4 82.2 1997.2 2004.5 2003.7 2008.5 INTIMATE STRUCTURE ON COOLING. 89 TABLE 32. — Frequencies and intervals of fluorescence bands in the spectra of the four double chlorides at —185° C. — continued. Series, and group Potassium uranyl chloride. Ammonium uranyl chloride. Rubidium uranyl chloride. Csesium uranyl chloride. ;xio' Inter- val. *X10' Inter- val. Jxio3 Inter- val. Jxio« Inter- val. Da' 5 6 7 1847.7 83.2 1930.9 84.0 2014.9 D2 - '2 3 4 5 6 J 1591.5 84.1 1675.6 85.8 1761.4 1769.3 83.7 84.4 1839.4 1845.1 1845.2 1853.7 83.5 84.1 84.7 83.3 1922.9 1929.2 1929.9 1937.0 84.5 84.2 83.3 83.2 2007.4 2013.4 2013.4 2020.2 E2'- '4 5 6 7 1772.7 83.0 1855.7 1859.0 84.2 82.9 1939.9 1941.9 1946.8 82.2 83.2 2024.1 2030.0 E2"- r2 3 4 5 6 7 1611.0 84.0 1695.0 83.1 1778.1 83.9 1862.0 1866.4 83.0 84.0 1945 . 0 1950.4 82.9 2033.3 Ai' 7 2028.4 2033.8 A, < 4 5 6 7 1784.6 1787.3 83.0 83.5 1867.6 1870.8 84.0 82.9 1951.6 1953.7 1957.9 82.6 82.1 2034.2 2040.0 A2< '4 5 6 7 1794.5 1797.2 83.2 83.2 1877.7 1880.4 83.9 83.5 1961.6 1963.9 2039.2 90 FLUORESCENCE OF THE URANYL SALTS. TABLE 33. — General list of absorption bands in spectra of the double uranyl chlorides at -{-20° C. Series b. Potassium uranyl chloride. Ammonium uranyl chloride. Rubidium uranyl chloride. Caesium uranyl chloride. \ X103 A Inter- val. ^xio3 A Inter- val. [ X103 A Inter- val. xio3 X Inter- va . 1970.1 1970.1 1974.0 1974.2 68.4 71.1 71.8 70.0 70.4 69.1 2038.5 2041.2 2044 . 0 2043.3* 70.3 70.6 2108.8 2113.0 2114.4 2113.9 70.8 72.0 71.2 70 2 70.4 71.0 2179.6 2185.0 2184.6 69.5 >70.7X5 73.0 2184.3 68.2 •71.0X5 72.0 2247 . 8 2256 . 2 2253 . 1 2255 . 3 73.3 71.7 2329.5 2327.0* 70.8 71.3X5 71.4 2400 . 3 2398.4 72.6 2471.0 68.7 2539 . 7 2603 . 0 2606.5 2675.0 2679 . 5 2756.8 Series b'r. 2620.9 71.9 2692 8 • Series j9. 2056.0 71.1 2127 1 72.2 2199.3 70.3 ^70. 9X2 1&9. 7X3 71.3 2269.6 2411.4 2620.5 2691 8 * These bands were doubled occasionally. INTIMATE STRUCTURE ON COOLING. 91 TABLE 33. — General list of absorption bands in spectra of the double uranyl chlorides at +20° C. — continued. Series c. Potassium uranyl chloride. Ammonium uianyl chloride. Rubidium uranyl chloride. Caesium uranyl chloride. ,-xio3 A Inter- val. IX1°3 Inter- val. ^XIO3 /\ Inter- val. ^xio3 A Inter- val. 1985.2 1988.5 1989 . 1 1993.1 68.6 69.0 68.5 71.3 2053 . 8 2057.5 2057.6 2064.4 71.6 •71.2X7 69.5 68.5 72.8 2125.4 2126.0 2130.4 68.9 70.8X6 2199.3 2623 . 8 2624 . 5 2693.3 Series y. 1997.0 2001.6 2001.0 70.4 69.0 69.5 2067.4 2070.6 2070.5 70.6 70.5X6 72.0 •70.4X6 72.0 70.8 2137.0 2142.6 2141.3 67.7 •71.6X3 69.4 2209.0 2423 . 7 2493 . 1 2560.2 2565 . 1 2637 . 1 71.5 2708 6 Series d'. 2071.3 170.4X2 72.8 2212.2 2284.0 70.6 2354.6 72.0 2426 6 70.0 2496 . 6 72.1 2568.7 68.1 2638.9 2642 . 0 2636.8 73.3 2712.2 2713.7 92 FLUORESCENCE OF THE TJRANYL SALTS. TABLE 33. — General list of absorption bands in spectra of the double uranyl chlorides at +30° C. — continued. Series d. Potassium uranyl chloride. Ammonium uranyl chloride. Rubidium uranyl chloride. Csesium uranyl chloride. ^XIO3 A Inter- val. ^-xio3 A Inter- val. ^xic3 A Inter- val. ,-xio3 A Inter- val. 2004.3 2007 . 1 2004.9 70.8 73.0 71.4 2075.1 2080.1 2076.3 71.3 69.1 X71.9X2 71.7 69.2 2146.4 2149.2 2145.5 2218.9 69.6 2288.5 2293.1 71.4 2359.9 2364.8 72.1 70.6 2432 . 0 2435.4 69.8 68.5 2501.8 2503.9 71.9 73.8 2573.7 2577.7 Series d". 2008.9 2013.7 •70.8X3 71.2 2080.1 69.4 2149.5 2221.8 2226.0 70.1 2291.9 69.4 2361.3 70.2 2431.5 71.6 2503.1 71.7 2574.8 Series e. 2021.4 2023.3 2025.3 2025.6 69.5 70.7 69.6 69.1 2090.9 2094.0 2094.9 2094.7 70.1 70.2X4 71.5 71.7X2 70.9 •69.6X3 73.0 71.5 70.9 2161.0 2165.5 2166.4 2165.6 69.2 71.3X5 72.4 2235.6 2237.0 73.0 2309.0 2310.0 68.2 2379.9 2378.2 69.8 2442.0 2448.0 72.2 2520.2 2588.7 2592.0 2661.7 INTIMATE STRUCTURE ON COOLING. 93 TABIJE 33. — General list of absorption bands in spectra of the double uranyl chlorides at -\-20° C. — continued. Series e". Potassium uranyl chloride. Ammonium uranyl chloride. Rubidium uranyl chloride. Caesium uranyl chloride. ^XIO3 A Inter- val. ^xio3 A Inter- val. ^xio3 A Inter- val. Jxio3 A Inter- val. 2173.5 71.0 2244.5 2248.5 70.1 69.5 2313.0 2314.6 2318.0 68.6 70.0 67.7 171.0X2 67.8 2381.6 2384.6 71.2 70.6 2460.0 2452.8 2455.2 71.2 72.2 p. 3X3 2524.0 2527.4 2741.2 2527.8 Series a. 2037.1 70.6X8 2602.2 72.0 2674.2 Series a". 2329 .4 2333.7 69.3 70.3X2 70.2 2398.7 2403.9 67.7 2471.6 2539.4 70.5 2542.1 TABLE 34. — General list of absorption bands in spectra of the double uranyl chlorides at —185° C. Series 61 '. Potassium uranyl chloride. Ammonium uranyl chloride. Rubidium uranyl chloride. Caesium uranyl chloride. -X103 X Inter- val. ixiO3 X Inter- val. ixiO3 X Inter- val. IxiO3 A Inter- val. 2029.1 2045.3 71.7 71.1 2100.8 2116.4 70.1 70.1 2170.9 2186.5 94 FLUORESCENCE OF THE URANYL SALTS. TABLE 34. — General list of absorption bands in spectra of the doullc wanyl chlorides at —185° C. — continued. Series b\. Potassium urany chloride. Ammonium uranyl chloride. Rubidium uninyl chloride. Caesium uranyl chloride. ^xio3 A Inter- val. Jxio3 A Inter- val. *xio3 A Inter- val. ^xio3 A Inter- val. 2038.5 71.5 2109.0 71.2 2180.2 70.7 2250 9 71.9 2322.8 71.2 2394 . 0 72.8 2466 . 8 70.5 2537.3 72.7 2609.0 Series b\" . 2044 . 2 70.7 2114.9 70.0 72.0X2 72.5 leg. 9x2 72.4 2184.9 2328 . 8 2401.3 2541.0 2613.4 73.0 2686.4 Series b^'. 2045.5 2051.6 71.4 71.3 2116.9 2122.9 70.8 70.6 2187.7 2193.5 69.5 2263 . 0 71.0 2334 . 0 70.7 2404 . 7 71.6 2476 . 3 69.2 2545.5 INTIMATE STRUCTURE ON COOLING. 95 TABLJB 34. — General list of absorption bands in spectra of the double uranyl chlorides at —185° C. — continued. Series 62. Potassium uranyl chloride. Ammonium uranyl chloride. Rubidium uranyl chloride. Csesium uranyl chloride. ixio3 A Inter- val. Jxio3 A Inter- val. Jxio3 A Inter- val. ^xio3 A Inter- val. 2043.6 2050.4 70.1 71.0 .2113.7 2121.4 70.3 70.2 2184.0 2191.6 70.3 71.1 2254.3 2262.7 2268.6 171.0X2 70.1 71.3 2325.6 2410.5 2480.6 69.8 2550.4 Series b?". 2051.1 71.6 2122.7 70.4 2193.1 72.8 2265.9 72.3 2338.2 70.5 2408.7 70.3 2479.0 71.1 2550.1 72.6 2622.7 Series 63. 2056.5 70.4X2 71.5X3 70.6 71.9X2 2061.9 70.6X3 70.9X2 2197.3 2273 . 8 2411.9 2415.5 2482.5 2626.3 Series C\ . 2064.3 70.6 2134.9 70.0 2204.9 96 FLUORESCENCE OF THE URANYL SALTS. TABLE 34. — General list of absorption bands in spectra of the double uranyl chlorides at —185° C. — continued. Series C\.. Potassium uranyl chloride. Ammonium uranyl chloride. Rubidium uranyl chloride. Caesium uranyl chloride. ixio3 A Inter- val. Jxio3 A Inter- val. ^xio3 A Inter- val. ,-xio3 A Inter- val. 2059.— 2067.8 70.3X3 72.0 2131.3 68.7 2200 0 72.0 69.9 2272.0 2278.7 71.0 2341.9 2349.7 69.5 •71.6X4 71.6 2411.4 2421.3 70.3 2491.6 68.9 2560.5 2997.6 Series Co'. 2141.3 71.1 2212.4 70.7 2264.2 2283 . 1 68.5 2332.7 71.3 [69.2X2 2404.0 2542 . 3 Series C?. 2057.6 2064.7 2064.7 69.8 71.8 69.8 2127.4 2136.5 2134.5 69.9 70.3X2 70.8 70.9 2197.3 2207.3 2205 . 4 71.0 2278.3 69.8 2337.8 2348.1 2356.0 70.9 71.8 2419.0 2427 8 70.9 70.3 2489.9 2498.1 71.7 69.3 2561 . 6 2567.4 71.4 2633.0 INTIMATE STRUCTURE ON COOLING. 97 TABLE 34. — General list of absorption bands in spectra of the double uranyl chlorides at —185° C. — continued. Series CV'. Potassium uranyl chloride. Ammonium uranyl chloride. Rubidium uranyl chloride. Caesium uranyl chloride. Jxio3 A Inter- val. ^XIO3 A Inter- val. [xio3 A Inter- val. ^XIO8 A Inter- val. 2346.1 2352 . 4 •69.7X3 72.9 68.8 [69.1X2 2414.9 2553 . 0 2561.5 2634 4 72.0 2706.4 Series di. 2068.8 2075.4 70.7 70.3 2139.5 2145.7 69.0 69 2 2208.5 2214.9 •72.7X2 70.4X3 70.0 2278.5 2360.2 2571.4 Series d\" . 2006.8 2009.6 71.1 72.0 2077.9 2081.6 71.0 70.8 2148.9 2152.4 69.3 66.7 2218.2 2219.1 70.3 72.2 70.5X2 69.6 2288 5 2291.3 70.6 2359.1 71.8 2430.9 2432.3 71.1 2502.0 2501.9 71.2 2573.2 72.3 2645.5 98 FLUORESCENCE OF THE URANYL SALTS. TABLE 34. — General list of absorption bands in spectra of the double uranyl chlorides at —185° C. — continued. Series d^'. Potassium uranyl chloride. Ammonium uranyl chloride. Rubidium uranyl chloride. Caesium uranyl chloride. ^XIO3 A Inter- val. Jxio3 A Inter- val. ^•xio3 A Inter- val. £xio3 A Inter- val. 2221.4 2086.2 71.2 70.2 2292.6 2156.4 70.7 71.3X3 67.3 2363.3 2223.7 72.5 2296.2 68.2 2364.4 72.0 2577.3 2436.4 72.1 2508.5 70.8 2579.3 Series (fa. 2079.0 69.9 2148.9 68.2 2217.1 2229.2 70.5 71.6 2287.6 2300.8 68.5 68.6 2356.1 2369.4 73.1 69.8X2 71.4 2429.2 2440.8 72.4 2513.2 71.4 2568.7 2584.6 Series dz". 2016.1 70.1 2086.8 2086.2 2092.7 68.7 72.0 71.1 2155.5 2158.2 2163.8 71.5 69.3 2227.0 2233 . 1 71.0 71.8 2298.0 2304.9 70.3 73.1 2368.3 2378.0 69.6 170.7X3 66.7 2437.9 2444.7 2650.0 INTIMATE STRUCTURE ON COOLING. 99 TABLE 34. — General list of absorption bands in spectra of the double uranyl chlorides at —185° C. — continued. Series dz. Potassium uranyl chloride. Ammonium uranyl chloride. Rubidium uranyl chloride. Caesium uranyl chloride. ^XIO3 A Inter- val. ^xio3 A Inter- val. ^xio3 A Inter- val. Jxio3 A Inter- val. 2014.0 70.6X2 69.2X3 73.6 2155.2 2362.7 2436.3 Series e\. 2021.6 72.0 [69.8X2 71.1 2093.6 2233.1 2304.2 70.5 2374.7 71.5 2446.2 71.1 2517.3 70.2 2587.5 Series e\. 2092.7 69.4 2162.1 2166.8 70.9 69.9 2229.4 •69.9X2 71.9 2233.0 2236.7 70.4 72.3 2303.4 2309.0 2369 . 1 72.6 68.7 2376.0 2378.7 69.0 71.9 2441.0 2445.0 2450.4 69.6 70.8 >70.9X2 2510.6 2515.8 71.4 2582.0 2657.5 100 FLUORESCENCE OF THE URANYL SALTS. TABLE 34. — General list of absorption bands in spectra of the double uranyl chlorides at —185° C. — continued. Series e-i . Potassium uranyl chloride. Ammonium uranyl chloride. Rubidium uranyl chloride. Caesium uranyl chloride. ^xio3 A Inter- val. Jxio3 A Inter- val. ixiO3 A Inter- val. ixiO3 A Inter- val. 2023.4 2029 . 6 71.1 70.8 2094 . 5 2100.4 70.5 •70.3X2 69.4 71.7X2 70.2 2165.0 2170.6 68.0 2238.6 71.7 2305 . 5 2310.3 2374.9 2518.3 Series c^. 2030.7 70.3 2101.0 71.5 2172.5 69.7 2242 . 2 71.0 2313.2 2314.0 2383.0 69.5 (71.7X3 69.0 [71.2X2 j 69.4 2382.7 2525 . 3 2597.8 2594 . 7 Series e-/'. 2031.0 2034 . 1 71.4 71.4 2102.4 2105.5 72.0 70.6 2174.4 2176.1 67.8 •72.7X2 72.2 68.9 2242.2 2245 . 0 72.8 2317.8 70.8 70.6X2 2387 . 5 2388.6 2459 . 7 71.0 2530.7 2529.7 INTIMATE STRUCTURE ON COOLING. 101 TABLE 34. — General list of absorption bands in spectra of the double uranyl chlorides at —185° C. — continued. Series a\ . Potassium uranyl chloride. Ammonium uranyl chloiide. Rubidium uranyl chloride. Caesium uranyl chloride. ^-xio3 A Inter- val. ^XIO3 A Inter- val. ^X103 Inter- val. ^XIO3 A Inter- val. . 2036.9 2038.1 70.6 70.3 2107.5 2108.4 70.7 71.1X3 71.9 70.7 2178.2 2179.1 70.6 2249 . 7 72.6 2322.3 71.2 •70.4X2 2391.5 2393 . 5 2463.4 70.2 2533.6 2534.3 Series a\. 2038.5 71.5 2109.0 71.2 2180.2 70.7 2250 9 71.9 2322.8 71.2 2384 . 4 2394 0 72.0 72 8 2456.4 2466.8 69 2 70.5 2525.6 2537 . 3 69.1 71.7 2594.7 2609 . 0 Series aa. 2038.1 2044 2 2045 . 3 70.4 70.7 71.1 2108.5 2114.9 2116.4 70.4 170.4X2 70.0 70.1 2178.9 2184.9 2186.5 2319.6 Series 02". 2391.9 70.7X2 74.2 2533.3 2607.5 VI. THE POLARIZED SPECTRA OF THE DOUBLE CHLORIDES AND DOUBLE NITRATES. The polarization of the fluorescent light from crystals, first rioted by Grailich in 1857,1 has since been studied by Maskalyne,2 von Lom- mel,3 E. Wiedemann,4 Sohncke,5 Schmidt,6 H. Becquerel,7 and Pochet- tino.8 With the exception of the work of Becquerel on the ruby, in which low temperatures were employed, the authors cited above dealt chiefly with fluorescence of the usual type, consisting of broad bands. In such cases the most that can be done is to determine the direction of vibration and estimate the proportion of polarized light. The uranyl salts afford a much more favorable field for such investi- gations. Well-formed crystals of certain of these salts show a marked pleochroism. When viewed through a Nicol prism their color changes from a yellow-green to a very pale yellowish-white when the plane of the Nicol is turned through 90°. In the case of the double chlorides of uranyl (i. e., U02C12-2NH4C1+2H20; UO2C12-2KC1+2H20; U02C12- RbCl+2H20; and U02C12 • 2CsCl), these changes of color are connected with striking and significant variations in the fluorescence and absorp- tion spectra, and this is true also of certain of the double nitrates. These double chlorides, as has been shown in Chapter V, differ from the other uranyl salts thus far studied in the greater degree of resolu- tion exhibited by their spectra at +20°. The further resolution effected by cooling the crystal to the temperature of liquid air is in general the same for all; i.e., the bands are resolved into doublets the components of which in some cases, particularly noticeable in the absorption spectra, show indications of further complexity. The doublets, moreover, are polarized,9 the planes of vibration of the com- ponents being at right angles to one another, so that two entirely dis- tinct spectra of fluorescence and absorption may be observed by the use of a Nicol prism. For the study of these remarkable phenomena the apparatus depicted in figure 70 was devised. Within the collimator of a spectroscope of constant deviation a rhomb of calcite R was so mounted as to give two vertically displaced images of the slit, and these by suitable adjustment of the length of the slit could be rendered contiguous without overlapping. 1 Grailich, Krystall-optische Untersuchungen, 8 G. C. Schmidt, Wiedemann's Annalen, LX, p. Wien, 1858. 740. 2 Maskalyne, Proc. Royal Society, xxviu, 7 H. Becquerel, Comptes Rendus, CXLIV, p. p. 479. 671. Jvon Lommel, Wiedemann's Annalen, vni, 8Pochettino, Nuovo Cimento (v), 18. 1909. p. 634. 9 Nichols and Howes, Proce. Nat. Acad. Sci. 4 E. Wiedemann, Wiedemann's Annalen, ix, p. i, p. 444, 1915; and more fully in Phys. 158. Rev. vin, p. 364. 1916. 6 Sohncke, Wiedemann's Annalen, LVIII, p. 417. 102 POLARIZED SPECTRA OF DOUBLE CHLORIDES. 103 The crystal C was mounted before the slit and turned about the axis of the collimator until the planes of vibration of the transmitted light coincided with planes of transmission of the rhomb. For the study of fluorescence, the light from a carbon arc A, after passage through the condensing lens L, the water-cell W, and a light- filter F, was employed for excitation. The filter was opaque to light of a wave-length greater than 0.45ju, so that the fluorescence appeared on a black background. When absorption spectrographs were required a pale-blue screen was substituted, and the carbon arc was replaced by a 1,000- watt nitrogen-filled tungsten lamp. C r 9 L A. X FIG. 70. Many crystals were produced before any were found which gave complete separation of the two polarized components. A mere inspec- tion of the crystals was not a sufficient criterion; but when trans- mitted light polarized parallel to one of the planes of vibration of the crystal was used, the presence of only one of the two absorption spectra was found to afford a very delicate test, both for the adjustment of the apparatus and the homogeneity of the crystal. In accordance with the usage adopted we shall call that component of the spectrum due to vibrations in the more transparent direction of the crystal, the white component, while the component at right angles to this will be designated as the green component. The stronger fluorescence, as might be expected, is that of the green component, since light polarized in that plane is more strongly absorbed. The four double chlorides upon which observations were made crys- tallize in triclinic plates. These were so mounted that the flat faces were at right angles to the transmitted light. The flat faces of the potassium, ammonium, and rubidium uranyl- chloride crystals correspond to the (c) crystallographic face, while the flat face of caesium-chloride crystals corresponds to the (&) crystallo- graphic face. The caesium chloride crystallizes in gypsum-like plates, which were mounted with the longest (c) crystallographic axis vertical. Since the plane of polarization of the white light is also vertical within 104 FLUORESCENCE OF THE URANYL SALTS. a degree or two, light vibrating horizontally is, in this arrangement, less absorbed than light vibrating vertically. As to the direction of vibration of the white light within the crystal, it can be said to be more nearly parallel to the (a) crystallographic axis than to the (b) axis. Rubidium chloride crystallizes in long six sided plates. As mounted, plane polarized light was transmitted most freely when the direction of vibration was parallel to the (a) crystallographic axis. Potassium and ammonium chlorides crystallize in thin plates which approximate more nearly the hexagon in form. Examination of the transmitted light with the aid of a Nicol shows that the same relations exist between the directions of vibration and the crystallographic axes as for the rubidium chloride.1 Two polarized fluorescence spectra are always present, provided the crystal is mounted as previously described. It is a remarkable fact that these spectra remain unchanged, whether the exciting light is unpolar- ized or is polarized in a white or green direction, or any other direction. Their character, moreover, appears to be independent of the direction from which the exciting light enters the crystal. This is in agreement with a general principle established by the study of fluorescence spectra,2 that the character and location of a fluorescence band is independent of the nature of the excitation. Changes in the polarized spectra occur, however, as might be expected, if different crystallo- graphic faces are placed at right angles to the axis of the collimator. Although visual observations were made, the spectra were mapped for the most part from the photographic plates. Occasionally, the fluorescence and a portion of the absorption spectrum could be photo- graphed simultaneously to advantage, but more often different screen- ing and various times of exposure were necessary in order to bring out different regions of the absorption. The exposures varied in time from 30 seconds to an hour. A STUDY OF TYPICAL GROUPS OF BANDS FROM THE FLUORESCENCE AND ABSORPTION SPECTRA AT +20° C. In the study of the spectra of the double chlorides described in Chap- ter V, it has been shown that each group consists of 5 members and that each of these bands is double. Since polarization effects a resolution or separation, we should expect in general to find 5 components in each polarized fluorescence group. In that chapter the bands of one fluores- cence group have been designated as B, C, D, E, and A, and the same nomenclature will be employed here. A typical fluorescence group for each of the four salts is indicated in figure 71 . The bands above the horizontal line are the green polar- ization components (Bg, Cg, etc.) ; those below, the white polarization 1 The excellent specimens which were finally utilized in this investigation we owe to the per- sistent and skillful efforts of Mr. D. T. Wilber. 2 Nichols and Merritt, Physical Review, series i, 27, p. 373. 1908. POLARIZED SPECTRA OF DOUBLE CHLORIDES. 105 components (Bw, Cw, etc.). The lengths of the bands give an approxi- mate idea of their intensities, although the difference in intensity between a strong C band and a weak A band can not be shown to advantage in such a diagram. The positions of the crests of the bands are taken from the observed values, to be found in table 35, but the width and form of the bands are more or less arbitrary, being the expression of a judgment based on a large number of observations. GREEN URANYL POTASSIUM CHLORIDE C, POLARIZED FLUORESCENCE GROUPS 4-20° URANYL AMMONIUM CHLORIDE L \! WHITE V URANYL RUBIDIUM CHLORIDE GREEN WHITE v A,. GREEN WHITE URANYL CAESIUM CHLORIDE C, GREEN WHITE FIG. 71. From this figure it will be seen that bands C, E, and A of uranyl potassium chloride appear as doublets, polarized at right angles. Band B has no green component visible, but, as will be shown in a subsequent paragraph, at —185° a green component is present, which lies nearer the red than Bw. Bands Cg and Cw are the two components of band C, while no component of band D has been found on the white side. Bands E and A are also well resolved; the white component of band E is of longer wave-length than the green component, while the white components of C and A, and probably of B, are of shorter wave- length than their respective green components. 106 FLUORESCENCE OF THE URANYL SALTS. The uranyl ammonium chloride group shows a strong similarity to the preceding group. All except band B appear as polarized doublets. Components Dw and Aw were discerned only with the greatest diffi- culty. The uranyl rubidium chloride group is very similar to the uranyl potassium chloride group. Band Bg is missing, but as in the potassium chloride, there is a — 185° component to the red of Bw. Component Cw has a position nearer Dg than has Cw in the preceding spectra. This is also the condition existing in the caesium-chloride spectrum, and it is possible, since no Dw component is visible in either spectrum, that Dw is very dim, and hidden in Cw. Uranyl caesium chloride gives the most satisfactory set of fluores- cence bands, since both components of band B are present, and the C, E, and A components are very well separated. It is interesting to note that Bg is of longer wave-length than Bw, as is the —185° com- ponent of Bg in the preceding salts. It has been previously stated that the absorption spectra, like the fluorescence spectra, are composed of series, which begin with the bands which terminate the fluorescence series. The absorption bands which lie nearest the fluorescence region can also be arranged in recur- ring groups. The absorption series will be designated 6, c, d, e, a; since they join the B, C, D, E, and A fluorescence series, respectively. The e and A series are the strongest in the reversing region, but grad- ually vanish, while the D series becomes stronger toward the ultra- violet. Figure 72 gives a typical absorption group for each of the four salts. As before, the components above the line belong to the green ; those below to the white polarization. By comparing the uranyl potassium chloride absorption group in figure 72 with the fluorescence group of the same salt in figure 71, it will be seen that there is no bg component present, as there was no Bg component present, but that cg) dg, eg, and ag, corresponding to series Cg, Dg, Eg, and Ag are present and that there are no other series represented. Although the relative intensities of the absorption com- ponents are almost reversed when compared with the relative intensi- ties of the fluorescence bands, the same spacing exists between the green components of both fluorescence and absorption. In the white polarization group, cw corresponds, in position, to Cw, and ew to Ew, while bw aw serves both Bw and Aw series in the following way: Bw is the first member of each fluorescence group, while Aw is the band of the preceding group which is nearest to Bw. As the fluoresence intervals of both the A and B series are approximately 83 frequency units, and Aw is 12 units distant from Bw, the reversing band of the aw series must coincide with the second member of the bw absorption series, since it is 71 units from the reversing band Bw or bw. The dw component is absent, as is Dw, and there are no superfluous series. POLARIZED SPECTRA OF DOUBLE CHLORIDES. 107 The absorption group of the uranyl ammonium chloride is very similar to that of the potassium chloride. Again, the bg component, like the Bg component, is absent, but the other fluorescence series are represented by absorption series, save that no component of d was found to join the very weak Dw fluorescence band. Uranyl rubidium chloride shows a grouping analogous to that of the potassium and ammonium chloride, while the uranyl caesium chloride group is only slightly different. A bg series is present, which is properly related to the Bg series, so that bg and bw are the same relative positions as are Bg and Bw. POLARIZED ABSORPTION GROUPS +20° URANYl POTASSIUM CHLORIDE r\ GREEN WHITE URAMYL RUBIDIUM CHLORIDE ttREEN WHITE I URANYL AMMONIUM CHLORIDE GFEEN Wl ITE URANYL CAESIUM CHLORIDE GREEN WHITE FIQ. 72. No green polarized component joins the Cg component. The dotted line shows where an absorption component would have to be placed to have the proper relation, according to our theory. The cg band is evidently complex. cw is present, however, as a single band, and the d, e, and a components occupy positions which agree with their corre- sponding fluorescence components, bw and aw are here separate. 108 FLUORESCENCE OF THE URANYL SALTS. A DETAILED STUDY OF THE RELATION BETWEEN FLUORESCENCE AND ABSORPTION SERIES. In figure 73 are indicated 2 complete fluorescence groups and 2 complete absorption groups for each of the 8 spectra. The remark- able fact is that although no observed fluorescence bands have been omitted which fall within the frequency numbers plotted, each fluores- cence series has its properly related absorption series, and with the excep- tion of the complex Cg series of the csesium chloride not a superfluous IS|00 1 I9|00 1 20)00 1 2IJOO 1 URANYL POTASSIUM CHLORIDE +20* C C D D E ' . c deacdeacd GREEN 1 ? i: i i C E B i i ( . i * L$ °, e ba c e ba, c WHITE 1 1 It i : 1 1 1 c URANYL AMMONIUM CHLORIDE C D E A '«. cdeacd.ea.cci E A IE! II • GREEN 1 1 ii > ; B 1 B E I C * c _ . J | e oa c e » c e ab c t * , Ml II 1 WHITE 1 l r~i URANYL CAESIUM CHLORIDE C C 1 GREEN 1 D 1 I; ) "r»« i d e ab c d e ab c d l , 1 11 1 i Ml 1 ! II 1 1 1 1 1 i B e A B I • * .' ", e Y c e o.t> c e G, o c WHITE 1 * \ 1 ie|oo i L»Jp_o i 2o|oo 2100 i FIG. 73. — Polarized bands of fluorescence and absorption; four contiguous groups showing the relation between the green and white components at +20°. Dotted lines show computed positions of absorption bands. Solid lines above the base indicate fluorescence; below the base, absorption. absorption series is present. Fluorescence bands are designated by the solid lines above the horizontal, and absorption bands by the solid lines below the horizontal. The dotted lines above the absorption bands represent the hypothetical positions of absorption bands, com- puted in the following manner: The average interval for the series in question was computed from all available observations on the bands which belong to it, carefully weighted. A hypothetical position for the band in the reversing group was then found by adding this interval to the average of the observa- tions on the position of the preceding band of the series. This hypo- POLARIZED SPECTRA OF DOUBLE CHLORIDES. 109 thetical position was taken as the starting-point of the corresponding absorption series, the hypothetical positions of the subsequent mem- bers being found by addition of the weighted average for the observed interval of that series. By reference to figure 73 and to tables 35 and 37, the reader can note the general agreement between observed and calculated positions ; also the occasional discrepancies. In the spectra of uranyl ammonium chloride and uranyl rubidium chloride for example, the Bw and Aw series are spaced at such an interval that aw and bw can not coincide, as will be seen from figure 73. The observed bwaw bands occupy posi- tions between the assumed positions of the b and a series, which tends to show that the ba band is a narrow doublet. The fact that a few of the observed absorption bands do not appear to be in their proper places can be readily explained when it is remembered that there is sufficient experimental evidence to lead to the belief that many of the absorption bands are doublets, consisting of a strong and a weak com- ponent. The breaks in a few of the absorption series, as in the eg and ag series of the uranyl ammonium chloride, are undoubtedly due to the sudden increase in strength of one component, accompanied by a corresponding decrease in the other component. Table 35 contains the observed positions of all the fluorescence and absorption bands at +20°, measured in our determinations of the spectra of the four salts. Figure 73 is a map of only the central portion, extending, as already stated, two groups into the fluorescence on the one side and two groups into the absorption on the other. TABLE 35. — Polarized series at +20° C. URANYL POTASSIUM CHLORIDE. « Fluorescence. Green component. White component. C. D. E. A. B. C. E. A. 1756.3 1838.6 1922.0 1774.6 1788. 1856.9 1871. 1940.6 1955. 9 1770.9 1853.5 1937.5 1821.0 1903.1 1984 . 6 2 1804.2 2 1887.1 1970 1 1827.2 1909.6 1992.0 1958.0 Absorption. Green component. White component. c. d. e. a. ba. c. e. 1985 . 7 2054.2 2125.4 2004 2073 2145 .4 .8 .5 2024 . 7 2094 . 7 2165.9 2036.7 2106.7 1968.1 2039 . 6 2111.0 2179.1 2398 1 1994.4 2064 . 8 2134.9 2019.8 2090.7 2160.8 2287 2359 .8 .0 2247.7 2602 4 2624.0 2695.4 2673 . 8 2743.5 2635.0 110 FLUORESCENCE OF THE URANYL SALTS. TABLE 35. — Polarized series at 20° C. — continued. URANYL AMMONIUM CHLORIDE. Fluorescence. Green component. White component. C. D. E. A. B. C. E. A. 1742.1 1824.0 1907.0 1990.0 1756.4 1840.3 1923.6 1775.8 1857.6 1941.6 1790. 1874. 1957. 3 1748.4 1829.6 1912.9 1770.9 1853.8 1937.0 1 1804.2 S 1886.8 1970 6 1959.0 Absorption. Green component. White component. c. d. e. a. ba. c. e. 1988.5 2057.3 2128.1 2007.9 2077.3 2146.9 2218.1 2289.7 2360.8 2431.4 2500.8 2571.4 2026.5 2098.1 2169.8 2240.1 2313.2 2383 . 3 2455.8 2525.9 2039.9 2110.9 2182.3 2256.8 2327.7 2399.2 1970.5 2042 . 6 2114.1 2185.0 1999.8 2070.2 2140.5 2022.4 2094.7 2166.0 2311.8 2381 . 8 . 2623.3 URANYL RUBIDIUM CHLORIDE. Fluorescence. Green component. White component. C. / D. E. A. B. C. E. A. 1739.1 1822.7 1905.0 1986.9 1757.6 1840.9 1923.9 1781.9 1865.3 1948.8 1750.6 1832.4 1916.4 1774.7 1858.0 1941.6 1793.6 1875.5 1959.5 1973.7 1956.9 1808.8 1891.1 1975.3 Absorption. Green component. White component. c. d. e. a. aft. c. e. 1991.2 2059.3 2130.4 2008.4 2079.4 2151.0 2030.0 2101.7 2040.4 2111.5 1973.7 2043.8 2114.9 2148.8 2253.8 1999.2 2069.1 2140.2 2209.5 2024.3 2094.7 2165.9 2294.1 2364.1 2434.9 2507.5 2579.3 2629.5 2590.7 2677.4 POLARIZED SPECTRA OF DOUBLE CHLORIDES. Ill TABLE 35. — Polarized series at 20° C. — continued. URANYL CAESIUM CHLORIDE. Fluorescence. Green component. White component. B. C. D. E. A. B. C. E. A. 1761.2 1843.3 1927.5 1778.1 1860.8 1944.4 1789.9 1873.7 1956.2 1729 . 8 1812.3 1894.3 1978.0 1751.3 1835.2 1917.9 2000.4 1775.6 1858.0 1941.4 1793.4 1875.8 1958.1 1807.3 1891.1 1973.6 1827.2 1909.9 1992.0 Absorption. Green component. White component. b. c. d. e. a. b. c. e. a. 1973.9 2045 . 6 2116.4 2187.0 1991.6 2057.6 2065 . 7 2127.9 2133.1 2198.3 2202 . 2 2007 . 4 2076.4 2028.6 2098.6 2036.4 2107.6 1978.2 2049.0 2002.8 2072.1 2140.9 2212.9 2024.5 2094.9 2164.9 2235 . 6 2037.1 2107.9 2179.1 2145.2 2170.1 2179.1 2214.6 2284 . 1 2355.4 2426.9 2496.3 2567.4 2467.9 2538.7 2002 . 1 2673 . 8 2624 . 0 2696.1 2637 . 1 THE EFFECT OF LOW TEMPERATURES ON THE RESOLUTION AND POSITION OF THE BANDS. It has been shown in Chapter V that low temperature tends to narrow all the bands in the spectra of the double chlorides, to resolve them into doublets, and to produce certain shifts in their position. These temperature shifts were explained by assuming that the bands at +20° are close overlapping doublets, the stronger components of which are weakened by lowering the temperature, while the weaker components are strengthened. Such shifts occur in all of the polarized spectra here under consideration and the same explanation is appli- cable. They will be considered in detail in a later paragraph. In table 36 are recorded the observed positions of the fluorescence and absorption bands in the two polarized components at --185°. Figure 74, like figure 73, is a map of four contiguous groups, two of fluorescence and two of absorption, inserted to facilitate the compari- son of the green and white components as regards the location of the bands. Since the arrangement repeats itself from group to group, it is unnecessary to include the outlying regions toward the red and toward the violet. 112 FLUORESCENCE OF THE URANYL SALTS. TABLE 36.— Polarized series at -185° C. URANYL POTASSIUM CHLORIDE. Fluorescence. Green component. White component. C. D. E. A. A'. B. C. E. A. 1738.4 1820.5 1903.3 1985.7 1785.4 1797.9 1868.8 1881.7 1953.5 1966.6 1741.3 1826.2 1911.7 1996.0 1771.9 1854.9 1938.5 1786.4 1870.9 1954.7 1842.3 1926.0 1858.1 1940.8 1807.3 1891.4 1975.5 Absorption. Green component. White component. c. d'. d. d". c. 6. e. a. 2010.5 2000.4 2067.8 2137.7 2207.5 2114.2 2184.8 2257.8 2021.0 2092.7 2164.0 2041.2 2112.2 2182.6 2252.7 2058.0 2128.1 2199.3 2076 2146 2218 2289 .6 .8 .7 .9 2086.4 2157.5 2228.8 2301.5 2371 . 1 2440.8 2512.3 2152.9 2222.2 2541.9 2485.7 e. e'. a. c'. 2024.3 2094.2 2164.5 2028 2099 2169 2241 .4 .1 .7 .4 2036.2 2106.6 2177.8 2248.5 2124.0 2193.5 2384 .9 2525 .3 2532.9 URANTL AMMONIUM CHLORIDE. Fluorescence. Green component. White component. C'. C. D. E. A. B'. B. C. C'. 1738.5 1821.2 1905.1 1988.9 1744 . 6 1828.0 1911.7 1994.8 1751.2 1835.5 1919.7 2004.2 1777.5 1861.6 1946.7 1843.5 1928.2 2012.5 1857.7 1941.0 2023.5 1783.6 1868.1 1952.4 1803.8 1888.4 1972.9 1810.1 1894.1 1978.2 Absorption. Green component. White component. c. d. e. a. a'. 6. c. e. e'. 2023.6 2094.9 2037.9 2108.8 2044.2 2115.5 2018.8 2030.3 2063.0 2066.0 2135.5 2138.4 2205.1 2208.7 2276.6 \2279.5 2085.8 2049.4 2121.2 2192.0 2263.8 2334.8 2077.6 2148.6 2217.8 2287.8 2359.0 2102.2 2174.0 2156.6 2166.5 2181.1 2233.1 2302.8 2227.9 2238.6 2249.7 2321.5 POLARIZED SPECTRA OF DOUBLE CHLORIDES. 113 TABLE 36. — Polarized series at —185° C. — continued. UBANYL RUBIDIUM CHLORIDE. Fluorescence. Green component. White component. B. C. D. A. B. C. A. 1746.4 1828.8 1912.0 1994.2 1755.0 1837.2 1920.9 2004.6 1804.7 1887.9 1971.4 1845.7 1929.9 2013.7 1874.8 1958.9 1812.4 1895.9 1979.0 1879.2 1962.3 Absorption. Green component. White component. b. c'. c. d". 6. c. e. (?. 2045.0 2115.5 2187.2 2066.1 2136.8 2207.5 2279.5 2350.7 2420.5 2491.3 2561.5 2004.8 2076.2 2146.6 2216.3 2030.5 2100.2 2172.0 2050.4 2122.2 2192.0 2263.0 2333.7 2200.7 2272.7 2223.0 2294.1 2362.9 2435.5 2504.4 2576.3 2238.1 2309.5 2378.0 2451.0 2359.0 2485.1 2541.3 2478.3 2548.4 2498.1 2570.0 2526.5 d. d'. a'. a. 2016.7 2085.5 2159.8 2229.2 2035.8 2108.4 2233.9 2305.0 2375.0 2446.2 2515.4 2246.7 2254.3 2325.8 2369.1 2391.6 2463.1 2532.9 2537.4 URANYL CAESIUM CHLORIDE. Fluorescence. Green component. White component. C. D'. D. E. A. B. C. E. A. 1750.7 1834.2 1916.8 1997.4 1794.1 1878.0 1962.1 1757.8 1841.3 1924.6 2007.8 1794.9 1879.0 1963.1 1847.1 1930.5 2013.7 1852.3 1935.8 2019.0 1866.7 1950.3 1816.6 1899.8 1983.6 1863.9 1947.0 114 FLUORESCENCE OF THE URANYL SALTS. TABLE 36. — Polarized series at —185° C. — continued. URANYL CAESIUM CHLORIDE — continued. Absorption. Green component. White component. b. 6'. c.' c. c". bo. c. e. 2050.8 2122.2 2192.7 2262.4 2333.7 2403.8 2475.2 2545.2 2615.7 2063 6 2009 . 6 2081.4 2152.9 2222 . 8 2294 . 1 2366.3 2435.2 2505 . 0 2033.4 2103.9 2174.9 2245.0 2317.0 2386.0 2458.2 2529.1 2126.8 2134.5 2204.4 2274.8 2345.8 2140.9 2211.9 /2280.5 \2284.7 2354.0 /2422 . 5 \2426 . 6 /2492.5 \2495.6 2566.1 2635.4 2707 . 1 2050.8 2122.2 2192.8 2262.4 2333.9 2404.1 2218.5 J2289.4 2359.0 J2431.3 J2500.6 2572.7 2644.8 2267.1 2337.5 2409 . 6 2482.6 2489.4 2561.5 2631.6 2701.2 2621.2 2695 . 1 2646.9 d'. d. e'. e. a. 2021.0 2091 6 2035 . 8 2107.0 2178.2 2044 . 6 2115.1 2184.8 2257.8 2327.7 2400.4 2467.9 2541.3 2163.1 2233.4 2227.5 2299 . 1 2368.8 2441.4 2510.0 2238.6 2310.5 2382.7 2321.3 2394 . 1 2464.9 2534.2 2378.7 2449 . 2 2520.2 2524 . 0 As in these spectra at +20°, so at — 185° we find the D bands only in the green component, the B bands chiefly in the white component. In the white component, at both temperatures, B, C, and A lie toward the violet, E toward the red. At — 185°, B is doubled in the uranyl ammonium chloride, D in the uranyl chloride. To aid in the direct comparison of the spectra at the two tempera- tures, they are plotted together in figure 75, in which diagram may be seen the direction of the shift for each series of the two components. The shift is nearly always toward the violet, the only exceptions being the Ag and aa series and possibly the Aw series in uranyl potas- sium chloride (see fig. 74), the eg and fg series of uranyl ammonium chloride and the Ay series of the latter salt. The change is greatest in uranyl caesium chloride and least in the potassium double chloride. In general the fluorescence bands shift in the same direction and by the same amount as the related absorption bands, but there are some puzzling exceptions to this rule to be considered in a following section. The increased resolution of the spectra upon cooling shows itself in the doubling of many bands which appear single at +20°, an effect particularly noticeable in the absorption spectra. (See plate 1, c.) Thus the ca series of the potassium salt tends to double at 2,058.0 POLARIZED SPECTRA OF DOUBLE CHLORIDES. 115 and becomes clearly double at 2,128.1 and 2,199.3. The dy and ea series of the same salt are doubled and the ba series of +20°, which was assumed from the relations of the spectrum to be an unresolved doub- let, is separated into a bw and an aw series at — 185°. Other examples of doubling may be noted in the case of Cg, cg, ag, and Bw of uranyl ammonium chloride, aa, aw, and bw of the rubidium salt, and bw, Dg, and da of the caesium salt. laloo isloo 20|00 21 00 URANYL POTASSIUM CHLORIDE -195° C C C GREEN E A B -U-L E A B I I I dea c d e a c WHITE C c Bi _ . B j e ab c e ab c EA. E * I !!!! !!!! I I I I I URANYL AMMONIUM CHLORIDE C C C D J D GREEN Cl I EA Cl , E A c, 1 1 I I I 1 1 III i d e a c dea C III ! ! I I ill i: 'ii n * e' b c e b III ! URANYL RUBIDIUM CHLORIDE c c c GREEN B A B I I A B I . d. ab c d ab WHITE URANYL CAESIUM CHLORIDE C C e b e b GREEN DO EA il I I dO E A Tl I I c d e ab c d e ab : ! 1 I j I J ' _\ } ,- 1 i. WHITE B A 6 A J l_ e ba, c e ba. IflOO _L 19100 _L 2000 2IJOO FIG. 74. — Polarized bands of fluorescence and absorption; four contiguous groups showing the relation between the green and white components at —185°. Dotted lines show computed positions of absorption bands; solid lines above the base indicate fluorescence, below the base absorption. ON THE FREQUENCY INTERVALS OF FLUORESCENCE AND ABSORPTION. During the preliminary study of the fluorescence and absorption of uranyl ammonium chloride described in Chapter V, the symmetry of the spectrum was such as to lead to the suspicion that the various homologous series would be found to have the same constant-frequency interval. The final tabulation of results, however, after many redeter- minations of what seemed to be discordant values, showed that while the departures from uniformity were in general scarcely larger than the errors of observation, they were to some extent systematic and indi- 116 FLUORESCENCE OF THE URANYL SALTS. cated slightly different values for the various series. The C bands in particular, which were a composite of what in these later studies we have designated as Cy and Cw of the polarized spectrum, were found to have an unquestionably smaller interval than the other series of the group. It will be seen from table 37 that this is true for both Cg and Cw in the case of all four salts at +20° and that with the possible exception of Aw, which is an exceedingly feeble component, visible only in two of the salts and very difficult of determination; all other series are very ie|oo I9|00 20JOO 2||00 GREEN +20° URANYL POTASSIUM CHLORIDE c d, e a, c A e a, c unAflTL. rulA33IUM V, TTTt t Mt aREEN-!85° . I I 1 1 i I I 1 1 I I I I T WHITE 4-20 .'if r i i " in i i MM ii e ba, c f ba c WHITE -tSS'l I I I I I I I I I I I ' I 111 GREEN 420° URANYL AMMONIUM CHLORIDE C D E A CDEA _| I || cdeac&eac GREEN -185° || | | WHITE 4-20° E B C E A B I I I I II e ba. c ba, WHITE -.85-1 I I I II I I IM 1 1 1 GREEN f 20° CDEA I M TTn II 1 , c d e a, c d ea c URANYL RUBIDIUM CHLORIDE EA C D EA GREEN-185'I I I I I I I I I I I M I II 0 WHITE -f 20 E A B C E A B I I I I I I e ah c e ab WHITE -185° 8REEN«o URANYL CAESIUM CHLORIDE BCDEABCDEA c| | | || I I 1 II 1 1 1 1 1 bcdeabcdeabc GREEN -185 WHITE 4-20° I II II I II I I I I I III I I II — — — — — — 1 1 — rn — n — ~i — i"1" f 9 E A B C E A I I III II I I b c e ab c e ab WHITE -185° 1 III III 18 100 oloo 1 20JOO 2IJOO Fia. 75. nearly of the same interval, not only in the same salt but in all the salts. When, however, we make further averages of the average intervals from table 37, taking the mean of all green components of fluorescence, then of all white components, for each salt separately, and do the same for the absorption intervals, we find an approach to systematic arrange- ment which is suggestive if not altogether conclusive. (See table 37.) Both components of the fluorescence spectrum show an average interval in the inverse order of the molecular weights, and while the absorption series do not give so decisive an indication the salts of lesser POLARIZED SPECTRA OF DOUBLE CHLORIDES. 117 molecular weight, NH4 and K show again a longer interval than do Rb and Cs. Averaging by series affords no such direct indication as to differences of interval, as will appear from table 38. It will be noted that while the averages for the green and white com- ponents of fluorescence are in very close agreement at +20° and also at - 185°, there is a difference of about 0.5 between the averages for +20° and those for -185°; also that the interval is greater for each individual series at —185° than at +20°, with the single exception of ew. This difference does not appear, however, in the case of the absorp- tion intervals. TABLE 37. — Average frequency intervals, +20° C. and —185° C. Fluorescence series. Green component. White component. Series. K NH4 Rb Cs Series. K NH« Rb Cs Bo 82.9 82.6 83.3 83.3 82.9 Bw. . . ^ W ' ' 83.0 82.1 83.4 82.9 83.6 83.3 82.9 82.8 Ca.... Dg.... Ea.... Ag.... 81.9 83.0 83.2 83.4 82.8 83.5 83.3 83.8 82.2 83.1 83.5 83.2 Ew... A 83.5 83.1 83.5 82.9 83.1 82.4 Absorption series. Series. K NH4 Rb Cs Series. K NH4 Rb Cs b0. 70.5 70.6 70.5 71.0 70.8 6«,... cw. .. 71.5 71.3 71.3 70.3 70.4 70.0 70.8 70.5 Cn 71.3 71.1 70.8 70.7 70.7 70.5 70.8 71.5 71.2 70.2 71.7 71.1 dg.... e0. . ^w • • • aw. . . 70.4 71.5 71.7 71.3 70.8 70.4 70.4 71.4 dg Average frequency intervals, —185° C. Fluorescence series. Green component. White component. Series. K NH4 Rb Cs Series. K NH4 Rb Cs Bg.... Cg.... Dg.... E0.... Ag.... 84.0 82.5 83.7 83.3 84.1 83.4 84.4 83.2 84.4 83.4 82.6 83.9 82.2 83.3 83.6 84.1 Bw. . . Cw. . . 84.1 84.8 84.1 84.4 83.2 83.9 83.6 83.3 83.0 83.1 84.1 84.1 .Aj^* . . 84.0 83.1 Absorption series. Series. K NH4 Rb Cs Series. K NH4 Rb Cs ba. 70.9 70.7 71.3 70.5 70.8 71.4 71.0 71.1 &„... cw. .. 71.3 69.8 71.3 70.6 70.8 70.3 70.6 70.7 Ca. . 71.5 70.3 71.8 71.0 70.9 71.1 71.5 70.9 dn aa e/i ew. . . 71.4 70.4 70.7 71. 1 70.9 70.6 "tt 70.6 uo 118 FLUORESCENCE OF THE URANYL SALTS. TABLE 37. — Average frequency intervals, +20° C. and 185° C. — continued. General averages of intervals (by salts). Fluorescence. NH4. K. Rb. Cs. Green +20° and -185°... White +20° and -185°... All fluorescence 83.60 83.78 83 . 25 83.43 83.25 83.42 83.19 83.16 83.69 83.34 83.33 83.17 Absorption. Green +20° and - 185°. . . White +20° and -185°... All absorption 70.99 71.02 71.07 70.96 70.96 70.55 70.74 70.70 71.00 71.01 70.75 70.72 TABLE 38. — General averages of intervals (by series). Fluorescence. Series. Green. Av. Series. White. Av. +20° -185° +20° -185° Eg.... Be.. 83.33 83.37 83.70 84.18 83 . 83 82.68 83.35 83.70 83.72 83.53 82.53 Ew... Bw . . . Aw. . . Dw. . . 83.37 83.23 82.80 83.05 83.75 83.73 83.17 83.49 83.74 Ag.... Dg.... Cg.... Av. . . . 83.26 83.23 82.38 Cw • • • Av. . . . 82.78 84.10 83.44 83.05 83 . 54 83.37 83 . 05 83.66 83.36 Absorption. +20° -185° Av. +20° -185° Av. 6n. 71.07 71.40 71.23 ew b,n. . 70.82 71.00 70.90 71.02 71.00 70.70 70.90 71.00 70.80 ba Q.n 71.02 70. 5S 70.95 70.90 71.02 70.98 70.96 70.75 70.96 aw dy, da Cn . cw .... Av. . . . 70.50 70.35 70.42 Av... . 70.90 71.07 70.97 70.81 70.77 70.78 On the other hand, differences so large are not to be regarded as errors of observation, it being possible to determine the average inter- val of any series, excepting possibly Ag and Aw, which are very weak and rather vague, within about 0.2. It does not follow, however, that the bands are really thus irregularly placed. The discrepancies are due rather to the fact that resolution is not equally complete in all portions of the spectrum and that on cooling the crystal structure was more or less disturbed and the polarization always much less complete. The anomalous values above 84 frequently observed at —185° (see table 37) are probably due to varying components of the opposite polar- ization superimposed on the bands in question and producing a false POLARIZED SPECTRA OF DOUBLE CHLORIDES. 119 shift. Thus, for example, the position of Cw would be modified by the presence of the overlapping of Dg or Cg; Dg by Cg, etc. In short, it is probable that if observations could be had on crystals which at — 185° preserved their structure, the difference in interval between +20° and — 185° would disappear. THE INFLUENCE OF MOLECULAR WEIGHT UPON THE POSITION OF BANDS. While some doubt may be felt as to the validity of the suggestion, based upon the averages presented in the foregoing paragraphs, that there is a relation between frequency intervals and the molecular weight, there can be no question as regards the influence of molecular weight upon the position of the bands. .60 .55 u. .50 jt J L Rb Cs K_ NH_, Rb_ Cs K_ NH_, Rt) J L J L Rb_ Cs K Rb_ Cs J L Rb_ Cs K NH, Rb_ Cs K_ NhU Rb_ Cs 20 21 22 23 24 25 FIG. 76. If we select a typical region in the spectrum and arrange the bands belonging to a single group as in table 39, we find a general drift of the various bands toward the violet as we pass from salt to salt in the order K, NH4, Rb, Cs. 120 FLUORESCENCE OF THE URANYL SALTS. The same drift occurs quite systematically throughout the entire fluorescence and absorption spectrum, as may be seen from figure 76. In this chart such of the fluorescence and absorption series as are present in all four salts at +20° are plotted on the frequency scale. The solid lines represent observed fluorescence bands; the dotted lines represent observed absorption bands; no hypothetical values are indicated. The order of the salts is the same as in table 39 and follows TABLE 39. Green polarization, 185°. White polarization, 185°. Cg. Dg. Eg. Ag. Bw Cw EW Aw. K 1903.3 1911.7 1912.0 1916.8 1842.3 1843.5 1845.7 1852.3 1940.8 1941.0 1868.8 1868.1 1874.8 1878.5 1891.4 1894.1 1895.9 1899.8 1911.7 1919.7 1920.9 1924.6 1854.9 1870.9 NH4. . . Rb.... Ca 1879.2 1879.9 1950.3 1862.5 that given by A. E. Tutton in his Treatise on Crystalline Structure and Chemical Constitution (London, 1916). He found for both single and double salts of the alkali metals that several of their optical properties, such as refractive index, etc., follow the order of the molecular weights, but that in the ammonium salts the NH4 radical often acts as if it were much heavier than the combined weights of its components would indicate, so that its position is quite close to rubidium and sometimes on the side toward caesium. It will be observed that there are several examples of this in figure 76, particularly in the case of the Cg series. SUMMARY. (1) The four double chlorides, uranyl ammonium chloride, uranyl potassium chloride, uranyl rubidium chloride, and uranyl caesium chlo- ride, crystallize in the triclinic system. The crystals are pleochroic and their fluorescence spectra and absorption spectra are polarized. (2) The spectra differ from those of other uranyl compounds thus far examined in that both in the fluorescence and absorption regions each band is resolved at +20° C. into a group of five bands forming homologous series of constant frequency interval. (3) The structure of the fluorescence spectrum is essentially the same in the different salts, the spacing of the bands of each group repeating itself in the successive groups, excepting in the reversing region, the appearance of which is modified by the overlapping of fluorescence and absorption. (4) Each of the five bands which constitute a group is a doublet, the two components of which are polarized at right angles to one another. (5) The frequency interval is the same or nearly the same for each series in a given salt. POLARIZED SPECTRA OF DOUBLE CHLORIDES. 121 (6) Variations in the average interval for the four salts are scarcely greater than the errors of observation, but there are indications of a very slight decrease of interval with increase of molecular weight, and this applies alike to fluorescence and absorption series. (7) The position, in the spectrum, of a given band varies slightly but systematically with the molecular weight of the salt. The order of diminishing wave-lengths is K, NH4, Rb, Cs; the shift from K to Cs being of the order of 5 A. u. This shift is in the same direction — from red toward violet — for all the homologous series and of the same size within the errors of observation. (8) Cooling to the temperature of liquid air produces the usual narrowing of bands, apparent shifts of position, and apparent changes of interval, all of which changes are explained by the relative enhance- ment or diminution of components of the bands. VII. THE NITRATES AND PHOSPHATES; INFLUENCE OF WATER OF CRYSTALLIZATION AND OF CRYSTAL FORM. I. URANYL NITRATE AND EFFECT OF WATER OF CRYSTALLIZATION. The spectra of the different uranyl salts are so similar in their general characteristics that we can scarcely doubt that the nature of these spectra is chiefly determined by the radical UO2. Apparently the uranyl radical contains a group of electrons whose arrangement is such as to permit of vibrations that give this type of spectrum; and although U02 is not stable in the chemical sense and must be com- bined with some acid in order to form a stable compound, yet the effect of the acid radical is merely to modify the constants of this vibrating system in the U02 radical without changing the type of vibration. It is natural to expect that the addition of water of crystallization would produce a similar effect, and it is our intention to present in this section of Chapter VII the results of a study of the influence of water of crystallization upon the fluorescence and absorption spectrum in the case of uranyl nitrate. The nitrate is particularly suited for such an investigation because of the fact that several different hydrates are formed. The crystals grown from a water solution contain 6 molecules of water. In an acid solution crystals are formed with 3 molecules of water. In both cases crystals may be obtained which are large enough to permit of observations being made with a single crystal. By methods described later, small crystals containing only 2 molecules of water are readily obtained. It is a matter of some difficulty to push the dehy- dration further, but specimens have been prepared for us by Mr. D. T. Wilber which we have reason to believe are either anhydrous or formed of a mixture of the anhydrous salt and the monohydrate. The fluorescence of the nitrate, like that of the other uranyl salts, with the exception of the double chlorides, the resolution of the bands of whose spectra into groups of five at +20° has been described in Chapter VI, is unresolved at ordinary temperatures. Careful spectro- photometric measurements of what appear to be unresolved bands reveal, however, indications of overlapping components, as has already been shown in Chapter III. At the temperature of liquid air the resolution into narrow bands characteristic of the uranyl spectra in general takes place, and it is to these resolved spectra that the following discussion refers. In the case of the hexahydrate, wave-lengths were in most cases determined photographically. \ isual observations, however, were also made, although these could not be extended throughout the whole spectrum. The agreement between measurements made by the two methods was surprisingly good. In the case of weak bands lying near 122 THE NITRATES AND PHOSPHATES. 123 to bands of great intensity the visual observations were found to be best. The results given for the fluorescence spectra of other hydrates and for the anhydrous salt are based upon visual observations exclu- sively. THE HEXAHYDRATE: U02(N03)2+GH20. The hexahydrate crystallizes in the rhombic system with the axial ratio a: b: c = 0.6837 : 1 : 0.6088. The crystals were grown in the form of plates by using a water solution whose depth was equal to the thick- ness of the plate desired. Single crystals as large as 15 mm. in diameter were obtained with relatively little difficulty. All of the results here discussed are based upon observations made with single crystals. In selecting the data to be used in taking a final average, each nega- tive was carefully studied and measurements that seemed for any reason doubtful were discarded. The elimination of doubtful observa- tions was made without reference to the agreement or lack of agree- ment between the different measurements, and was, in fact, completed before the measurements of the different negatives were compared. About 40 negatives were used, although the number for any one line was rarely more than 10. The errors of calibration of the spectrograph and spectrometer can hardly exceed 1 A. u., except perhaps in the extreme red end of the spectrum. The uncertainties due to the faintness of certain bands, to their finite width, and to photographic broadening are more difficult to estimate and undoubtedly differ greatly with the character of the band and its position in the spectrum. In the case of the sharper bands of moderate intensity we feel that the averages that are here tabulated are reliable within 1 A. u. In other words, the reciprocal wave-lengths are accurate to within about 0.02 per cent. For the faint or hazy bands the possible error is undoubtedly much greater. Of the 55 fluorescence bands observed, 46 can be arranged in 9 series, as tabulated below, the frequency interval being nearly constant in each series. Two of the remaining bands have the same interval, and apparently form part of a series whose other members were too weak to detect. The 7 bands that do not fall in any series arrangement are all extremely weak, and since in most cases they are recorded only once, their existence is subject to considerable doubt. Estimates are given in table 40 of the intensities of the different bands and of the reliability of the measurements. In some cases the series seem to extend into the region of absorption, and in such cases the absorption bands that seem to form part of the series are also given. The data for series B, D, E, and F, which are made up of the stronger bands and those of medium intensity, are undoubtedly the most reli- able. The values of the average interval between bands in these series are 86.0, 85.8, 85.9, and 86.1 respectively. In taking these averages, 124 FLUORESCENCE OF THE URANYL SALTS. the first band in the case of series D and E has been left out of con- sideration on account of its relative uncertainty. For the other series the interval, although less certain, has nearly the same value. It will be noticed that there is nothing to indicate any change in the interval as we pass from the longer to the shorter waves. TABLE 40. — Series in the fluorescence spectrum of uranyl nitrate hexahydrate [UO*(N03)i+6H20}. Inten- sity.1 Relia- bility.2 I3 X 1 AX Inten- sity.1 Relia- bility.2 I3 X 1 AX ( v. d. 2 F 1760.1 m. 1? F 1631.3 3 EF i i Hi jKL M 1 ill FIG. 80. — A single group from each of the four spectra. 1. Mono-potassium uranyl nitrate — trigonal. 2. Di-potassium uranyl nitrate — monoclinic. 3. Mono-ammonium uranyl nitrate — rhombic. 4. Di-ammonium uranyl nitrate — mono- clinic. The bands occupy their natural positions in the left-hand panel, but have their strongest bands in vertical alignment in the right-hand panel. THE NITRATES AND PHOSPHATES. 139 That the crystal system to which a salt belongs is an important factor in determining the position of the bands can be seen in figure 80. In the left-hand panel a single group is shown in its natural position; in the right-hand panel the strongest bands of each group are placed in the same vertical line, to show the resemblance in grouping. This similarity is probably due to the fact that all four belong to the same chemical family. If we compare this grouping with that of the uranyl nitrate spectra in section i we find little resemblance, hence the grouping is probably characteristic of the double uranyl-nitrate family. In the left-hand panel it will be seen that the second and fourth groups occupy almost identical positions, while the first and third occupy positions which differ from one another and from the second or fourth. As has previously been stated, the second and fourth groups belong TABLE 49. — Series of the fluorescence spectrum of di-potassium uranyl nitrate. l/X A(1/X) iA A(1/X) l/X A(1/X) •I 1775.2 1861.9 1948.9 2034.6 86.7 87.0 85.7 F 1723.8? 1808.6 1894.1 1980.3 2068.8 84.8 85.5 86.2 88.5 I' 1651.5 1737.6 1823.8 1911.2 1998.5 86.1 86.2 87.4 87.3 07 n 1621.3 O7 7 2085.5 Ql . U D, 1708.0 1793.7 1880.0 1966.9 2053.7 0* . / 85.7 86.3 86.9 86.8 G 1554.0 1640.4 1727.6 1813.2 1899.0 86.4 87.2 85.6 85.8 1 1831.8 1919.5 2007.0 87.7 87.5 R7 9 1986.2 Of ,1—1 1663.3 QQ C E- 1631.6 1717.9 1802.7 1889.3 1975.8 2062.2 86.3 84.8 86.6 86.5 86.4 H /1903.7 \1906.8 /1989.7 \1993.8 /2075.8 86.5 86.6 K 1751.8 1837.8 1925 . 1 2011.9 1670.8 1757.8 oo.O 86.0 87.3 86.8 87.0 86 . 8 \2080.7 L 1844.6 Rfi c 1931.4 Q 2018.3 O\J , »7 Series in the absorption spectrum of di-potassium uranyl nitrate. 1A A(1/X) i/x A(1/X) l/X A(1/X) 4 2196.6 2269.3 72.7 •i 2152.6 2224.3 71.7 ,{ 2105.9 2179.9 74.9 -{ 2210.9 2285 . 7 74.8 J 2169.2 2240.1 2310.9 70.9 70.8 n»r -( 2253.4 2325.1 2396.9 71.7 71.8 /( 2369.4 7c n [ 2382.6 .7 \ 2444.4 / o . u f 2361.6 76 2 *i 2437.8 *7 CT A / o 4 1 2513.2 140 FLUORESCENCE OF THE URANYL SALTS. to the monoclinic crystal systems, the first to the trigonal and the third to the rhombic system. Since all four spectra vary slightly in their frequency intervals, the relative positions would change slightly if we compared homologous groups in the other end of the spectrum, but this gradual and slight shifting would not change the general condition, which indicates that the absolute position of a group is largely deter- mined by the crystal system. This is not entirely new, as the four triclinic crystals of the double uranyl chlorides exhibit spectra which are as nearly coincident as could be expected of salts which vary in molecular weight. TABLE 50. — Average intervals. Mono-ammonium uranyl nitrate. Fluorescence series . Absorption seiies Ratio of fluorescence to absorption . A 86.6 a 73.7 1.18 D 88.3 d 74.5 1.19 G 87.7 g 74.2 1.18 I 88.1 i 75.1 1.18 Di-ammonium uranyl nitrate. Fluorescence series Absorption series. Ratio of fluorescence to absorp- tion . . A 84.4 a 69.2 1.22 B 84.4 b 71.6 1.18 C 84.3 c 69.8 1.21 D 84.5 d 68.9 1.23 E 85.0 e 68.7 1.22 G 83.7 a 68.8 1.22 I 83.9 i 69.6 1.21 J 83.8 3 69.7 1.20 K 84.8 k 70.9 1.20 L 84.0 I 71.5 1.18 Mono-potassium uranyl nitrate. Fluorescence series. Absorption series Ratio of fluorescence to absorption. D 86.9 d 73.2 1.18 I 87.2 i 71.9 1.16 K 86.6 fc 74.1 1.17 Di-potassium uranyl nitrate. Fluorescence series . Absorption series Ratio of fluorescence to absorption . D 86.6 d 72.7 1.19 E 86.2 e 74.8 1.16 F 87.2 75.0 1.16 H 86.5 h 71.7 1.21 K 86.9 k 71.3 1.22 L 86.9 I 74.9 1.16 THE NITRATES AND PHOSPHATES. 141 Again, in the case of the uranyl nitrate, the crystals of the hexa- hydrate are of the rhombic system, while those of the trihydrate and dihydrate are of the triclinic system. In spite of slight shifts due to changing molecular weight, the strong bands of the two spectra pro- duced by the crystals of the triclinic system agree fairly well, while the strong bands of the spectrum produced by the rhombic crystal reside in entirely different positions. There is one more bit of evidence which adds weight to the above view. The chemical formulae of the two potassium salts are more nearly alike than those of the two ammonium salts, since the di- ammonium salt has 2 molecules of water of crystallization, while the other salts have none, yet there is a greater difference between the first and second spectra than there is between the second and fourth spectra. SUMMARY OF SECTION II. (1) The spectra of the double uranyl nitrates resemble those of the previously studied uranyl salts in that the bands can be arranged in series having constant frequency intervals. (2) These intervals, while constant for any given series, are different for different series. (3) In the mono-ammonium uranyl nitrate and the mono-potassium uranyl nitrate the ratio of the interval of a fluorescence series to the interval of the absorption series which joins that fluorescence series is approximately a constant. (4) Although the grouping of the bands shows a strong family resemblance in the four spectra, yet the absolute position of a group is largely determined by the crystal system. III. RESOLUTION ON COOLING AND ITS DEPENDENCE ON CRYSTALLINE STRUCTURE. The crystal system of any uranyl compound is an important factor in determining the character of its fluorescence and absorption spectra, as we have endeavored to show in the foregoing section. There is equally good evidence that resolution is dependent on the existence of a crystalline condition. TABLE 51. — Bands of fluorescence in canary glass.1 At +20° C. At -185° C. X 1/XX103. X 1/XX103. 5280 5180 1894 1931 5330 5140 1876 1946 1 R. C. Gibba, Physical Review (1), vol. 30, p. 382. 142 FLUORESCENCE OF THE URANYL SALTS. Not all uranyl fluorescence spectra are well resolved on cooling. In the case of a piece of canary glass, for example, the rather unusually broad, vague doublet occurs at +20° (see table 51) . At the temperature of liquid air the doublet is partially resolved, but no narrow components appear. The solid solution of uranyl phosphate in microcosmic salt, the phosphorescence of which has already been described in Chapter IV, yields a narrowing of the bands on cooling and a shift, but no resolution. (See table 52.) TABLE 52. — Bands of uranyl phosphate in microcosmic salt.1 At +20° C. At -185° C. X 1/XX103. X 1/XX103. 5670 1764 5680 1761 5421 1845 5430 1842 5183 1929 5190 1927 4970 2012 4980 2008 1 The bands are 160 A. u. in width. The inference that the failure to obtain resolution of the bands is due to the non-crystalline structure of the substance is confirmed by the observations described below. EXPERIMENTS ON THE SPECTRA OF SODIUM URANYL PHOSPHATES.1 Stokes,2 in an early paper on the ultra-violet spark spectra of the metals, described a fluorescent screen prepared by treating the ordi- nary uranyl phosphate with a solution of phosphoric acid and sodium or ammonium phosphate. While the uranyl phosphate is only feebly fluorescent, the double salts thus produced were very brilliant. To investigate the fluorescence spectra of these double phosphates, the following preparations were made : (1) A mixture of uranyl phosphate and sodium phosphate in the ratio of 4 molecular weights of HU02P04.3|H20 to 1 molecular weight of HUo2P04. (2) A similar mixture in proportions 2 to 1. (3) A similar mixture in proportions 1 to 1. These three specimens, when cooled to —180° C. and excited by radiation from the carbon arc, yielded precisely similar and well- resolved spectra. (See fig. 81, 1, 2, and 8.} In addition to the above, four further specimens were made by mixing increasing amounts of phosphoric acid with sodium uranyl phosphate, i. e.: (4) One molecule of phosphoric acid to 2 molecules of uranyl phosphate and 1 molecule of sodium phosphate, giving the composition H3NaUO2(P04)2 This was a powder, similar to preparations 1, 2, and 3. 1 Howes and Wilber, Physical Review (2), vn, p. 394. 1916. 2 Stokes, Philos. Trans., 152, p. 599. 1862. THE NITRATES AND PHOSPHATES. 143 (5) One molecule of phosphoric acid to 1 molecule of uranyl phosphate and 2 molecules of sodium phosphate. When dried, this contained much free sodium phosphate. (6) Two molecules of phosphoric acid to 1 molecule of uranyl phosphate and 1 molecule of sodium phosphate. This specimen did not dry, but remained syrupy at room temperature and appeared to be vitreous at — 180°. (7) A solution of uranyl phosphate in a considerable excess of syrupy phosphoric acid. This gave a glass-like mass even at +20°. The fluorescence spectra of these 7 substances are plotted to the usual frequency scale in figure 81 . Table 53 gives the location of the narrow bands, and approximately of the crests of the broad, unresolved groups; also the frequencies and frequency intervals. It will be seen that the spectra of 1, 2, and 3 consist of recurring groups of narrow bands and that homologous members of these groups TABLE 53. — Wave-lengths and frequencies of the line series of the fluorescence of the sodium uranyl phosphates. X 1/X Al/X X 1/X Al/X Series A 5640 1773.1 Series F 6015 1662.4 (very dim) 5396 5171 1853.1 1933.8 80.0 80.7 (dim) 5739 5484 C94Q 1742.6 1823.4 IQO^ 3 80.2 80.8 81.9 Average 80.4 5034 1986.3 81.0 Series B 6153 1625.2 Average. . . . 81.0 fj:rr.\ t;»RO 1 7n^ 8 80.6 5599 1785.9 80.1 Series G 5460 1831.6 5359 5136 1865.9 1947.0 80.0 81.1 (dim) 5227 5014 1913.0 1994.6 81.4 81.6 Average 80 5 Average .... 81.5 Series C 6114 1635 7 Series H .... 5429 1841 8 (very dim) 5827 5568 HQO7 1716.0 1796.0 1 877 1 80.3 80.0 81.3 (very dim) 5199 4989 1923.4 2004.6 81.6 81.2 Average 81 4 80 5 stcCorioa "D/ fil AQ i A9i n Series D 6075 1646 0 (medium). 5877 1701 5 80.5 (medium) 5794 5538 5299 1726.0 1805.6 1887.1 80.0 79.6 81.5 5607 5363 5136 1783.5 1864.6 1947.0 82.0 81.1 82.4 Average 80 4 Average Series E 6350 1574 9 (very strong) 6040 5760 5506 5270 5057 1655.7 1736.2 1816.2 1897.4 1977.6 80.8 80.5 80.0 81.2 80.2 Average 80 5 * Series B' is found in spectrum No. 5 only. 144 FLUORESCENCE OF THE URANYL SALTS. TABLE 53. — Wave-lengths and frequencies of the broad-band series of the fluorescence of the sodium uranyl phosphates. X 1/X Al/X X 1/X Al/X Spectrum No. 4 .... 6219 5909 5641 5390 c;i CLQ 1608.0 1692.6 1772.7 1855.3 locje 7 83.6 80.1 82.6 83.4 Spectrum No. 6 .... 5932 5644 5388 5147 1685.8 1771.8 1856.0 1942.9 86.0 84.2 86.9 4948 2021.0 82.3 Average 85.3 Average 82.6 Spectrum No. 7 .... 5958 1678.4 tifiRI 1 7AR c; 88.1 Spectrum No. 5 .... 5927 5647 5398 nil 74 1687.2 1770.9 1852.5 1 Q*}9 7 83.7 81.6 80.2 5400 5157 4935 1851.9 1939.1 2026.4 85.4 87.2 87.3 4956 2017.8 85.1 Average 87.0 Average 82.7 form the usual constant-interval series. The interval, which is the same for all within the errors of observation, is the shortest yet observed in the study of the fluorescence of the uranyl salts. Position as well as the arrangement of the bands is identical, and it is highly probable that we have to do with the same crystalline fluorescent compounds in these three preparations. The broad bands of specimens 4 and 5 form series with a constant interval of 82.5 units. Evidently the increase in the proportion of phosphoric acid tends to suppress the strongest line series and merge the dimmer series into broad bands. With the increas- ing predominance of the broad bands, caused by the increasingly larger pro- portion of acid present, there is a si- multaneous increase in interval from 82.5 units to 85.1 units for specimen No. 6, and 87.0 units for specimen No. 7. Experiments similar to the above were made in which ammonium phos- phate was substituted for sodium phos- phate. The results were in all respects analogous to those above described. That in general the resolution of the uranyl spectra by cooling occurs only when the fluorescing substance is in crystalline form is further substantiated by numerous experiments on frozen solutions to be described in detail in Chapter X. 1. 1 II , III , Mil ii i 1 1 ll 1 ll 2. III! 1 .III , ,1,1 ,,,,1,1 III. 1, III 3. II 1 1 ll.l ,Ml! In. 1, ill 4. 1,1 ,Ai 1 /Ill J\ „ ll 5. f\ M ,/] 6. / i V A. A L \ / i V s i V y \ | I7|00 i leloc 1 i 19 00 THE NITRATES AND PHOSPHATES. 145 SUMMARY OF SECTION III. (1) Where uranyl compounds occur in solid solution, as in canary glass, or in a bead of microcosmic salt, the banded fluorescence spec- trum with constant frequency intervals, as observed at +20° C., is not further resolved into groups of narrow, line-like bands by cooling to the temperature of liquid air. (2) Sodium uranyl phosphate or ammonium uranyl phosphate, when prepared in the form of crystalline powder, gives fluorescence spectra which are fully resolved at low temperatures. (3) In the presence of an excess of phosphoric acid, where the above compounds, or uranyl phosphate, form solid solutions of vitreous structure, resolution does not occur on cooling. (4) There is reason to think that the dependence of resolution by cooling upon the existence of crystalline structure applies in general to the fluorescence of the uranyl salts. VIII. THE ACETATES. The uranyl acetates afford a broader field for investigation than the chlorides or nitrates, the spectra of which have been considered in previous chapters. In addition to two forms of the single acetate UO2(C2H302)2, we have the double salts of all the alkali metals except caesium; the double salts of calcium, barium, strontium, magnesium, zinc, lead, silver, and manganese; the triple salt NaMg UO2(C2H302)5. In the fluorescence spectra of the acetates, as in the case of all uranyl salts thus far studied, the broader bands observed at room temperature are resolved into groups when the substance is excited at the temperature of liquid air, and the constitution of these groups, which repeat themselves at regular intervals from the red to the region in the blue, where absorption begins to replace fluorescence, is very similar in the acetates to that of the groups in the spectra of the com- pounds already discussed. THE SINGLE ACETATE. Two distinct varieties of this salt were available for observation — the finely powdered anhydrous form, U02(C2H302)2, and the crystalline form, U02(C2H302)2.H2O. The spectra of the two are very similar in appearance; each being characterized by two strong, well-defined series forming a set of doub- lets. They are easily distinguished, however, by the widely different location of the doublets. In the spectrum of the anhydrous variety these occur near the group centers of the alkaline double salts, whereas in the crystalline form they fall nearly midway between these groups. The strong series of the crystalline salt, which we have denoted as E and F, frequently appear in greatly reduced intensity in the spectra of the double salts, due doubtless to the presence of traces of the single acetate. The strong doublets C and D of the anhydrous acetate, if they ever appear in the spectra of the double salts, would be more difficult to detect, as they would overlap bands in the groups of the latter. Wave-lengths and frequencies of these two forms of uranyl acetate are given in tables 54 and 55. Intensities are designated as very strong (vs), strong (s), medium (m), dim (d), very dim (vd), and very very dim (wd). STUDIES OF A SINGLE GROUP. Since the acetates, like the chlorides and nitrates discussed in pre- vious chapters, have spectra consisting of similar recurring groups, it is convenient and sufficient, in the study of the structure of the ensemble of the fluorescence, to consider a single group. For this 146 THE ACETATES. 147 TABLE 54. — Fluorescence bands in spectrum of uranyl acetate (anhydrous), —185°. Group. Series. M 1/MX103 Int. Group. Series. M i/Vxio3 Int. f C" 0 . 6037 1656.4 m. A 0.5295 1888.5 d. I o ! D .6011 1663.5 m. B .5273 1896.6 d. E .5975 1673.6 vd. C .5229 1912.5 d. 1 F .5950 1680.6 vd. C' .5223 1914.4 s. 6- D .5202 1922.4 s. A .5822 1717.5 d. E .5183 1929.4 vd. B .5799 1724.3 d. F .5161 1937.5 m. C .5749 1739.4 d. G .5117 1954.4 vd. 4- C' .5739 1742.6 m. H .5088 1965.5 vd. D .5713 1750.5 m. E .5687 1758.5 vd. A .5062 1975.4 vd. F .5663 1765.7 vd. B .5041 1983.6 vd. C' .5006 1997.4 d. A .5548 1802.6 d. 7, 1 D .4979 2008.6 s. B .5523 1810.5 d. E .4961 2015.6 vd. C' .5481 1824.6 d. F .4940 2034.3 d. C .5469 1828.5 s. 5- D .5445 1836.4 s. E .5424 1843 . 6 d. F .5401 1851.5 d. G .5352 1868.5 vd. H .5318 1880.5 vd. TABLE 55. — Fluorescence bands in spectrum of uranyl acetate [UO-i(C-iHzO-t)O}, at +185° C. Group. Series. M 1/MX103 Int. Group. Series. M i/Vxio3 Int. J E 0.6158 1623.9 d. A 0.5166 1935.7 vd. 3\ F .6133 1630.5 m. B .5149 1942.3 d. C .5130 1949.2 d. f E .5860 1706.6 m. C' .5122 1952.0 vd. 4 E' .5849 1709 . 8 m. D .5107 1958.0 vd. I F .5825 1716.8 s. 7, Ei .5096 1962.3 d. i ' E .5090 1964.6 m. B .5648 1770.5 vd. Fi .5075 1970.4 s. C' .5630 1776.1 vd. F .5067 1973.5 s. Ei .5583 1791.2 vd. F' .5059 1976.7 vd. E .5575 1793.6 m. G .5027 1989.1 vd. 5- F .5550 1801.9 s. H .5005 1998.2 vd. F" .5529 1808.8 vd. G .5504 1816.9 vd. A .4962 2015.3 d. H .5473 1827.0 vd. B .4930 2028.5 m. I .5442 1837.6 vd. C .4913 2035.5 m. C' .4904 2039 . 2 d. A! .5416 1846.4 vd. S' D .4892 2044 . 1 d. B .5385 1857.0 d. Fi .4863 2056.3 m. C .5367 1863.3 d. F .4857 2058.7 m. C' .5357 1866.7 vd. F' .4848 2062.7 g. D .5342 1872.1 vd. G .4823 2073.2 vd. 6 Ei .5329 1876.5 d. E' .5322 1878.9 m. F .5305 1885.0 vd. F' .5300 1886.7 s. F' .5289 1890.7 vd. G .5258 1901.9 vd. H .5231 1911.7 vd. 148 FLUORESCENCE OF THE URANYL SALTS. URANYL ACCTATE purpose group 7, which is in the brightest part of the spectrum and is free from the complications due to the overlapping of fluorescence and absorption in the reversing region, is most favorable. In figure 82 the spectral region of this group is plotted for the anhydrous and crystalline forms of the single acetate to depict the remarkable dis- placements brought about by the presence of water of crystallization and the consequent modifi- cation of crystal structure. The effect is very similar, both as regards the direction of the shift of the groups and the amount of shift, to that already described in the case of the nitrates. (Compare fig. 78 in Chapter VII.) In both instances it is not the transfer of the groups toward the blue without change in their structure that p 82 occurs, but something much less obvious. In fact, it is not possible to identify any of the bands in the spectra of the hydrated form with those belonging to the an- hydrous salt. To produce this change in the spectrum it is only necessary to add a drop of water to a small portion of the anhydrous powder and to compare the fluorescence of the dry and moistened substance when excited in the usual way at — 185°. FREQUENCY INTERVALS OF THE SINGLE ACETATES. The frequencies and frequency intervals of the series occurring in the spectra of the two forms of the single acetates are given in tables 56 and 57, from which it will be seen that the two forms of the acetate TABLE 56. — Uranyl acetate (anhydrous). Anhudroos C 0 F A B 1 1 1 ^ G M 1 I III Crystalline F B C O G lii! 1 1 1 | ,1:1. 1 i 1900 1950 Series. Group 3. Group 4. Group 5. Group 6. Group 7. Average interval. A 1717.5 1802.6 1888.5 1975.4 85.97 B 1724.3 1810.5 1896.6 1983.6 86.40 Ci 1739.4 1824.6 1912.5 1997.4 86.00 C 1656.4 1742.6 1828.5 1914.4 2000.6 86.00 D 1663.5 1750.5 1836.4 1922.4 2008.6 86.27 E 1673.6 1758.5 1843.6 1929.4 2015.6 85.50 F 1680.6 1765.7 1851.5 1937.5 2024. 3 85.91 G 1868.5 1954.4 85.90 H 1880.5 1965.5 85.00 General average 85.96 THE ACETATES. 149 appear to have the same interval. The difference between the weighted averages is much less than the uncertainties in the determination of the intervals of the dim bands of the weaker series. TABLE 57. — Uranyl acetate (crystalline; 2HiO). Series. Group 3. Group 4. Group 5. Group 6. Group 7. Group 8. Average interval. A2 2015 3 Ai 1846.4 A 1935.7 B 1770.5 1857.0 1942.3 2028 . 5 86 0 C 1776.1 1863 3 1949 2 2035.5 86 45 C' 1866.7 1952 5 2039 2 86 25 D 1872 1 1958 0 2044 1 86 0 Ei 1706.6 1791 2 1876 5 1962 3 85 23 E 1623 9 1709.8 1793 6 1878 9 1964 6 85 18 F! 1885.0 1970.4 2056 . 3 85 65 F 1630.5 1716.8 1801.9 1886.7 1973 5 2058 8 85 66 F' 1890 7 1976.7 2062 7 86 00 F" 1808 8 G 1816 9 1901 9 1989 1 2073 2 85 42 H 1827 0 1911 7 1998 2 85 60 I 1837 6 General average .... 85 72 THE DOUBLE ACETATES. The fluorescence spectra of these salts have as a rule lower frequency intervals than the two forms of single acetate. The average interval is below 85, as compared with 85.7 for U02(C2H302)22H20 and 85.9 for the anhydrous single acetate. The group structure is in general less symmetrical than that of the double chlorides or the double nitrates and precise comparisons are therefore more difficult. Corresponding groups in the majority of cases, however, occupy very nearly the same position in the spectrum, and the system of designating the various bands employed in the discussion of the chlorides and nitrates has been used. If we neglect some of the weaker outlying bands, the group structure of several of the double acetates is found to consist of 4 nearly equi- distant bands the wave-length of which is almost if not quite inde- pendent of the metal which enters into the composition of the double salt. The substances which most nearly conform to this type are the double acetates containing lithium, potassium, calcium, and strontium. Manganese uranyl acetate differs from these only in the absence of band B in some groups. (See fig. 83.) In the spectrum of the barium double acetate the groups are shifted bodily toward the red about 5 frequency units. In the spectra of the ammonium and rubidium salts band D is doubled. 150 FLUORESCENCE OF THE URANYL SALTS. The double acetates of sodium, magnesium, zinc, silver, and lead (fig. 84, a) are made up of groups which, while they overlap, are by no means identical, either as to the location or arrangement of their bands. The spectra of these 5 salts agree, however, in this: They contain in each group 5 bands which correspond so closely with the bands B, C, D, E, and F of the double acetates depicted in figure 83 that by a bodily shift of the group as a whole they may be made to conform to the uniform arrangement, so far as those bands are concerned, quite as well as do the latter. This may be seen from figure 84 6, in which the dotted vertical lines indicate the positions of the bands in the uni- form type, while the group in each case has been shifted so as to register approximately. DOUBLE ACETATCS DOUVLC ACCTATCA D ' ' r Li C H 1 K 1 Co 1 Mn 1 Sr 1 1900 l«0 FIG. 83. No. 1 1 No, 1 C 0 't r | | II 1 1 1 M< 1 Mq 1 1 1 1 Zn Zn I 1 1 II 1 1 1 1 1 A. 1 I 1 1 I Pb Pb | | JL FIG. 84a. FIG. 846. The distinction between the spectra under discussion and those previously considered, which were described as having group spectra conforming to an essentially uniform type both as to location of bands and group-structure, is twofold : (a) there is a shift of the groups as a whole; (6) there are additional series, varying in intensity, some of which are among the strongest in the spectra and which are charac- teristic of the individual salt. It should be reiterated in this connection that neither the bands B, C, D, E, and F, which, although sometimes uniformly shifted, are common to the spectra of the double acetates, nor the additional bands, are found in the spectra of the single acetates. The spectrum of neither the anhydrous acetate nor the crystalline form can be made to conform to the uniform type by a general shift. THE ACETATES. 151 A POSSIBLE RELATION TO THE METALLIC SPARK SPECTRA. It appears from the foregoing that any metal capable of forming a double uranyl acetate modifies the constitution of the fluorescence spectrum both as to the composition of the groups and their location. Certain metals, such as lithium, potas- sium, calcium, manganese, and strontium, produce one and the same modification, irrespective of the metal which is present. Other metals shift the group slightly (e. g., barium) or vary slightly the relative distances between neighboring bands without otherwise changing the struc- ture of the group. The presence of still other metals, such as sodium, magnesium, zinc, silver, and lead, results in a considerable general shift and the introduction of new series into the spectrum characteristic of the partic- ular metal in question and existing only in the doublet salt of which it forms a part. Some of these groups are much more complex than the uniform type depicted in figure 83. The others are accompanied by strong bands or minor groups of bands lying outside the usual boundaries. One might imagine, to account for this type of spec- trum, that in addition to the metal in combination as a part of the double salt, there are in solution certain other radiators. If these are uncombined particles of the metal existing in a condition akin to the gaseous state, one might expect a type of radiation, under exci- tation, similar to that discovered by Wood1 in sodium- vapor; i. e., series of constant frequency made up of bands instead of lines because of damping. One member of each such series should coincide or nearly coincide with some line in the arc or spark spectrum of the metal. Now, there are in fact various coincidences or approxi- mations thereto close enough to bring lines of the emission spectrum well within the brighter portion of one of the fluorescence bands in question. In the spec- trum of silver uranyl acetate, for example, there is a strong series which does not coincide with any series in the fluorescence spectra of the other acetates thus far observed. One member of this series coincides with the brightest visible line in the spark spectrum of silver (Haschek 0.54655 /JL; frequency number 1,829.6). Our reading of the corresponding band, made before we had any suspicion of the possible relation here suggested, was 1,829.8. The rather bright line (0.51838) and the neigh- boring doublet (0.51729-0.51675) in the spark spectrum of magnesium correspond similarly to bands 1,928.9 and 1,934.4 of the fluorescence spectrum. FIG. 85. 1 R. W. Wood. Physical Review (2), xi, p. 76. 152 FLUORESCENCE OF THE URANYL SALTS. In the spark spectrum of lead, of the 9 lines listed by Haschek which lie in the fluorescence region, 7 are within one frequency unit of our readings of the corresponding bands; 4 of these are in practically perfect coincidence, the departures from the crests of the bands being only one or two tenths of a unit. Of the 25 spark lines of zinc within the fluorescence region, 15 are certainly not related to fluorescence in the manner here under con- sideration, 4 in somewhat doubtful coincidence, and 6 are in close approximation. Of these last, 5 are consecutive lines of the spark spectrum, all of which are in group 7 of our fluorescence system. The evidence of any significant relation based upon these coincidences is obviously far from conclusive. The matter is mentioned here solely in view of possible developments in the further study of the connection between fluorescence and temperature radiation. The search for possible coincidences in the case of sodium led to the discovery of a striking arrangement, which seems to be peculiar to that element. The doublets and triplets of the spark spectrum, while they do not form constant-frequency series, are so located that they could be excited to radiation of the type described by Wood, with a common interval equal to the fluorescence interval of the acetates; i. e., about 85, the result would be a well-defined group spectrum of the type of the fluorescence spectrum of the uranyl salts. (See fig. 85.) There are, however, only two individual coincidences with bands of the sodium uranyl acetate. In the figure, the actual arc-lines of sodium are elongated. The shorter lines are derived from them by assuming constant-frequency series having the interval 85, as described above, FLUORESCENCE SERIES IN THE SPECTRA OF THE DOUBLE ACETATES. In tables 58 to 70 the fluorescence bands in the spectrum of the double salts are arranged in the order of their wave-length. In tables 71 to 83 the frequencies and average intervals of each series in the various salts are given. It will be seen by comparison with tables 56 and 57 that the average interval for the double acetates is less by more than one frequency unit than for the single acetates ; also that the av- erage for the various double salts differ from the general average of all (table 84) by an amount no greater than the difference between the intervals of the various series present in the spectrum of a given salt. In brief, whatever real differences may exist are too small to be deter- mined from our data. THE ACETATES. 153 TABLE 58. — Lithium uranyl acetate. Group Group Group and M 1/juXlO3 Int. and M i/Vxio3 Int. and M 1/MX103 Int. series. series. series. fA 0.6105 1638.0 d. 'C 0.5481 1824.5 m. C 0.5016 1993.7 m. JG .6033 1657.4 vd. D .5451 1834.4 d. D .4987 2005.1 8. 31T? Hi .5963 1677.0 m. K., E .5423 1844.0 s. 7'E .4967 2013.1 ms. IH .5858 1707.0 vd. O F .5399 1852.0 m. F .4948 2021 . 1 ms. G .5363 1864.7 vd. H .4889 2045.6 d. c .5740 1742.1 d. H .5336 1874.1 vd. D .5711 1751.0 d. (C .4808 2080.0 ms. 4' E .5680 1760.5 m. C .5238 1909.0 m. 8 D .4784 2080.6 8. F .5653 1769.0 d. D .5209 1919.8 s. IF' .5751 2104.8 ms. H .5590 1789.0 vd. A< E .5184 1928.9 s. \JN F .5166 1935.8 m. G .5128 1950.2 vd. H .5102 1960.2 d. TABLE 59. — Sodium uranyl acetate. JG 0.6262 1597.0 d. B! 0.5510 1815.0 vd. 'B 0.5027 1989.2 vd. 2\I .6107 1637.5 s. B .5494 1820.2 vd. C .4998 2001.0 m. C .5468 1828.8 m. C' .4988 2004.9 d. Bi .6085 1643.5 m. C' .5452 1834.1 m. D .4972 2011.3 vs. B .6068 1648.0 d. D .5432 1840.9 m. D' .4965 2014.2 vs. C .6028 1659.0 d. D' .5422 1844.3 m. 7, E .4949 2020.8 8. C' .6007 1664.8 d. E .5404 1850.6 s. t F .4931 2028.0 8. D .5978 1672.8 d. 5- F .5383 1857.8 m. F' .4917 2033.6 vvd. 3 E .5948 1681.1 6. F' .5371 1862.0 vvd. G .4900 2040.9 d. F .5924 1688.0 m. Gi .5359 1866.0 vvd. G' .4891 2044.5 vd. G .5898 1695.5 vd. G .5345 1871.0 vd. H .4874 2051.5 m. G' .5875 1702.0 vvd. G' .5332 1875.5 vd. I .4844 2064.3 vd. H .5846 1710.5 d. H .5313 1882.0 m. ,1 .5805 1722.5 m. I .5283 1893.0 vd. B! .4821 2074 . 1 vd. ir .5269 1897.8 vd. C .4796 2085 . 1 8. B .5782 1729.5 d. C' .4787 2089.2 d. B' .5764 1735.0 d. B .5249 1905.0 vd. 8- D .4769 2097.0 s. C .5731 1744.9 m. B' .5242 1907.5 vvd. D' .4762 2100.0 8. C' .5717 1749.0 m. C .5221 1915.3 s. E .4734 2112.5 m. D .5693 1756.5 m. C' .5210 1919.3 m. G .4707 2124.5 vvd. D' .5687 1758.5 m. D .5191 1926.3 s. E .5662 1766.1 s. D' .5182 1929.9 m. 4 F .5642 1772.5 m. 6< E .5166 1935 . 6 vs. F' .5625 1777.8 vvd. F .5148 1942.5 vs. G! .5614 1781.4 vvd. G .5115 1955.0 vd. G .5599 1785.9 vd. G' .5099 1961.0 vd. G' .5590 1789.0 vd. H .5083 1967.5 m. H .5565 1796.9 m. I .5048 1980.8 d. . I .5529 1808.6 d. 154 FLUORESCENCE OF THE TJRANYL SALTS. TABLE 60. — Magnesium uranyl acetate. Group and series. M i/Vxio3 Int. Group and series. M 1/MX103 Int. Group and series. M 1/AiXlO3 Int. 2 I 0.6147 1627.0 m. 'A 0.5537 1806.1 d. A 0.5063 1975.1 vvd. B .5507 1815.9 d. B .5036 1985.8 m. A .6115 1635.4 m. C .5475 1826.5 m. C .5007 1997.1 m. B .6072 1647.0 vd. R< D .5447 1836.0 vs. 7, D .4983 2006.8 m. D .6002 1666.1 m. *J E .5423 1844.0 s. / > E .4966 2013.8 a. 3< E .5971 1674.8 d. F .5408 1879.0 vvd. F .4951 2019.6 vd. G .5926 1687.6 vd. G .5382 1858.0 d. G .4929 2028.7 d. H .5897 1695.7 vd. H .5358 1866.5 d. H .4908 2037 . 4 d. I .5831 1715.0 m. A .5289 1890.8 vd. B .4828 2071.4 m. rA .5810 1721.2 d. B .5261 1900.9 d. C .4802 2082.6 m. B .5774 1731.8 vd. C .5230 1912.0 m. Di .4791 2087.1 m. D .5708 1751.8 s. ft, D .5206 1920.9 s. 8< E .4764 2099.0 m. 4 E .5684 1759.2 m. \J i E .5184 1928.9 d. F .4754 2103.6 d. F .5672 1763.2 vd. F .5170 1934.4 vd. G .4733 2112.6 d. G .5640 1773 . 1 vd. G .5145 1943.8 d. H .4707 2124.5 d. H .5613 1781.6 d. H .5128 1950.1 d. I .5556 1799.8 d. TABLE 61. — Ammonium uranyl acetate. A 0.6097 1640.2 m. 'B 0.5520 1811.5 d. 'E 0.5048 1981.0 d. C .6044 1654.5 vd. C .5484 1823.5 m. C .5020 1992.0 m. 3< E .5970 1675.5 m. D' .5453 1834.0 m. D' .4994 2002 . 5 s. F .5945 1682.0 d. 5 E .5424 1843.5 s. 7 D" .4983 2007.0 8. I .5867 1704.5 1 F .5404 1850.5 m. < E .4970 2012.0 9. G .5379 1859.2 vd. F .4953 2019.0 9. 'B .5789 1727.5 vd. H .5360 1865.5 vd. G .4929 2029.0 d. C .5749 1739.3 d. I .5338 1873.5 d. J .4897 2042.0 d. D' .5709 1751.5 d. 4- E .5681 1760.3 d. 'B .5271 1897.0 vd. 'C .4816 2076.5 m. F .5661 1766.5 m. C' .5242 1907.5 m. 8, D' .4787 2089.0 vs. H .5613 1781.5 d. D' .5212 1918.5 m. D" .4778 2093 . 0 vs. I .5589 1789.2 vd. D" .5200 1923.0 m. ,F .4756 2102.5 m. 6 E .5189 1927.0 s. F .5171 1934.0 m. G .5142 1944.8 vd. H .5127 1950.5 vd. J .5105 1958.9 d. TABLE 62. — Potassium uranyl acetate. 'A 0.6009 1641.2 m. 'B 0.5512 1814.2 vd. f& 0.5041 1983.9 vvd. C .5975 1673.7 d. C .5484 1823.4 m. C .5016 1993.8 3. 3 E .5958 1673.5 m. D .5444 1837.0 m. D .4986 2005.7 a. F .5936 1684.7 d. 5< E .5420 1845.0 s. 7, E .4967 2013.3 3. H .5858 1707.8 vd. F .5399 1852.2 m. t F .4948 2021.0 VS. H . 5330 1876.0 d. G .4923 2031.3 d. 'C .5738 1742.8 d. G' 2036 . 7 d. D .5701 1754.0 d. rB .5262 1900.3 vd. ^H .4891 2044.7 d. E .5677 1761.5 m. C .5237 1909.3 m. 4< F .5652 1769.1 d. D .5202 1922.3 m. 'C .4811 2078.9 3. G' .5614 1781.3 vd. fi< E .5182 1929.6 s. s< D .4781 2091.5 S. H .5582 1791.3 d. \J i F .5163 1937.0 s. F .4749 2105.9 8. G .5138 1946.3 vd. .G .4724 2116.8 vd. G' .5121 1952.7 vd. IH .5101 1960.4 d. THE ACETATES. 155 TABLE 63. — Calcium uranyl acetate. Group and series. M 1/MX103 Int. Group and series. M 1/MX103 Int. Group and series. M i/Vxio3 Int. c 0.6017 1662.0 d. 'C' 0.5483 1823.8 d. C' 0.5017 1993 . 2 m. D .5987 1670.3 vvd. D .5451 1834.5 m. D .4990 2004 . 0 m. Q, E .5968 1675.6 d. K., E .5427 1842.6 s. E .4970 2012.1 s. O i F .5939 1683.8 vd. O F .5403 1850.8 m. 7^F .4953 2019.0 s. H .5869 1703.9 vd. G" .5361 1865.3 d. G" .4918 2033.3 d. .J .5821 1717.9 d. H .5339 1873.0 d. H .4895 2042.9 d. I .4852 2051.0 vvd. C .5740 1742.2 d. 'C .5240 1908.4 m. D .5703 1753.5 d. D .5212 1918.6 m. fC .4875 2076 . 8 m. 4, E .5682 1759.9 m. E .5190 1926.8 s. 8 ID .4790 2087.7 m. F .5659 1767.1 d. 6- F .5169 1934.6 m. F .4755 2103.0 8. G" .5612 1781.9 vd. G" .5130 1949.3 d. IG .4729 2114.6 vd. H .5590 1788.9 vd. H .5107 1958.1 d. ,K .5059 1976.7 wd. TABLE 64. — Manganese uranyl acetate. 4(D 0.5718 1748.9 vd. 'C 0.5240 1908.4 d. fC 0.4812 2078.1 m. \E .5692 1757.0 vd. D .5213 1918.4 m. s-m .4786 2089.4 m. 6- E .5187 1927.9 s. IF .4751 2104.8 m. 'C .5491 1821.0 vd. F .5168 1935.0 s. D .5454 1833.5 vd. H .5108 1957.9 d. 5JE .5428 1842.3 m. F .5408 1849.1 m. 'C .5018 1992.8 m. H .5339 1873.0 vd. D .4994 2002.6 s. 7, E .4971 2011.7 s. I F .4952 2019.4 s. G .4936 2025.9 vd. H .4895 2042.9 vd. TABLE 65. — Zinc uranyl acetate. 2 E 0.6288 1590.3 d. D 0.5460 1831.6 vd. A 0.5050 1980.3 vvd. D' .5449 1835.0 d. C .5026 1989.7 m. A .6234 1640.0 vd. E .5430 1841.6 s. C' .5011 1995.8 m. C .6045 1654.3 vd. E' .5420 1845.0 m. D .4998 2000.9 d. E' .5967 1675.8 m. 5' F .5411 1848.0 m. D' .4988 2004.9 s. 3 F .5942 1682.9 d. O F' .5400 1851.9 m. E .4977 2009 . 1 s. G .5921 1689.0 vd. G .5382 1858.0 vd. 7- E' .4965 2014.0 m. H .5893 1697.0 vvd. H .5362 1865.0 vd. F .4957 2017.2 s. T .5870 1703.6 vd. I .5342 1872.0 m. F' .4946 2021.9 s. J .5331 1875.9 d. G .4935 2026.3 vd. A .5800 1724.0 d. H .4918 2033.4 vd. B .5780 1730.1 vd. rA .5275 1895.7 vd. I .4899 2041.4 d. C' .5749 1739.3 d. C .5249 1905.0 d. J .4888 2045.9 d. E .5701 1754.0 d. C' .5238 1909.1 d. 4' E' .5685 1759.0 s. D .5219 1916.1 d. 'A .4843 2064.8 vd. F .5661 1766.6 m. D' .5209 1919.9 m. C .4819 2075.1 m. G .5640 1773.1 vd. 6< E .5193 1925.6 s. C' .4809 2079.6 m. H .5624 1780.6 vvd. E' .5182 1929.9 m. D .4795 2085.5 m. [I .5618 1788.0 d. F .5171 1934.0 m. 8< D' .4786 2089 . 3 s. F' .5160 1938.0 m. E .4776 2094 . 0 m. rA .5528 1809.0 d. G .5149 1942.0 vvd. F .4758 2101.9 s. B' B .5512 1814.1 vd. H .5131 1949.1 wd. ,Gi .4746 2107.0 d. C' .5482 1824.0 m. I .5110 1956.9 d. J .5099 1961.3 d. 156 FLUORESCENCE OF THE URANYL SALTS. TABLE 66. — Rubidium uranyl acetate. Group and series. M i/Vxio3 Int. Group and series. M 1/MX103 Int. Group and series. M 1//1X103 Int. [B 0.6093 1641.2 vd. rc 0.5489 1821.8 s. JH 0.5129 1949.7 vd. 3E .5977 1673.1 s. C' .5476 1826.2 vd. 6\I .5109 1957.3 m. 3 F .5955 1679.3 d. D .5458 1832.3 m. IH .5879 1701.0 wd. D' .5445 1836.5 d. C .5022 1991.2 8. 5- E .5428 1842.3 vs. D .4995 2002.0 8. 'A' .5811 1720.9 vd. F .5405 1850.0 s. D' .4984 2006.4 m. C .5753 1738.2 m. G .5377 1859.8 vd. E .4971 2011.7 s. C' .5739 1742.5 wd. H .5360 1865.7 vd. 7< F .4953 2019.0 s. D .5717 1749.2 d. I .5338 1873.4 s. G .4927 2029.6 vvd. A, D' .5703 1753.3 d. H .4916 2034 . 2 wd. *±\ E .5685 1759.0 vs. rc .5241 1908.0 8. I .4893 2043.7 m. F .5662 1766.2 s. C' .5229 1912.4 wd. G .5639 1773.4 wd. D .5214 1917.9 s. C .4813 2077.7 8. H .5618 1780.0 wd. 6- D' .5202 1922.0 m. D .4790 2087.7 B. I .5592 1788.3 m. E .5188 1927.5 vs. 8< D' .4780 2092.1 m. F .5170 1934.2 s. F .4751 2104.8 B. 5/A .5554 1800.6 d. G .5144 1944.0 vd. G .4730 2114.2 vd. IB .5525 1810.0 d. TABLE 67. — Strontium uranyl acetate. 2 I 0.6112 1636.1 d. 'C 0.5483 1823.9 m. 'C 0.5017 1993.2 m. D .5453 1834.0 vd. D .4990 2004.0 m. E .5974 1674.0 m. E .5427 1842.6 m. E .4969 2012.5 m. Q F .5951 1680.5 m. 5- F .5407 1849.4 m. 7' F .4950 2020.4 s. o H .5872 1703.0 d. G .5363 1864.5 d. G .4916 2034.0 vd. I .5812 1720.7 d. H .5334 1874.9 d. H .4888 2045.9 vd. I .5300 1886 . 8 vd. C .5752 1738.6 d. C .4812 2078.2 8. D .5721 1747.9 d. C .5239 1908.6 m. 8< D .4789 2088.1 m. 4< E .5687 1758.5 m. D .5211 1919.0 d. F .4751 2105.0 B. Tt i F .5663 1765.8 m. E .5187 1928.0 s. G .5615 1780.8 vd. 6< F .5169 1934.5 s. H .5589 1789.1 vd. G .5131 1949.0 d. H .5104 1959 . 1 d. I .5078 1969.4 d. TABLE 68. — Silver uranyl acetate. (E' 0.5979 1672.5 d. 'C 0.5500 1818.4 d. 'c 0.5027 1989.2 d. S]F .5961 1677.5 vd. D .5465 1829.7 m. D .5000 2000.2 s. IH .5878 1701.2 vd. E .5437 1839.4 s. E .4979 2008.6 m. 5' F .5417 1846.0 m. i F .4960 2016.1 8. A .5806 1722.5 vd. G .5379 1859.0 vd. G .4930 2028.6 vd. C .5764 1735.0 d. H .5345 1871.0 vd. LH .4902 2040.0 vd. D .5730 1745.1 d. 4- E .5699 1754.7 m. C .5250 1904.9 d. fC .4824 2073.0 d. F .5678 1761.1 d. D .5227 1913.2 m. 8-m .4796 2085.1 s. G' .5625 1777.7 vd. E .5198 1923.9 8. IF .4759 2101.5 m. ,H .5600 1785.7 vd. 6< F .5183 1931.0 m. G .5141 1945.1 vd. H .5113 1955.7 vd. THE ACETATES. 157 TABLE 69. — Barium uranyl acetate. Group Group Group and M 1//*X103 Int. and M 1/MX103 Int. and M 1/MX103 Int. series. series. series. 'C 0.6055 1651.5 vd. C 0.5498 1818.8 d. 'C 0.5025 1990.0 d. E .5993 1668.6 d. D .5471 1827.8 d. D .5000 2000.0 m. . F .5963 1677.0 d. E .5441 1837.9 s. JE .4978 2008.8 m. 3 G .5923 1688.3 vd. 54 F .5415 1846.7 s. 7 F .4959 2016.5 s. H .5888 1698.4 d. G .5387 1856.3 d. G .4933 2027.2 vd. H .5349 1869.5 d. H .4901 2024.4 d. C .5764 1734.9 vd. D .5733 1744.3 d. 'C .5250 1904.8 d. fC .4878 2075.5 m. 4< E .5700 1754.4 m. D .5226 1913.5 m. JD .4795 2085.5 8. F .5672 1763.0 m. E .5201 1922.7 8. 8 F .4759 2101.3 m. G .5638 1773.7 d. 6< F .5180 1930.5 S. [G .4731 2113.7 vd. H .5602 1785.1 d. G .5150 1941.7 vd. ^H .5116 1954.7 d. TABLE 70. — Lead uranyl acetate. 2 K 0.6202 1621.6 m. fc 0.5489 1821.9 m. A/1 0.5100 1960.8 vd. C' .5472 1827.5 m. 6\L .5089 1965.0 vd. B .6086 1643 . 1 vd. D .5459 1831.7 s. C .6048 1653.4 vd. E .5439 1838.7 s. B .5042 1983.5 m. D .6015 1662.5 m. E' .5423 1844.0 vd. C, .5028 1989.0 vd. E .5991 1669.1 m. 5 F .5410 1848.3 vd. C .5019 1992.4 s. 3< G .5938 1684.1 vd. G .5390 1855.1 d. C' .5008 1997.0 s. I .5909 1692.4 d. H .5369 1862.4 m. D .4996 2001.8 8. H' .5891 1697.5 vd. H' .5352 1868.5 d. 7- Et .4983 2006.8 wd. I .5859 1706.8 m. I .5333 1875.1 vd. E .4976 2009.6 vst. L .5841 1712.1 d. L .5320 1879.6 vd. F .4948 2021 . 1 vd. G .4936 2025.8 vd. B .5788 1727.7 d. B .5269 1897.8 m. H .4916 2034 . 0 m. C .5751 1738.7 d. Ci .5252 1904.0 vd. I .4886 2046.7 vd. C' .5741 1741.8 d. C .5242 1907.6 s. D .5722 1747.7 s. C' .5230 1911.9 s. rB .4837 2067.4 m. 4. E .5700 1754.3 s. D .5219 1916.2 vs. Ci .4824 ' 2073.0 vd. *i G .5649 1770.1 d. E! .5201 1922.7 vd. C .4814 2077.4 s. H .5621 1778.9 m. • E .5195 1925.0 vs. D .4790 2087.5 s. H' .5611 1782.3 d. E' .5186 1928.3 vd. 8< E .4772 2095.7 s. I .5591 1788.7 vd. F .5172 1935.0 vd. E' .4766 2098.2 wd. L .5567 1796.3 d. G .5149 1942 . 1 d. F .4749 2105.9 d. H .5130 1949.1 m. G .4737 2111.0 vd. s(B .5518 1812.1 m. H' .5119 1953.4 vd. &\d .5500 1818.2 vd. 158 FLUORESCENCE OF THE URANYL SALTS. TABLE 71. — Lithium uranyl acetate. Series. Group 2. Group 3. Group 4. Group 5. Group 6. Group 7. Group 8. Average interval. A 1638 0 c 1657.4 1742.1 1824.5 1909.0 1993.7 2080.0 84.52 D 1751.0 1834.4 1919.8 2005.1 2090.5 84.89 E 1677.0 1760.5 1844.0 1928.9 2013.1 84.03 F 1769.0 1852.0 1935.8 2021 . 1 84.05 F' 2104.8 G 1864.7 1950.2 85.50 H 1707 0 1789.0 1874 . 1 1960.2 2045 . 6 84.65 84.50 TABLE 72. — Sodium uranyl acetate. B, 1643 5 1729.5 1815.0 2074.1 86.12 B 1648 C 1735.0 1820.2 1905.0 1989.2 86.43 B' 1907.5 c 1659.0 1744.9 1828.8 1915.3 2001.0 2085 . 1 85.22 C' 1664 . 8 1749.0 1834 . 1 1919.3 2004.9 2089.2 84.88 D 1672.8 1756.5 1840.9 1926.3 2011.3 2097.0 84.84 D' 1758.5 1844.3 1929.9 2014.2 2100.0 85.37 E 1597 0 1681 1 1766.1 1850.6 1935.6 2020.8 84.76 F 1688.0 1772.5 1857.8 1942.5 2028.0 2112.5 84.90 F' 1777 8 1862.0 2033.6 85.27 Gi 1695 5 1781 4 1866 0 85.25 G 1702.0 1785.9 1871.0 1955.0 2040.9 2124.5 84.50 G' 1789.0 1875.5 1961.0 2044.5 85.17 H 1710.5 1796.9 1882.0 1967.5 2051.5 85.20 I 1637 5 1722 5 1808 6 1898.0 1980.8 2064.3 85.36 I' 1897 8 85.22 TABLE 73. — Magnesium uranyl acetate. A 1635 4 1721 2 1806.1 1890.8 1975 . 1 84.93 B 1647.0 1731.8 1815.9 1900.9 1985.8 2071.4 84.88 c 1826.5 1912 0 1997.1 2082 . 6 85.37 C' 2087.1 D 1666.1 1751.8 1836.0 1920.9 2006.8 85.18 E 1674.8 1759.2 1844.0 1928.9 2013.8 2099.0 84.84 F 1763.2 1849.0 1934.4 2019.6 2103.6 85.10 G 1687.6 1773 . 1 1858.0 1943.8 2028.7 2112.6 85.00 H 1695.7 1781.6 1866.5 1950.1 2037.4 2124.5 85.76 I 1627 0 1715 0 1799 8 86.40 85.19 TABLE 74. — Ammonium uranyl acetate. A 1640 2 B 1727.5 1811.5 1897.0 1981.0 84.50 C 1654.5 1739.3 1823.5 1907.5 1992.0 2076.5 84.40 D 1751.5 1834.0 1918.5 2002.5 2089.0 84.38 D" 1923.0 2007.0 2093.0 85.00 E 1675.5 1760.3 1843.5 1927.0 2012.0 84.13 F 1682.0 1766.5 1850.5 1934.0 2019.0 2102.5 84.10 G 1859.2 1944.8 2029.0 84.90 H 1781 5 1 865 . 5 1950.5 84.50 I 1704.5 1789.2 1873.5 1958.9 2042.0 84.38 84.40 • THE ACETATES. 159 TABLE 75. — Potassium uranyl acetate. Series. Group 3. Group 4. Group 5. Group 6. Group 7. Group 8. Average interval. A 1641.2 B 1814.2 1900.3 1983 . 9 84 83 C 1656.1 1742.8 1823.4 1909 . 3 1993.8 2078 9 84 56 D 1754.0 1837.0 1922.3 2005.7 2091 5 84 63 E 1673.5 1761.5 1845.0 1929 6 2013 3 84 95 F 1684.7 1769.1 1852.2 1937.0 2021 0 2105 9 84 24 G 1946 3 2031 3 2116 8 85 25 G' 1781 3 1845 0 1952 7 2036 7 85 13 H 1707.8 1791.3 1876.0 1960.4 2044.7 84.26 Gen. av. 84 57 TABLE 76. — Calcium uranyl acetate. Series. Group 3. Group 4. Group 5. Group 6. Group 7. Group 8. Average interval. C 1742.2 1823 8 1908 4 1993 2 2076 8 83 87 C' 1662.0 D 1670.3 1753.5 1834.5 1918.6 2004 0 2087 . 7 83 91 E 1675.6 1759 . 9 1842.6 1926.8 2012 1 84 12 F 1683 . 8 1767.1 1850 8 1934.6 2019 0 2103 0 83 85 G' 2114.6 G" 1781.9 1865.3 1949 . 3 2033 . 3 83 80 H 1703.9 1788.9 1873.0 1958 . 1 2042.9 84.66 I 2051.0 J 1717.9 Ki 1976.7 Gen. av . 83.88 TABLE 77. — Manganese uranyl acetate. Series. Group 3. Group 4. Group 5. Group 6. Group 7. Group 8. Average interval. C 1821 0 1908 4 1992 8 2078 1 85 70 D 1748 9 1833 5 1918 4 2002 6 2089 4 85 12 E 1757.0 1842.3 1927.9 2011.7 84.90 F 1849.1 1935 0 2019 4 2104 8 85 23 Gi 2025 9 H 1873 0 1957 9 2042 9 84 95 Gen. av. . 85.19 160 FLUORESCENCE OF THE URANYL SALTS. TABLE 78. — Zinc uranyl acetate. Series. Group 2. Group 3. Group 4. Group 5. Group 6. Group 7. Group 8. Average interval. A 1640.0 1724.0 1809 0 1895.7 1980.3 2064 . 8 84.96 B 1730 1 1814 1 84.00 c 1654.3 1905 0 1989.7 2075 . 1 84.16 C' 1739 5 1824 0 1909 1 1995.8 2079 . 6 85.03 D 1831 6 1916 1 2000 9 2085 5 84.48 D' '1835 0 1919 9 2004 9 2089 3 84.77 E 1590.3 1754 0 1841 6 1925.6 2009 . 1 2094 . 0 83.95 E' 1675.8 1759 0 1845 0 1929 9 2014.0 2101.9 85.22 F 1682.9 1766 6 1848 0 1934 0 2017.2 83.80 F' 1851 9 1938 0 2021.9 85.00 Gi 2107.0 G 1689 0 1773 1 1858 0 1942 0 2026 3 84.33 H 1697 0 1780 6 1865 0 1949 1 2033 4 84.20 I 1703.6 1788 0 1872 0 1956.9 2041.4 84.45 J 1875 9 1961 3 2045.9 85.00 Gen. av 84.51 TABLE 79. — Rubidium uranyl acetate. Series. Group 2. Group 3. Group 4. Group 5. Group 6. Group 7. Group 8. Average interval. A 1800 6 A' 1720 9 B 1641.2 1810 0 84.40 C 1738 2 1821 8 1908.0 1991.2 2077.7 84.88 C' 1742 5 1826 2 1912 4 84.45 D 1749.2 1832.3 1917.9 2002.0 2087.7 84.62 D' 1753.3 1836.5 1922.0 2006.4 2092 . 1 84.70 E .. 1673 1 1759 0 1842 3 1927 5 2011 7 84.65 F 1679 3 1766 2 1850 0 1934.2 2019.0 2104.8 85.10 G 1773.4 1859.8 1944.0 2029 . 6 2114.2 85.20 H 1780 0 1865 7 1949 7 2034 . 2 84.73 H'.. 1701 0 I 1788 3 1873 4 1957 3 2043 7 85.13 Gen. av .... 84.86 TABLE 80. — Strontium uranyl acetate. Series. Group 2. Group 3. Group 4. Group 5. Group 6. Group 7. Group 8. Average interval. C 1738 6 1823.9 1908.6 1993.2 2078 . 2 84.90 D 1749 9 1834 0 1919 0 2004 . 0 2088.1 85.05 E 1674 0 1758 5 1842 6 1928 0 2012.5 84.63 F 1680.5 1765.8 1849.4 1934.5 2020 . 4 2105.0 84.90 G. 1780 8 1864 5 1949 0 2034.0 84.40 Hi 1703 0 H 1789 1 1874.9 1959.1 2045 . 9 85.60 I 1636 1 1720 7 1886.8 1669.4 83.32 Gen. av 84.74 THE ACETATES. 161 TABLE 81. — Silver uranyl acetate. Series. Group 2. Group 3. Group 4. Group 5. Group 6. Group 7. Group 8. Average interval. A 1722 . 5 c 1735.0 1818.4 1904.9 1989.2 2073.0 84.50 D 1745 . 1 1829.7 1913.2 2002.2 2085 . 1 85.00 E 1754.7 1839.4 1923.9 2008 . 6 84.63 E' 1672.5 F 1677.5 1761.1 1846.0 1931.0 2016.1 2101.5 84.80 G 1859.0 1945.1 2028 . 6 84.80 G' 1777 7 H 1701.2 1785.7 1871.0 1955.7 2040 . 0 84.70 Gen. av . . 84.74 TABLE 82. — Barium uranyl acetate. Series. Group 2. Group 3. Group 4. Group 5. Group 6. Group 7. Group 8. Average interval. C 1651 5 1734 9 1818 8 1904.8 1990 0 2075 . 5 84.80 D 1744.3 1827.8 1913.5 2000.0 2085.5 85.30 E 1668.6 1754.4 1837.9 1922.7 2008 . 8 85.05 F 1677.0 1763 0 1846.7 1930.5 2016.5 2101.3 84.86 G 1688 3 1773 7 1856 3 1941 7 2027.2 2113.7 85.08 H . 1698 4 1785 1 1869 5 1954 7 2040 4 85.50 Gen. av .... 85.08 TABLE 83. — Lead uranyl acetate. Series. Group 2. Group 3. Group 4. Group 5. Group 6. Group 7. Group 8. Average interval. B 1643 1 1727 7 1812 1 1897 8 1983 5 2067 4 84 86 Ci 1818 2 1904 0 1989 0 2073 0 84 93 C 1653 4 1738 7 1821 9 1907 6 1992^ 4 2077 4 84 75 C' 1741.8 1827 5 1911.9 1997.0 85.07 D 1662.5 1747.7 1831 7 1916 2 2001 . 8 2087.5 85.00 Ei 1922 7 2006 8 84.10 E 1669.1 1754.3 1838.7 1925.0 2009.6 2095 . 7 85.32 E' 1844.0 1928.3 2098 . 2 84.30 F 1848 3 1935.0 2021 1 2105.9 84.40 G 1684.1 1770 1 1855 1 1942 1 2025 8 2111.0 85.38 H 1692 4 1778 9 1862 4 1949 1 2034 0 85 40 H' 1697 5 1782 3 1868 5 1953 4 85 30 I 1621.6 1706 8 1788 7 1875 1 1960 8 2046 7 85 02 L 1796 3 Gen. av 85.12 162 FLUORESCENCE OF THE URANYL SALTS. TABLE 84. — Summary of average intervals of the double acetates. Substance. Interval. Substance. Interval. Li(UO2)(C2HsO2)3.3H;2O 84.50 Zn(UO2)2(CiH3O2)6.7H2O 84.51 Na(UO2) (C2H3O2)3 85.22 Rb(UO2)(C2H3O2)3 84.86 Mg(UO2)2(C2H3O2)6 7H2O .... 85.19 Sr(UO2)2(C2H3O2)e . 6H2O 84.74 NH4(UO2)(C2H3O2)3 84.40 Ag(UO2) (C2H3O2)3 84.74 K(UO2)(C2H3O2)3 84 57 Ba(UO2) (CtKiOds . 6H2O 85.08 Ca(UO2)2(C2H3O2)6 8H2O . 83 88 Pb(UO2)(C2H3O2)4.4H2O 85.12 MnCTTOoVPoTToOolj fiTToO OK iq General average 84.76 ABSORPTION SPECTRA OF THE ACETATES. The fluorescence and absorption of the acetates are related to each other in a manner entirely similar to that already established in the case of the other uranyl compounds. The absorption bands occur in series of constant interval and this interval is much shorter than that of the fluorescence series. Fluores- cence and absorption overlap in the reversing region, with numerous coincidences and an interlocking of the fluorescence and absorption intervals. Reversals, both exact and of the well-known displaced type, are more frequent, perhaps, than in any family of uranyl salts as yet studied. A notable example occurs in the spectrum of lead uranyl acetate (see fig. 86). RtVEBSAI FIG. 86. The absorption spectra fall into two fairly well defined classes : (1) Double acetates of Li, NH4, Na, K, Ca, Zn, Rb, Sr., Ag., Ba. In this class the system of bands having a series interval of 70 + ends at about 2,180, where is located the head of the strongest series. Band E of the fluorescence series is usually missing in group 8 and is supplanted by a strong absorption band (1) located 85 =*= frequency THE ACETATES. 163 units from the terminating absorption band mentioned above, des- ignated as e in accordance with convention used in previous papers. An excellent example of this type of change from fluorescence to absorption is afforded by the spectrum of barium uranyl acetate (fig. 87) . Here an exact reversal of band Eg1 occurs and the strong absorption band e, which takes the place of E in that group, is 85 frequency units from the first member of the strong e series which extends toward the ultra-violet with the usual absorption interval of 70 units. Displaced reversals F, G, and H also occur — an indica- tion of the probably complex structure of these bands. The corre- sponding absorption bands are likewise 85 units from the first members of the/', g', and hf series of the absorption spectrum. There is almost as notable a resemblance between the absorption spectra of this class as between their fluorescence spectra. The resolu- tion is, however, not so good, and all the members of the various series are not so easily located. Almost without exception the bands which can be observed are definitely related, in the manner just described, to the fluorescence series. (2) The single acetates U02(C2H302)2 and U02(C2H302)2+2H20; double acetates of Mg, Mn, Pb. Here the absorption system (interval 70 =*=) distinctly overlaps the fluorescence system extending into the region of groups 8 and 7 beyond, without change of interval. Ml )U P 7 I iR OUP 8 GROUP 9 1 f 1 t 1 ) D 4 • r e 1 H 1 1 ? 1 a JA' ' 1 ' 1 r' _»SL 00 e 83 e 70 ? _70 20JOO 2 l|00 22|00 FIQ. 87. The various series of absorption bands located in our visual and photographic studies of the acetates are contained in tables 85 to 97 inclusive. Frequencies and average intervals are given for each salt, the series being designated as usual by small letters, which indicate their relation to fluorescence series denoted by the corresponding capital letter. The three examples of bands or series not thus related to visible fluorescence are indicated by means of the Greek letter 7. 1 This band Eg may appear either as fluorescence or as absorption according to the conditions of illumination, etc. It is commonly seen as fluorescence in the spectrum of the zinc uranyl acetate and as absorption in the spectra of other salts of this class. 164 FLUORESCENCE OF THE URANYL SALTS. TABLE &5—IA Series. Frequencies. Interval. c. ... 2234.5 2085.0 2097.8 2100.0 2026.5 2110.8 2113.0 2117.8 2124.0 2129.0 2375.0 2446.5 2514.8 2584.8 70.06 c'. e 2185.0 2256.6 2325.0 2396.2 70.40 e' /' 2108.8 f". Q\ . n 2201.5 2623 .8 70.39 h h' Weighted ave rage . . 70.27 TABLE 8Q.—NH,(U02)(C2H^02)3. Series. Frequencies. Interval. 0i 2145.6 1996.5 2232.4 2096.2 2107.8 2114.8 2119.8 2047.8 •y 2082.4 2374.0 2183.2 2163.4 2299.2 2511.6 69.62 71.73 70.12 c 2446 3 e 2253 . 8 2322 . 8 2394 .2 /' . f] h 2207.0 2130.0 2278.8 2348.5 2422.8 2487.5 70.13 i' Weighted ave rage . . 70.19 TABLE 87—Na(UO2)(C2H,02-),. Series. Frequencies. Interval. b 2229.2 2375.3 2239.6 2093.4 2395.8 2328.3 2102.6 2265.0 2111.0 2117.7 2126.8 2137.7 2336.3 b' c 2311.1 2382.1 2454.0 2524.0 2591.8 69.95 di d d . . 2472 .8 70.46 70.30 e 2190.1 2336.4 2478.9 2277.4 2211.4 2217.8 2259.4 2409.1 2332.1 2403.8 2542.4 2475 .9 e' f f 2343.0 2285.7 2291.0 2415.3 70.40 71.54 70.15 g 2350.7 2423.7 2569.1 2358.5 2430.1 2499.4 h { Weighted ave rage . . 70.46 THE ACETATES. 165 TABLE 88.— Mg ( UOJ ,. ( . 7H-f). Series. Frequencies. Interval. Cl 2161 0 c 2168 3 2239 2 70 90 c' 2172 8 2315 0 . 71 10 d' 2092 . 8 e 2182 8 f 2102 2 01 2108 4 n 2266 0 2336 2 70 20 a' 2115 3 2199 3 2269 .0 69.70 Weighted average 70.60 TABLE 89.— K(UO,)(C2H,02). Series. Frequencies. Interval. c 2166 0 2234 .0 68.00 cf 2084 0 2000 0 d 2176.1 £, 2011 5 el 2017 0 2100.0 2186.0 2256.3 2326.5 70.25 f. 2025 0 2109 3 n 2116 0 2203 0 a' 2124 0 22116 2292.2 2349.0 68 70 h 2131 8 h' 2061.8 Weighted average 69 18 TABLE 90.— Ca( UO . 8H20. Series. Frequencies. Interval. 2023.1 2173.0 2180.1 2104.8 2185.0 2198.8 2032.1 2123.6 2126.3 2436.6 2092.5 2245.2 2249.2 2166.8 2312.7 2317.5 71.85 70.63 70.00 d 2384.9 c 2390.1 f f. 2254.3 2340.3 2115.1 2278.4 2325.6 2409.6 2203.6 2349.1 2395 .8 70.27 70.27 70.63 70.70 /TI n 2274.3 2415.5 h' hi i Weighted ave rage 70.54 TABLE Ql.— Series. Frequencies. Interval. c 2425 0 $ 2011.5 2094.0 2178.4 2236.2 2307.5 71.62 el 2311 3 n 21105 2181.2 2251.5 70.50 Weighted average 71.25 166 FLUORESCENCE OF THE URANYL SALTS. TABLE 92.— . 7H?0. Series. Frequencies. Interval. c 2372.5 2096.4 2188.2 2263.0 2178.6 2205.6 2129.9 2442.6 2181.5 70.10 70.10 e 2251.7 2322 9 2392 5 /' g h 2343.6 2277.9 2415.0 71.40 72.80 h' 2350 .7 .... i Weighted ave rage 70.77 TABLE Q3.—Rb(UO^(C2H30,-)3. Series. Frequencies. Interval. a 2059.3 bi 2070.8 , c 2158.9 2292. 2 2299.4 2373.6 24408 . ... 70 27 c' 2081.6 d' 2093 . 8 2246 . 7 2317.0 70 35 d" 2392.3 e 2186.7 2259. 4 2326 . 1 2399 .8 71 03 gi 2027.2 a'.. 2115.1 ii 2041.7 i 2129.0 2209. 5 2279.5 2350.2 2423.1 71.20 Weighted average 70.78 TABLE ^.— . 6H,0. Series. Frequencies of absorption bands at —185° (groups 8 to 12). Interval. d 2371.2 c 2375 .6 Cl . 2390.0 e 2094. 5R 2181.4 2251.9 2322.5 70.55 e' 2185.7 2254. 1 68.40 e" 2399.9 h 2273.8 2412.7 69 45 h' 2206.0 2347.8 70 90 i 2209.0 2279.1 2350.0 2420.0 2490.0 70.25 General weighted average 70.11 TABLE 95.— AgU02(C2H30-i). Series. Frequencies. Interval. c 22314 2369.7 69 15 e 2180.0 2251.3 2321.2 2390.1 70.03 /'. . 2107.0 2191.4 0 . . 2116.0 h 2125.0 i 2133.0 2204.1 2276.3 2344.7 70.39 Weighted average 70.01 THE ACETATES. 167 TABLE 96.— Ba( UOt . 6H2O. Series. Frequencies. Interval. b 2294.1 2227.2 2094.7 2105.3 2115.1 2204.1 c 2370.8 2180.1 2187.2 71.82 71.13 72.80 e 2250.2 2322.5 2393.5 /' 2260.0 a' gh 2275.8 2419 0 2486 9 70.70 Weighted aver age. . 71.36 TABLE Q7.—Pb(U02)(C,H302)H2O. Series. Frequencies. Interval. 5 2065 2072 2365 2094 2098 2101 2105 2110 2119 2125 2201 0 0 0 5 0 5 5 5 5 0 5 2142 2437 2236 2168 2389 2176 5 5 5 5 0 5 2355 2506 2309 2240 .0 .0 .8 .0 2426 5 2495 .0 70. 71. 71. 71. 71. 71. 5 5 43 83 87 75 c' d e 2380 2313 0 2450.0 2521.0 2594.5 5 e' e" f 2248 .4 2321 5 2392 .5 h h' 2194 2272 5 0 2264 2341 .0 .0 2332 2413 5 71. 71. 00 00 k 5 2482.0 2552.5 Weighted average 71. 21 TABLE 98. — General weighted averages of intervals of absorption series in the spectra of the acetates at -185° C. Substance. Li N: Na M K 2H2O... ;)s . 3H2U . .7H2O Interval. 71.04 72.35 70.27 70.19 70.46 70.60 69.18 70.54 71.25 Substance. Zn(UO2)2(C2H3O2)6 - 7H2O . Rb UO2(C2H3O2)3 O /TT/~\ \ //"^ TT /"\ \ (* ' TT f~\ oT\\j\J2)'i(\-'itiyJ'i)§ • ori2U . Ag UO2(C2H3O2)3 Ba(UO2)2(C2H3O2)6 - 6H2O . Pb(UO2)(C2H3O2)4.4H2O. A.V. interval for all acetates . Interval. 70.77 70.78 70.11 70.01 71.36 71.21 70.68 From the list of general, weighted averages of the intervals for the various salts (tables 97 and 98) it appears that the frequency interval of the single acetates is larger than the general average, corresponding in this respect with the larger interval of their fluorescence spectra, as has been previously noted. The determination of intervals is, however, somewhat less accurate than in the case of the fluorescence bands, and, as in that instance, no difference between various series, or various salts, can be considered as positively established. 168 FLUORESCENCE OF THE URANYL SALTS. SUMMARY. (1) The spectra of the uranyl acetates consist of the usual equi- distant fluorescence bands. (2) When excitation occurs at the temperature of liquid air, these bands are resolved into groups the homologous members of which form series of constant-frequency intervals. (3) There are two single acetates — a finely powdered, anhydrous variety and a crystalline form with 2 molecules of water of crystalliza- tion, whose spectra differ widely, particularly as to the groups of fluorescence bands. (4) Of the double acetates, those containing lithium, potassium, calcium, manganese, and strontium have spectra which may be regarded as essentially identical both as regards the location of the principal bands and the structure of the fluorescence groups. The only distinctions between their spectra are in the sharpness of reso- lution and relative brightness of the various components. (5) The spectrum of barium uranyl acetate differs from the above in that the groups are shifted, as a whole, about 5 frequency units toward the red. (6) In the spectra of the double acetates of ammonium and rubidium, band D in each group is doubled, but there is no shift of the groups. (7) The presence of sodium, magnesium, zinc, silver, and lead modifies the group structure by the addition of bands characteristic of the metal and causes slight relative displacements of the group system as a whole. Otherwise the spectra resemble those mentioned under (4). (8) The frequency interval for the fluorescence series of the double acetates is probably the same for all series and for all salts, the depar- tures of the general averages for the various salts being less than one frequency unit from the average for all, i. e., 84.76. The same is probably true of the absorption series, the general average for which is 70.68. (9) The frequency intervals, both in the fluorescence and absorp- tion spectra, are larger by more than one frequency unit for the single acetates than for the double acetates. IX. THE SULPHATES. Uranyl sulphate (U02S04.3H20) and the double uranyl sulphates of the alkaline metals are among the most brilliant of known fluores- cent substances. Their spectra are characterized by an unusual com- plexity 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 already considered, 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° vanishes with the group 7 (frequency 2000 to 2070), which is the reversing region for this family of salts, and the eighth group lies entirely within the absorption region. 1 II NH4 I I I I II NH* No. ii II _u jli Rb 1 Ca I , 1 1 I II Na 11 l_l Rb Ca 1 I I II n lin i i FIG. 88. FIG. 89. (2) Absorption of the type having the usual 70 ± frequency interval extends without change of interval into group 7. In discussing the acetates, what we have called the heads of the prominent absorption series lie in the region between 2040 and 2060 instead of at about 2170, as in the spectra of the acetates. 169 170 FLUORESCENCE OF THE URANYL SALTS. (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 terminates toward the violet, where it meets the head of the corresponding absorption series mentioned above. (4) The location in the spectrum of the fluorescence groups in the spectrum of the sulphates is not approximately the same for the different salts, as is the case with the corresponding double acetates. On the contrary, there is in general a shift toward the violet with increasing molecular weight, as may be seen from figure 88, in which group 5 of the 6 sulphates under consideration are depicted. This shift is larger than that observed in the double nitrates, but not quite so systematic. TABLE 99. — Uranyl sulphate: U02.SO*3HzO. Fluorescence at —185° C. Prepared by extracting an excess of uranium oxide (UsOs) with sulphuric acid and oxidiz- ing the solution to UC>2 . SO* by means of B^Oj. This neutral solution was evaporated to crys- tallization. The crystals were needles, some being 1 by 2 by 5 mm. in size, apparently ortho- rhombic, with three good pinacoidal cleavages. The angle of the optical axes is very nearly 90° and the double refraction is positive. Group and series. • 1/MX103 Int. Group and series. ' i/Vxio3 Int. /E 0.6254 1599.0 d. 'A 0.5256 1902.6 d. 2\F .6223 1606.9 d. B .5237 1909.5 vd. B' .5228 1912.7 vd. B .6046 1654.0 vd. C .5210 1919.4 d. C .6009 1664.2 vd. C' .5203 1921.9 d. D .5976 1673.4 d. D .5182 1929.8 d. 3- E .5942 1682.9 vd. 6' E .5157 1939.2 m. F .5914 1691.0 d. F .5135 1947.5 s. H .5857 1707.4 vd. F' .5123 1951.8 m. I .5827 1716.1 vd. H .5091 1964.4 m. I .5071 1972.0 d. B .5740 1742.2 vd. J .5054 1978.6 d. C .5716 1749.4 d. D .5686 1758.8 d. A' .5027 1989.2 d. E .5657 1767.7 m. B .5014 1994.4 vd. 4- F .3630 1776.1 m. B' .5004 1998.5 m. G .5600 1785.7 vd. C .4990 2004.0 m. H .5574 1794.1 d. C' .4981 2007.6 vd. I .5551 1801.3 vd. C" .4978 2008.8 vd. J .5534 1807.0 vd. C'" .4974 2010.5 vd. D .4964 2014.3 vd. A .5506 1816.5 d. D' .4955 2018.2 vd. B .5478 1825.6 vd. 7' EI .4941 2023.9 vd. C .5450 1834.9 d. E .4938 2025.3 m. C' .5441 1837.9 d. E' .4933 2027.2 d. D .5423 1843.9 vd. Fj .4926 2030 . 0 vd. 5, E .5394 1853.8 m. F .4917 2033.9 s. * F .5369 1862.4 s. F' .4912 2035.8 vd. F' .5357 1866.7 vd. F" .4905 2038.7 m. G .5346 1870.7 vd. H .4878 2049.9 m. H .5321 1879.2 d. J .4843 2064.8 vd. I .5301 1886.4 d. J .5280 1893.9 vd. THE SULPHATES. 171 If the above groups are aligned by bringing band F into vertical registration, as in figure 89, it will be seen that the apparent dissimi- larity in the composition of the group in the various salts is due rather to the occurrence of various weak bands than to the arrangement of the stronger bands, which, while not identical, approximates to identity almost as closely as in the acetates or the nitrates. As in previous diagrams (see the chapters dealing with the spectra of the chlorides, nitrates, and acetates), the vertical lines indicate the position of the crests of the bands and, qualitatively only, their relative intensities. They are estimated in making observations merely as very strong (vs), - U02. (SOt)i.8HiO. TABLE 100. — Uranyl ammonium sulphate: Fluorescence at —185° C. Prepared by crystallizing a solution of the two component salts in the proportions of the double salt. The composition has been determined by Rimbach (Ber. d. d. Chem. Ges., 37, 479 (1904) ; the crystallization by de la Provastaye (Ann. Chem. Phys. (3), 5, 51 (1842), who described it as being monoclinic. The preparation consisted of square and rounded plates of diameter from 0.025 to 0.050 mm. The needle-like crystals showed distinct pleochroism from colorless to yellow, the greatest absorption being in the direction of greatest index. Group and series. M 1/MX103 Int. Group and series. M 1/MX103 Int. (E 0.6214 1609.2 d. 'H! 0.5300 1886.8 vd. 2^F .6185 1616.7 d. H .5295 1888.7 d. IH .6100 1639.3 d. 5' H' .5290 1890.4 d. I .5280 1839.3 vd. 'B .6007 1664.7 d. J .5265 1899.3 vd. C .5977 1673.1 d. D .5941 1683 . 2 d. A .5249 1905.2 d. 3' E .5911 1691.8 s. Bi .5230 1912.0 vd. F .5883 1699.7 s. B .5214 1917.8 d. H .5809 1721.6 vd. Ci .5198 1923.8 m. C .5194 1925.4 m. A .5755 1737.6 vd. C' .5190 1926.8 m. B .5717 1749.2 d. Di .5177 1931.6 d. C .5697 1755.3 d. D .5169 1934.6 d. C' .5689 1757.8 d. 6< Ei .5152 1941.0 . vd. D .5663 1765.8 m. E .5147 1943.0 8. 4< E .5632 1775.5 B. Fi .5128 1950.1 vd. F .5608 1783.3 S. F .5123 1951.9 vs. G .5579 1792.3 vd. F' .5116 1954.7 vd. H! .5549 1802.1 vd. Gx .5106 1958.6 vd. H .5539 1805.5 m. H .5073 1971.2 vd. J .5508 1815.5 vd. H' .5066 1974.0 d. J .5038 1984.9 vd. A .5491 1821.2 d. B! .5472 1827.5 vd. Ai .5025 1990.0 d. B .5456 1833.0 in. A .5015 1994.0 vd. C, .5440 1838.2 m. B .4998 2000.9 d. C .5435 1840.1 m. C .4975 2010.0 m. C' .5430 1841.6 m. Di .4960 2016.1 d. 5 Di .5415 1846.7 vd. 7 D .4955 2018.3 d. D .5406 1849.8 d. E .4933 2027.1 s. E .5380 1858.8 s. Fi .4916 2034.2 vd. F! .5361 1865.3 vd. F .4912 2036.0 vs. F .5354 1867.6 s. F' .4905 2038.7 vd. F' .5346 1870.6 d. Gi .4893 2043 . 7 vd. Gi .5337 1873.7 vd. 172 FLUORESCENCE OF THE URANYL SALTS. strong (s), medium (m), dim (d), very dim (vd), and very very dim (vvd) respectively. No attempt is made in the diagram to indicate the width of the bands. The spectrum of the single sulphate resembles those of the double sulphates much more nearly than is the case with the single and double salts of the other acids. Wave-lengths, frequencies, and relative intensities of the bands observed in the fluorescence spectra of uranyl sulphate and the double sulphates of ammonium, sodium, potassium, rubidium, and caesium are given in tables 99 to 104. Similar measurements of the bands in the absorption spectra are given in table 10. The determination of wave-lengths were made by the visual and photographic methods de- scribed in the foregoing chapters. TABLE 101. — Uranyl sodium sulphate: Naz . UO2 . (804)2 . 2H-f). Fluorescence at —185° C. Prepared by crystallizing a solution containing the two component salts in the proportions of the double salt. (See O. de Coninck, Chem. Centralblatt, ix, I, 919, 1905.) The preparation consisted of crystalline grains about 0.5 mm. in diameter, with much mother liquor or deliques- cence. The crystals are apparently monoclinic, with positive double refraction. Group and series. M i/Vxio3 Int. Group and series. M 1/juXlO3 Int. B 0.6296 1588.3 vd. D 0.5374 1860.8 d. C .6255 1598.7 vd. E .5355 1867.3 d. E .6182 1617.6 d. F .5330 1876.0 m. 2, < F .6151 1625.8 m. G .5311 1882.7 vd. G .6122 1633.5 vd. 51 G' .5301 1886.4 vd. H .6093 1641.2 vd. H .5287 1891.4 d. H' .5278 1894.7 d. B .5976 1673.3 d. I .5260 1901.1 d. B' .5963 1677.0 vd. C .5945 1682.1 d. A .5226 1913.5 d. 3i D .5908 1692.6 d. B .5197 1924.2 vd. < E .5880 1700.7 d. Ci .5181 1930.1 vd. F .5851 1709.1 s. C .5167 1935.4 m. G .5828 1715.9 d. D! .5152 1941.0 d. H .5797 1725.0 d. D .5141 1945.1 vd. E .5123 1951.8 d. A .5729 1745.5 vd. 6< Fi .5110 1956.9 vd. B .5698 1755.1 m. F .5101 1960.4 s. Ci .5677 1761.5 vd. Gi .5087 1965.8 vd. C .5665 1765.1 m. G .5077 1969.5 vd. D! .5642 1772.4 d. H, .5061 1975.9 d. D .5629 1776.5 d. H .5050 1980.2 m. 4' E .5607 1784.1 d. I .5038 1984.9 vd. F .5579 1792.3 m. G .5560 1798.6 vd. 'A .5006 1997.6 vd. H .5532 1807.7 d. Bi .4977 2009.2 vd. H' .5522 1810.9 vd. B .4965 2013.9 s. Ii .5509 1815.9 vd. Ci .4955 2018.2 m. I .5501 1817.9 vd. C .4943 2022.9 m. 7< E .4910 2036.5 m. A .5468 1828.8 d. F .4890 2045.0 d. B .5439 1838.4 vd. G .4873 2052.3 vd. 5< Ci .5418 1845.7 vd. H .4857 2058.9 d. C .5406 1849.9 m. H' .4847 2063 . 1 vd. Di .5388 1856.0 d. THE SULPHATES. 173 TABLE 102. — Uranyl potassium sulphate: Ki.UO^SOJz.SHzO. Fluorescence at -185° C. Prepared by crystallizing a solution of the two component salts in the proportions of the double salt. The composition has been determined by Rimbach (Ber. d. d. Chem. Ges., 37, 478 (1904). The crystals obtained in this laboratory were orthorhombic. The preparation con- sisted of 6-sided plates and rounded grains about 0.045 mm. in diameter, the plane of the optical axis being a (100) and b the acute bisectrix. Double refraction positive. Group Group and M I/MX 10s Int. and M 1/MX103 Int. series. series. C 0.6267 1595.7 vd. F' 0.5332 1875.5 d. D .6229 1605.9 vd. Gi .5324 1878.3 vd. E .6188 1616.0 d. G .5319 1879.9 vd. 2< F .6164 1622.3 m. < G' .5314 1881.8 vd. F' .6150 1625.9 vd. H .5295 1888.6 vd. G .6129 1631.6 vd. I .5276 1895.2 vd. B .6010 1663.9 vd. A' .5240 1908.4 vd. Ci .5981 1672.0 d. B! .5235 1910.2 vd. C .5957 1678.8 d. B .5226 1913.4 d. Di .5941 1683.2 vd. Ci .5199 1923.4 vd. D .5921 1688.0 vd. C .5189 1927.2 m. 3< E .5886 1698.9 vd. Di .5173 1933.1 vd. F .5859 1706.7 m. D .5164 1936.5 d. F' .5851 1709.1 d. D' .5157 1939.1 vd. G .5830 1715.3 vd. 6- Ei .5144 1944.0 vd. H .5804 1723.0 vd. E .5137 1946.7 m. F .5115 1955.4 s. B .5718 1748.8 vd. F' .5107 1958.1 d. Ci .5697 1755.3 d. Gi .5097 1961.9 vd. C .5680 1760.7 d. G .5093 1963.4 m. D! .5662 1766.3 vd. G' .5088 1965.4 vd. D .5644 1771.9 d. H .5069 1972.6 d. 4< E .5616 1780.5 m. I .5054 1978.6 d. F .5589 1789 . 1 s. F' .5583 1791.3 d. A' .5023 1991.0 m. G .5563 1797.5 vd. B! .5015 1994.0 vd. H .5539 1805.5 vd. B .5007 1997.3 d. I .5519 1811.9 vd. Ci .4988 2004.7 vd. C .4973 2010.9 s. A' .5481 1824.5 vd. D! .4959 2016.5 vd. B .5461 1831.0 d. 7 D .4951 2019.7 m. Ci .5438 1838.9 d. E! .4935 2026.5 m. C .5424 1843.8 m. E .4923 2031.3 d. 5 Di .5405 1850.1 vd. F .4906 2038.5 s. D .5391 1854.9 d. Gi .4894 2043 . 3 vd. E! .5374 1860.8 vd. G .4889 2045 . 2 d. E .5366 1863.5 m. G' .4884 2047.5 vd. F .5342 1871.8 vs. 174 FLUORESCENCE OF THE URANYL SALTS. TABLE 103. — Uranyl rubidium sulphate: Rb2. U02. (SOJi- 2H20. Fluorescence at— 186° C. Prepared by crystallizing a solution containing the two component salts in the proportions of the double salt. The composition has been determined by Rimbach (Ber. d. d. Chem. Ges., 37, 479, 1904). The crystallization is in every way like the potassium salt, although the solubil- ity is less and the crystals smaller. The preparation consisted of 6-sided plates about 0.02 by 0.04 mm. in size. Group and series. ' 1/MX103 Int. Group and series. • 1/MX103 Int. 1 F 0.6485 1542 . 0 vd. 'F' 0.5332 1875.8 vd. Gi .5320 1879.7 vd. I/-. .6269 1595.1 d. 5- G .5310 1883.2 vd. D .6225 1606.4 d. H .5292 1889.5 vd. E .6187 1616.4 vd. I .5276 1895.4 vd. F .6157 1624.1 d. A' .5240 1908.4 vd. B .6004 1665.5 vd. B .5223 1914.5 d. C .5975 1673.7 d. Ci .5198 1923.8 d. 3, C' .5954 1699.5 d. C .5187 1927.8 m. D .5919 1689.5 d. D .5163 1937.0 d. E .5887 1698.7 m. E .5136 1946.9 m. F .5860 1706.5 s. 6< F .5115 1955.2 vs. F' .5105 1958.7 vd. B .5719 1748.5 vd. Gi .5096 1962.3 vd. Ci .5692 1757.0 vd. G .5087 1965.8 vd. C .5679 1760.9 vd. H .5068 1973.3 vd. D .5643 1772.2 d. I .5053 1979.0 vd. 4, E .5616 1780.6 m. < F .5591 1788.7 s. A' .5021 1991.8 vd. F' .5580 1792.0 vd. B .5008 1996.7 d. G .5560 1798.6 vd. Ci .4987 2005.2 d. H .5538 1805.7 vd. C .4973 2010.9 m. I .5520 1811.6 vd. 7* D .4953 2018.9 d. < EI .4930 2028.4 d. B .5461 1831.2 d. E .4922 2031.8 d. Ci .5438 1838.8 d. F .4906 2038.4 s. C .5424 1843.7 d. Gi .4890 2045.1 vd. D .5392 1854.6 d. G .4880 2049.2 vd. E .5364 1864.3 m. F .5342 1871.8 vs. THE SULPHATES. 175 TABLE 104. — Uranyl ccesium sulphate: Cs-zUO^SO^ 2HiO. Fluorescence at —185° C. Prepared by precipitating uranyl sulphate by adding caesium sulphate in calculated amount to form the double salt, which is very insoluble. The composition of the crystals is given as above by O. de Coninck (Chem. Centralblat, ix, 1, 1306, 1095). The preparation consisted of very small square plates about 0.01 mm. on a side, the largest of which showed an apparently uniaxial negative figure. The crystals are therefore presumably tetragonal. Group and series. M i/Vxio3 Int. Group and series. A» 1/MX103 Int. 2 F 0.6129 1631.8 d. 'E, .5336 1874.2 d. E 0.5321 1879.3 m. A .5989 1669.7 vd. F .5299 1887.0 s. B .5964 1676.7 vd. 5' G .5276 1895.4 vd. Ci .5939 1683.8 d. G' .5261 1900.8 vd. C .5916 1690.3 d. H' .5239 1908.6 d. < D .5894 1696.6 vd. I .5228 1912.9 vd. Ei .5869 1703.9 vd. E .5850 1709.3 d. A .5189 1927.2 d. F .5825 1716.6 m. B .5168 1935.0 vd. Ci .5148 1942.5 vd. A .5695 1755.9 vd. C .5140 1945.7 m. B .5671 1763.4 vd. D .5118 1953.7 vd. Ci .5652 1769 . 2 . d. D' .5110 1956.9 vd. C .5636 1774.3 d. 6 Ei .5099 1961.0 d. D .5612 1782.0 d. E .5088 1965.5 m. 4 Ei .5593 1788.0 d. F .5067 1973.6 vs. E .5574 1794.0 m. G .5047 1981.4 vd. F .5550 1801.7 s. G' .5035 1986.1 vd. G .5517 1812.5 vd. H .5019 1992.6 d. H .5490 1821.4 vd. I .5003 1999.0 m. I .5472 1827.4 vd. A .4970 2012.1 d. 'A .5434 1840.3 d. C .4920 2032.6 m. B .5410 1848.3 vd. D .4902 2040.0 vd. 5' Ci .5390 1855.2 d. 7< E! .4888 2045.8 d. C .5374 1860.8 m. E .4874 2051.6 m. D .5353 1868.0 d. F .4858 2058.6 s. FREQUENCY INTERVALS OF THE FLUORESCENCE SERIES. The average frequency intervals of the various series, as derived from the foregoing tables, are given in table 105, together with the weighted average for each salt. It will be noted that the intervals of the single sulphate and the double salt of caesium are distinctly greater than the intervals of the other four sulphates. There is nothing fortuitous about these differences, for, as will be seen from the table, the different series for each salt have intervals within one frequency unit of the general average for that salt, with three exceptions. These exceptions are series Ci in the ammonium and sodium double sulphates and GI in the ammonium salt. Such occasional apparent discrepancies are not uncommon in the fluorescence spectra of the uranyl salts. They are not due to accidental errors, but are probably ascribable to the complexity of bands having overlapping components the relative intensity of which in different portions of the spectrum varies progressively. Many such cases are known. A doublet ill- resolved and appearing as a single hazy band, the component of longer 176 FLUORESCENCE OF THE URANYL SALTS. TABLE 105. — Average frequency intervals in the fluorescence spectra of the sulphates. ] ntervals. Weightec 1 averages. Series. UO2SO4 +3H2O. (NH4)2U02(S04)2 +2H2O. Na2UO2(SO4)3 +2H2O. K2UO2(SO4)2 +2H2O. RbsUCMSOOu +2H2O. Cs2UO2(SO4)2 +2H2O. A 84.1 84.0 85.6 A' 83.3 Bi 84.5 B B' 85.1 85 8 83.5 84.2 84.2 83.3 82.8 86.1 Ci 85.6 85.6 83.2 82.7 86.2 c 84.2 84.8 83.1 83.2 85.6 C' 84.9 84.5 Di 83.2 84.3 83.3 85.8 D 85.2 83.8 84.2 82.8 82.5 Ei 83.2 85.7 E Fi 85.0 83.6 84 4 83.8 82.1 83.1 85.6 F F' 85.4 84 6 83.9 84.0 83.8 83.3 83.0 82.7 83.3 85.4 Gi 85 0 82.5 G 85 2 84.4 82 7 83.5 86.0 G' 82.9 85.3 H. 84 6 H H' 85.5 83.0 83 6 84.4 84 1 83.2 83.8 85.6 I 85 2 84 9 83 3 83.7 85.8 Ji 85 2 j 84 7 Average 85.2 83.7 84.3 83.0 83.2 85.7 wave-length being much stronger in the bands toward the red and dying away gradually in subsequent bands as we approach the blue, while the other component steadily increases, will give the effect of an increased frequency interval for the series. The increase might easily be of the general order observed in this case. There is also always the possibility of the presence of a trace of another uranyl compound which would yield additional series. Such cases, for example, are not uncommon in the study of the acetates, where an admixture of the single acetate occurs. ABSORPTION SPECTRA. The difficulties in obtaining a complete record of the absorption bands of the uranyl sulphates are similar to those described in the preceding chapters. The transmission, like that of the other uranyl salts, ranges progressively from almost complete transparency in the red, yellow, and green to a high degree of opacity in the ultra-violet. Large, clear crystals of the sulphates are not obtainable and there- fore it is not possible to use very thick layers and thus to follow the selective absorption far beyond the reversing region toward the red, as has been done in the case of the chlorides.1 The bands which we were able to locate lie approximately between 2,000 and 2,600 fre- quency units. They belong almost exclusively to the system having 1 H. L. Howes. Physical Review (2), xi, p. 66. 1918. THE SULPHATES. 177 the shorter frequency interval of 70=^. A few end members of the reversing system, which presumably extends throughout the fluores- cence region, were discernible. Determinations were made in part by photographing the spectrum of the light transmitted by thin layers, in part by the method of reflection. In table 106 the frequencies of the bands in the spectra of the 6 sulphates are arranged by series. Each series, as usual, is designated by a small letter corresponding to the capital letter which denotes the fluorescence series to which it is related. TABLE 106. — Absorption spectra of uranyl sulphates at —180° C. Salt. Series. Frequencies. Average interval. UO2SO4+3H2O. / (NH4)2U02(S04)i +2H2O. Na2UO2(SO4)2 +2H2O. ai a 6' 4 d e h f f hi h a b c di e e' /i / /' ffi g h 'a a' b c d, d e e' f 0i a g' h h' 2056.8 2128.1 2399.8 68.6 70.8 70.1 2060.6 2202 2 2068.7 2140.5 2209.2 2218.3 2016.1 2081.2 2152.9 68.4 68.0 70.1 70.1 2093.6 2160.3 2229.5 2029.9 2170 . . ... 2102.6 2236.9 2373.0 2035.8 2102.6 2045.8 2115.9 2186.7 2327.2 70.3 2260 .4 General average 69.6 2061.2 2135.3 2205.8 2275.3 71.3 69.9 71.2 69.9 69.6 69.5 70.2 69.3 70.4 2072.5 2212.4 2082.8 2155.1 2226.2 2295.2 2510.0 2016.5 (R)2199. 8 2263.0 2409.6 2096.0 2236.2 2305.7 2444.4 2031.8 2448.6 2244.4 2315.6 2383.3 2524.0 2595.4 2107.7 2178.7 22500 2454.3 2039.6 2253.8 2323.8 2392.0 2464.0 2532.8 2044.0 2116.0 2187.1 71.5 70.2 70.3 2330.5 2401.0 25410 2127.7 2198.8 2236.0 2409.6 2477.1 . . 2549 4 General average 70.3 2069 .5 2144.5 2214.7 70.2 72.5 70.5 2080.8 2153.3 .... 2093.7 2306.8 2375.9 2167.3 2172.5 2243.8 2314.8 2385.5 71.0 71.4 2035.8 2107.5 2250.1 2039 .2 2043.9 2114.3 2184.5 70.3 70.1 2050.0 2120.3 2260.4 2190 1 2054.7 2055.2 2063.4 2128.6 2199.9 71.3 72.1 2135.2 2207.7 General average 71.3 178 FLUORESCENCE OF THE URANYL SALTS. TABLE 106. — Absorption spectra of uranyl sulphates at —180° C — continued. Salt. Series. Frequencies. Average interval. K2U02(S04)2 +2H2O. Rb2UO2(SO4)2 +2H2O. Cs2UO2(SO4)2 +2H2O. '&! b b' Cl c di d d' e\ e e' h f /' g\ Q Q' h fti b Cl c di d d' e\ e e' fi f f Oi Q hi h 'a a' c c' di e f f a o' h i 2063.1 2412.3 69.8 70.8 68.8 70.4 70.2 71.1 2064.4 2136.4 2204.6 2276.9 2065.7 2341.0 2071.9 2353.5 2078.6 2289.4 2017.3 2444.1 2375.3 2158.2 2174.0 2095.5 2307.0 2379.8 71.1 71.6 70.6 70.2 2241.7 2312.1 2385.0 2035.0 2106.8 2246.7 2039.6 2109.3 2179.4 2248.2 2390.6 2250.7 2293 .5 2116.4 2188.5 2256.7 2323.7 69.1 2047.9 2056.8 2266.3 69.8 General average 70.3 2065.6 2136.6 2205.7 2275.1 69.8 2071.2 2218.8 2434 . 3 .... 2014.9 2230.2 2301.0 24432 71.0 2370.5 2028 . 4 2096 . 2 2375 . 1 2449 .8 70.2 68.8 2241.2 2307.3 2378.7 2312.2 2106.6 2174.5 2249.2 71.3 69.7 69.1 69.2 69.3 2037.9 2107.6 2040.3 2109.7 2179.4 2385.7 2045.4 2322.0 2391.3 2048.5 2117.7 2187.7 2256.7 2325.8 2193.9 2085.5 2267.3 2340.8 2408.5 70.0 General average 69.8 2096.0 2165.4 2236.9 2375.9 2445.0 2517.7 2311.9 70.3 2103.5 2035.8 2106.4 70.6 2115.1 . . 2120.0 2261.0 2329.6 2399.8 2471.0 2542.8 2613.4 70.5 2061.2 2130.8 22006 ... . ... . 69.7 2133.4 .... 2065.5 2206.5 2279.5 71.3 70.8 2071.9 2143.6 2215.8 2353.1 2426.9 2498.3 2567.2 2077 .2 2084 . 6 2294 .1 69.8 General average 70.4 THE SULPHATES. 179 TABLE 107. — Average frequency intervals for the six sulphates. UO2SO4 69.6 (NH4)2U02(SO4)2 70.3 Na2UO2(SO4)2 71 .3 K2UO2(SO4)2 70.3 RB2UO2(SO4)2 69.8 Cs..UO2(SO4)2 70.4 As is frequently the case in these absorption spectra, one or more bands of a given series are commonly missing or at least not discerni- ble in the negatives. On the other hand, nearly all the bands are found to be members of a series which is definitely related to a fluores- cence series and has the proper frequency interval. The occasional isolated bands, moreover, are so located that they may be definitely associated with a fluorescence series and may reasonably be classed as the sole visible member of an absorption series the remainder of which fails to appear in our photographs. These show no systematic departure from the general average (70.3) for the entire group. Lying as they do within one frequency unit of the average, we may fairly conclude that within the errors of observa- tion, which are rather large on account of the lack of definition and incomplete resolution of these absorption groups, the various sulphates have a common frequency interval. The frequency intervals of the various series of a given salt depart somewhat more widely from the average for that salt, but again there is no systematic variation, and it is probable that all the series would be found to have the same interval, were it possible to locate the bands with greater certainty. SUMMARY. (1) The fluorescence spectrum of the uranyl sulphates consists of 8 equidistant bands, the first and eighth of which disappear at the temperature of liquid air. (2) The remaining bands are resolved into groups of narrow line- like bands, the homologous members of which form series having constant frequency intervals, ranging from 85.7 in caesium uranyl sulphate to 83.0 in potassium uranyl sulphate. (3) The fluorescence groups are distinguished by a strong pair of bands about 8 frequency units apart and 7 weak bands, some of which are doublets. (4) There is a shift of all bands toward the violet, with increasing molecular weights, of about 15 frequency units in passing from the spectrum of uranyl sulphate to that of caesium uranyl sulphate. (5) The absorption spectra of the sulphates are made up of series of bands having a frequency interval of 70.3 (general average) . (6) These absorption series extend into group 7 of the fluorescence without break of interval . There are many reversals where fluorescence and absorption overlap. The reversing region is therefore one group farther toward the red than in most spectra of the uranyl compounds. X. THE FLUORESCENCE OF FROZEN SOLUTIONS. The fluorescence spectra of solutions generally consist of only one or two very broad bands. Such bands undoubtedly possess component bands in considerable number, but spectrum analysis often fails to reveal them because of extensive overlapping. Chlorophyl in alcohol possesses a series of absorption bands1 which resemble the absorption bands of the uranyl solutions. The very broad fluorescence band in the orange and red probably consists of several components which form a similar series. Anthracene in solution2 presents a fluorescence series of at least 4 bands which strongly resembles the series found in the fluorescence spectra of uranyl solutions. Probably the first observer to note the fact that uranyl solutions yield fluorescence spectra consisting of several bands was G. C. Stokes.3 He states that "a solution of nitrate of uranium is decidedly sensi- tive," i. e., fluorescent. Later, in the same paper, he writes "I have observed seven of these bands arranged at regular intervals." E. Becquerel,4 in his monumental work on the uranyl salts, makes this observation : " Certain solutions of the salts of uranium give, in the violet rays, a luminous emission scarcely less brilliant than the crystals themselves .... several [bands] appear to correspond to the bands given by the solid salts; the sul- phate and the double sulphate of potassium and uranium are in this class." In the same year Hagenbach,5 who was studying many fluorescence materials, observed that the uranyl oxide in nitric acid shows 8 very sharply outlined maxima in the fluorescence spectrum. Morton and Bolton6 studied the absorption of the uranyl solutions and noted the fluorescence. These investigators were the first to recognize the possibility of the existence of more than one hydrate of the same salt, which, they state, "enables us to explain some discrepancies of authori- ties on this point." Our present work has brought out the necessity, of such a hypothesis. Jones and Strong,7 following the same method as Morton and Bolton, have published the most extensive data on the absorption spectra, but their work does not include temperatures below the freezing-point. This chapter contains the results of experiments which were de- scribed in two papers, together with some additional data heretofore unpublished. The first, a preliminary paper,8 showed that the bands of 1 Nichols and Merritt. Carnegie Inst. Wash. Pub. No. 130, p. 85. 1910. 2 Louise MacDowell. Physical Review (1), 26, p. 155. 1908. 3 Stokes. Philosophical Transactions, p. 463. 1852. 4 E. Becquerel. Comptes Rendus, 75, p. 296. 1872. 6 Hagenbach. Poggendorf Annalen, 146, p. 582. 1872. 6 Morton and Bolton. Chemical News, pp. 113, 164. 1873. 7 Carnegie Inst. Wash. Pub. No. 130. 8 Nichols and Merritt. Physical Review (2), 3, p. 457. 1914. 180 FROZEN SOLUTIONS. 181 the solutions, even at — 180°, resembled in breadth and regular spacing those of the solid salts at room temperature. The uranyl acetate in alcohol proved to be the exception, since at —180° it resolved into faint lines, which did not, however, coincide in position with those of the solid acetate at that temperature. The variety of shifts with systematic dilution and temperature change led to the second investigation,1 in the hope that some general law of shift might be deduced. It was also planned to study the funda- mental relations between concentration and frequency interval, temperature, and state of resolution, etc. With these relations in view much work was done which led to the discovery of many beautiful and unique spectra. EXPERIMENTAL METHOD. For the study of the spectra, except where otherwise specified, a Hilger constant-deviation spectrometer was used. The apparatus for the cool- ing and excitation of the sub- stances under observation was designed to enable the observer to hold the tempera- ture of the specimen con- stant at any temperature between 0° and -180° C. The mounting consisted of a cylindrical copper block M (fig. 90), the top of which was bored to receive a small test-tube F, which contained the fluorescent solution. The side of the block was channeled to let the exciting light fall on the specimen and to let the fluorescent light out. To the bottom of this copper block was soldered a cylinder of sheet copper, which could be partially or completely covered by the liquid air in the unsilvered Dewar bulb D, thus produc- ing different temperatures in the specimen. This mount- ing was rigidly suspended from above by partially non- FIG. 90. 1 Howes. Physical Review (2), vol. 6, p. 193. 1915. 182 FLUORESCENCE OF THE URANYL SALTS. conducting material. The Dewar bulb was fastened to an adjustable support H, and the mounting could be submerged in the liquid air to various depths by raising or lowering the bulb by means of a cord R. Fluorescence was excited by the rays from a carbon arc A. The light passed successively through a water-cell W, a large short-focused condenser C, and a solution of ammonio-copper sulphate B. This solution absorbed all of the exciting light of a wave-length greater than 0.4780 fj,, so that the fluorescent light, which entered the colli- mator slit S of the spectrometer, could be viewed on a black back- ground. A small resistance coil T was inserted in a glass tube, which was always placed in the middle of the solution. The temperatures were recorded on a Callender recorder. The massive copper block M served to reduce the vertical temperature gradient in the frozen solu- tion to less than 1° per centimeter. It made the apparatus rather slow in responding to changes, but afforded an excellent control of tempera- ture. The salts were carefully weighed and "normal" solutions were prepared.1 Acid solutions were made by adding a definite volume of the commercial concentrated acid to a definite volume of a water solution of known concentration. Although readings were taken and tables made in units of wave- length, the diagrams of spectra are plotted on an arbitrary scale of frequencies, i. e., I/JJL X 103. URANYL SULPHATE IN AQUEOUS SOLUTION. The uranyl sulphate in water, upon excitation with the carbon arc, yields 4 bands at +20° C.; but when cooled 6 bands are visible, the new bands being of longer wave-lengths. This phenomenon — increase of intensity with cooling — is a very fortunate one, for otherwise the study of the more dilute solutions would be limited to the lowest temperatures. In table 108 will be found the bands of the 1/1, 1/10, 1/100, and 1/1000 normal aqueous solutions. In the spectrum of the 1/1 normal solution band 7 is at 0.4927 IJL at +20°, which is of interest because the crystalline salt was found to give a fluorescence band at 0.4925.2 If a reasonable error is assumed, these bands may be considered to be coincident. In this region they are approximately 75 A.U. in width; hence measurements were taken on the crest rather than on the middle of the band, the crests being located slightly nearer the violet edge. The absorption spectrum3 of the normal solution presents a band in this region at 0.4910, which is 17 A.U. nearer the violet than the fluores- 1 The term "normal" solution, as used in this paper, means one which contains the same num- ber of grams of solute to the liter of solvent as the number which represents the molecular weight of the particular salt dissolved. 2 Nichols and Merritt. Physical Review (1), 33, p. 354. 1911. 3 Jones and Strong. Carnegie Inst. Wash. Pub. No. 130, p. 109. FROZEN SOLUTIONS. 183 cence band. Jones and Strong employed the continuous spectrum of the Nernst lamp, which produces fluorescence in this region. Such luminescence, although masked by the more intense background, tends to shift the crest of the absorption band toward the violet. A comparison of the wave-lengths of the bands of the solid with those of the solution indicates a progressive difference. When the spectra of the solution and of the solid are plotted in frequency units, either is found to include only one series of bands, those of the solution being of a slightly smaller interval than those of the solid. TABLE 108. — Uranyl sulphate in water.1 Solution and temp. Band 2. Band 3. Band 4. Band 5. Band 6. Band 7. '+ 20 0.5641 0 . 5383 0.5143 0.4927 0 .5641 .5383 .5141 .4926 - 35 0.5934 .5641 .5378 .5141 .4927 Normal solution. - 60 - 90 -120 -150 .-180 f+ 20 0.6229 .6227 .6230 .6235 .6241 .5928 .5923 .5923 .5924 .5924 .5637 .5635 .5634 .5634 .5636 .5636 .5376 .5370 .5370 .5373 .5375 .5379 .5137 .5131 .5133 .5136 .5140 .5139 .4924 .4919 .4919 .4919 .4921 .4918 - 30 .5631 .5373 .5136 .4913 1/10 — 60 .5911 .5629 .5368 .5130 .4911 normal — 90 .5904 .5621 .5364 .5125 .4910 solution. -120 .5904 .5624 .5365 .5127 .4909 -150 -180 f- 35 .6228 .6231 .5917 .5921 .5629 .5633 .5633 .5367 .5371 .5373 .5130 .5133 .5135 .4913 .4916 .4913 - 60 .5634 .5373 .5129 .4911 1/100 - 90 .5631 .5368 .5125 .4907 normal < -120 .5629 .5365 .5126 .4907 solution. -150 .5924 .5631 .5368 .5129 .4909 -180 .5928 .5638 .5370 .5132 .4911 1/1000 f— 90 .5583 .5330 .5102 .4904 normal < -120 .5574 .5324 .5097 .4904 solution. -150 .5574 .5345 .5105 .4909 1 The numbers by which unresolved bands are designated in this and the following tables correspond to the group numbers used in previous chapters, since each band corresponds to a group in the resolved spectra of the crystallized salts. In figure 91, band 5 is seen to shift with falling temperature toward the violet, the shift amounting to 13 A.u. when a temperature of — 100° is reached. With further cooling to — 180° the band shifts back toward the red. It would be interesting to ascertain whether this shift toward the red would continue with further decrease in temperature. The other bands of the normal solution behave similarly with falling temperature, i. e., the entire spectrum undergoes a shift to the violet, followed by a reverse shift to the red. The wave-lengths of the bands at —180° are approximately the same as the wave-lengths at —60°. 184 FLUORESCENCE OF THE URANYL SALTS. Jones and Strong discovered a shift of 15 A.u. toward the red for the absorption band of wave-length 0.4910 when the temperature of their solution was raised from +5° to +84°. Our fluorescence band at 0.4927 shifts in the same direction, with a rise in temperature from -90° to +20°. H. Becquerel believed that any modification of the absorption spectrum is accompanied by a similar change in the fluorescence spec- trum, and these shifts lend strength to his generalization. A brief study of the fluorescence spectra of the 1/10 and the 1/100 normal solutions at different temperatures indicates that a similar temperature shift occurs. •60 -120 J L J L 30 70 J L 40 no TO BO TO BO /u ow *" "*w FIG. 91. — Uranyl sulphate — Temperature shift. (1) 1/1 normal, aqueous: (2) 1/10 normal, aqueous: (3) 1/100 normal, aqueoua; (4) 40 c.o. 1/10 normal, aqueous to 1 c.c. sulphuric acid; (5) 1 c.c. 1/10 normal, aqueous to 1 c.c. sulphuric acid. The increase in the amount of solvent produces a shift of the spec- trum toward the violet. For example, band 7 at —90° shifts as fol- lows: in the 1/1 normal solution the wave-length is 0.4919, in the 1/10 normal 04910, in the 1/100 normal 0.4907, in the 1/1000 normal 0.4904 M. The 1/1000 normal solution shows a spectrum which is very strongly shifted toward the violet The above comparison of the wave-lengths of band 7 fails to indicate the shift of the spectrum, because it is accom- panied by a marked decrease in interval, while of all the bands, number 7 is the least shifted. For example, band 4 of the 1/1000 normal at a temperature of —90° is of wave-length 0.5583, while band 4 of the 1/100 normal at —90° is of wave-length 0.5631 units. Bands 5, 6, and 7 show progressively less variance in wave-length with the correspond- ing bands of the 1/100 normal solution because of the shorter frequency of the 1/1000 normal interval. Measurements of these spectra plotted on a frequency scale indicate that while the bands are spaced by about 85.7 units in the 1/1, 1/10, and 1/100 normal solutions, the 1/1000 FROZEN SOLUTIONS. 185 normal bands are of only 82 units interval. (See table 109.) This may be due to a change in the ionization with dilution. To ascertain whether the rate of cooling caused a change in the spectra, a solution was suddenly plunged into liquid air and excited to fluorescence. Measurements were then taken on the bands, but no change in wave-lengths was observed. TABLE 109. — Uranyl sulphate in water. Frequencies and average intervals of fluorescence bands. Band. +20° 0° -35° -60° -90° -120° -150° -180° {?, 1605.4 1605.9 1605.1 1603 8 1602 3 Frequencies 3 1685.2 1686.9 1688.3 1688.3 1688 0 1688 0 in normal solution. 4 5 6 7 1772.7 1857.7 1944.4 2029.5 1772.7 1857.7 1945 . 1 2030.0 1772.7 1857.7 1945 . 1 2029.6 1773.9 1860.1 1946.7 2030.9 1774.6 1862.2 1948.9 2032.9 1774.9 1862.2 1948.2 2032.9 1774.9 1861.2 1947.0 2032.1 1774.3 1860.5 1945.5 2030.0 Average in1 t.. 85.6 85.8 86.1 85.1 85.4 85.6 85.7 85.5 ft 1605 6 1604 9 Frequencies 3 1691.8 1693 . 8 1693.8 1690 0 1688 9 in 1/10 4 1774.3 1775.9 1776.5 1779.0 1778.1 1776 5 1775 8 normal 5 1859.1 1861.2 1862.9 1864.3 1863 9 1863 3 1861 9 solution. fi 1945.9 1947.0 1949 3 1951.2 1950 5 1949 3 1948 2 7 2033 . 3 2035 . 4 2036 2 2036 . 7 2037 1 2035 4 2034 2 Average in1 86.3 86.5 86.1 85.7 85.8 86 0 85 9 Frequencies [3 1688 0 1686 9 in 1/100 4 1775.3 1774.9 1775.9 1776 5 1775 9 1773 7 normal 5 1861.2 1861 2 1862 9 1863 9 1862 9 1862 3 solution. 6 1947.4 1949.7 1951.2 1950.8 1949 . 7 1948 6 J 2035.4 2036.2 2037.9 2037 . 9 2037 1 2036 2 Average inl 86.7 87.1 87.3 87 1 87 3 87 3 Frequencies r4 1791.2 1794 0 1794 0 1794 0 in 1/1000 5 1876 2 1878 3 1870 9 1870 9 normal 6 ** 1960 0 1961 9 1958 9 1958 9 solution. 7 2039 . 4 2039 . 4 2037 1 Average in1 82.7 81 8 81 0 82 4 URANYL SULPHATE MIXED WITH SULPHURIC ACID. It has been observed that the bands of the aqueous solutions move toward the violet with progressive dilution with water; hence it was of considerable interest to ascertain the effect of dilution with sul- phuric acid. The addition of one volume of acid to 40 volumes of the 1/10 normal aqueous solution (table 110) produces a negligible effect, but a mixture of equal volumes shifts the bands back toward the red, in fact, the wave-lengths of the bands at +20° are longer than those of the normal solution — aqueous. This can be discovered from a comparison of the wave-lengths of the 1/10 normal solutions with those of the normal solutions in tables 108 and 110. The effect is not evident at low tern- 186 FLUORESCENCE OF THE URANYL SALTS. peratures, because the spectrum of the 1 to 1 acid solution persistently shifts toward the red instead of reverse shifting at —100° (see fig. 91). The frequency interval remains unchanged, with a proportionately large acid dilution. TABLE 110. — Uranyl sulphate in sulphuric acid. Temp., etc. Band 2. Band 3. Band 4. Band 5. Band 6. Band 7. '+ 10° 0.5634 0.5376 0.5140 0.4931 - 30° 0.5929 .5635 .5376 .5139 .4928 40 c.c. of 0.1 normal aque- - 60° 0.6238 .5924 .5631 .5373 .5135 .4919 ous solution with 1 c.c.- - 90° .6224 .5910 5621 .5365 .5129 .4919 of acid. -120° .6226 .5914 .5625 .5362 .5126 .4920 -150° .6234 .5918 .5627 .5365 .5128 .4925 .-180° .6242 .5919 .5630 .5367 .5130 .4925 + 20° .5945 .5657 .5388 .5149 .4921 - 30° .6246 .5935 .5640 .5376 .5139 .4916 1 c.c. of 0.1 normal aque- - 60° .6231 .5921 .5630 .5368 .5133 .4911 ous solution with 1 c.c.-1 - 90° .6215 .5906 .5616 .5359 .5124 .4905 of acid. -120° .6199 .5893 .5608 .5348 .5114 .4898 -150° .6203 .5893 .5607 .5347 .5113 .4897 -172° .6203 .5890 .5602 .5345 .5109 .4893 TABLE 111. — Uranyl sulphate in sulphuric acid. — Frequencies and average intervals of fluorescence bands. Band. + 10° -30° -60° -90° -120° -150° -180° '?, 1603.1 1606.7 1606.2 1604.1 1602.1 3 1686 7 1688 0 1692 0 1690.9 1689 . 8 1689.5 Frequencies of 40 c.c. of 0.1 normal aqueous- with 1.0 c.c. of acid. 4 5 6 7 1774.9 1860.1 1945.5 2028.0 1774.6 1860.1 1945.9 2029.2 1775.9 1861.2 1947.4 2032.9 1779.0 1863.9 1949.7 2032.9 1777.8 1865.0 1950.8 2032.5 1777.1 1863.9 1950.1 2030.5 1776.1 1863.2 1949.3 2030.5 Average interval 84.4 85.6 86.0 85.2 85.3 85.3 85.7 [? 1601 0 1604 9 1609 0 1613 2 1612.1 1612.1 Frequencies of 1 c.c. of 0.1 normal aqueous1 with 1 c.c. of acid. 3 4 5 6 7 1682 . 1 1767.7 1856.0 1942.1 2032.1 1684.9 1773 . 1 1860.0 1945.9 2034.2 1688.9 1776.2 1862.9 1948.2 2036.2 1693.4 1780.6 1866.0 1951.6 2038 . 7 1696.9 1783.2 1869.9 1955.4 2041.7 1696.9 1783.5 1870.2 1955.8 2042.1 1697.8 1785.1 1870.9 1957.3 2043.7 Average interval 87.5 86.6 86.3 86.0 85.7 86.0 86.3 URANYL POTASSIUM SULPHATE IN WATER. The spectra of the aqueous solutions of uranyl potassium sulphate, like those of uranyl sulphate, consist of a single series of bands. The temperature shift of the more concentrated solutions, e, g., the 1/15 and 1/150 normal, is at first toward the violet, followed by a reverse shift toward the red. The wave-lengths of the bands of the aqueous solutions are recorded in table 112. It will be seen that the shift toward the red is more marked than in the uranyl sulphate solu- tions. (See also fig. 92.) FROZEN SOLUTIONS. 187 The 1/1500 and 1/15,000 normal solutions yield bands which present a hazy appearance, lacking the pronounced crests of the more con- centrated solutions. For this reason the readings of such wave- lengths are more likely to be in error. The same tendency to first shift toward the violet and then shift toward the red is evident. TABLE 112. — Uranyl potassium sulphate in water. Solution and temp. Band 3. Band 4. Band 5. Band 6. Band 7. 1/15 normal solution. 1/150 normal < solution. 1/1500 normal < solution. 1/15000 normal • solution. '+ 20° 0° - 30° - 60° - 90° -120° -150° .-180° '+ 20° - 30° - 60° - 90° -120° -150° k-180° '- 30° - 60° - 90° -120° -150° k-180° - 60° - 90° -120° -150° -180° 0.5656 .5651 .5650 .5652 .5653 .5653 .5653 .5669 0.5388 .5387 .5386 .5386 .5386 .5387 .5393 .5406 .5398 .5391 .5388 .5393 .5394 .5403 .5411 .5378 .5368 .5373 .5382 .5382 .5385 .5257 .5338 .5233 .5234 .5235 0.5154 .5152 .5148 .5148 .5148 .5149 .5154 .5166 .5155 .5153 .5152 .5153 .5152 .5166 .5171 .5147 .5149 .5149 .5152 .5152 .5153 .5038 .5027 .5021 .5020 .5021 0.4932 .4928 .4926 .4927 .4925 .4928 .4933 .4945 0.5942 .5945 .5945 .5946 .5953 .5966 .5659 .5657 .5657 .5656 .5667 .5679 .5612 .5612 .5624 .5630 .5643 .5650 .5483 .5464 .5456 .5465 .5476 .4931 .4928 .4931 .4932 .4941 .4951 .4938 .4938 .4936 .4931 .4931 .4938 .5956 .5966 .4831 .4829 .4829 .4828 The frequency interval, as may be seen from table 113, suffers a marked change with dilution. The 1/15 and 1/150 normal solutions show spectral series of 86.8 and 85.5 average interval respectively, the 1/1500 series of 83.4 units, the 1/15,000 series of only 80.2 units. The 1/1500 series undergo marked increase of interval on cooling. The bands of the 1/15,000 normal are so greatly shifted that they lie approximately in the middle of the intervals between the bands of the 1/1500 normal solution. Such a "shift" of the entire spectrum must be due to a marked change in the molecular arrangement ; hence it can hardly be designated as a shift of the 1/1500 normal spectrum. Pre- sumably a new hydrate has been formed by the freezing of the 1/15,000 normal solution. The 1/150,000 normal solution gave a spectrum which was too dim to permit of measurement, except at the lowest temperatures. From the three bands which are visible, it appears that the frequency interval 188 FLUORESCENCE OF THE URANYL SALTS. is approximately 77 units. If so, it is the shortest frequency interval yet discovered in the fluorescence of the uranyl salts. A comparison of the location of the bands of the solid potassium sulphate with those of the 1/15 normal solution shows that the bands of the solutions are from 12 to 40 A.u. nearer the red, according to their wave-lengths. URANYL POTASSIUM SULPHATE IN SULPHURIC ACID. The addition of sulphuric acid in moderate proportions to the aqueous solutions of the potassium sulphate increases the intensity and improves the resolution of the bands. A solution of the 5 c.c. of 4/15 normal aqueous solution to 1 c.c. of acid was subjected to the cooling process. In figures 92 and 93 and table 114 the shifts will be observed. Band 5, which is typical of the other bands, shifted toward the violet by 21 A.u. at —120°, and then shifted 6 A.u. toward the red at — 180°. + 20 0° -60 -120 -180 1400 a 830C S400 5390 SI90 5370 J>»0 1170 StIO 5180 S3«4 FIG. 92. — Uranyl potassium sulphate — Temperature shift. (1) 1/15 normal, aqueous; (2) 1/150 normal, aqueous; (3) 1/1500 normal, aqueous; (4) 5 c.c. of 4/15 normal, aqueous to 1 c.o. sulphuric acid; (5) 1 c.c. of 4/15 normal, aqueous to 1 c.c. sulphuric acid. The addition of acid in larger proportion — 1 c.c. of acid to 1 c.c. of 4/15 normal solution — resulted in a spectrum which shifted toward the violet in a peculiar fashion with each decrement of temperature. (See curve No. 5 of fig. 92.) The total shift of band 5 amounted to 42 A.u. Further dilution with acid, e. g., 20 c.c. of acid to 1 c.c. of solution, resulted in another broad-banded spectrum at +20°. With cooling, however, partial resolution occurred. This is best observed in figure 94, where several narrow bands appear in the regions formerly occupied by the broad bands. Homologous components are lettered a, 6, and c. Still greater dilution — 50 c.c. of acid to 1 c.c. of aqueous solution — produced a spectrum which passed through the same development; FROZEN SOLUTIONS. 189 UR. POTASSIUM SULPHATE IN SULPHURIC ACID. 5:1 4-2 -30° -60° -90° -120* -wo* /T\ -180 leloo taloo zoloo •52 1 FIG. 93. TABLE 113. — Uranyl potassium sulphate in water. — Frequencies and average intervals of fluorescence bands. Band. +20° 0° -30° -60° -90° -120° -150° -180° [3 1682.9 1682 1 1682 1 1681 8 1679 8 1676 2 Frequencies in 1/15 normal solution. 4 5 6 7 1768.0 1856.0 1940.2 2027.6 1769.6 1856.3 1941.0 2029.2 1769.9 1856.7 1942.5 2030.0 1769.3 1856.7 1942.5 2029.6 1769.0 1856.6 1942.5 2030.0 1769.0 1856.3 1942.1 2029.2 1769.0 1854.3 1940.2 2027.2 1764.0 1849.8 1935.7 2022.2 Average ini 86.5 86.5 86.8 86.9 87.0 86.9 86.9 86.5 [3 1679 0 1676 1 Frequencies 4 1767.1 1767.7 1767.7 1768 0 1764 6 1760 9 in 1/150 5 1852.5 1854.9 1856.0 1854.6 1853 9 1850 8 1848 1 normal fi 1939.9 1940 . 6 1941 0 1940 0 1941 0 1935 7 1933 9 solution. 7 2028 . 0 2029 2 2028 0 2027 6 2023 9 2019 8 Average inl 87.4 87.0 87.2 86.8 86.5 86.2 85 9 Frequencies [4 1781.9 1781.9 1778.1 1776.1 1772 1 1769 9 in 1/1500 , 5 1860.1 1862.9 1861.2 1858 0 1858 0 1857 0 normal 6 1942.9 1942 1 1942 1 1941 0 1941 0 1940 6 solution. 7 2025 . 1 2025 1 2025 9 2028 0 2028 0 2025 1 Average in1 81 1 81 1 82 6 84 0 85 3 85 1 Frequencies f4 1823 8 1830 2 1832 8 1829 8 1826 2 in 1/15000 5 1902 2 1909 . 1 1911.0 1910 6 1910 2 normal fi 1984.9 1989.3 1991.6 1992 0 1991 6 solution. 7 2070 8 2070.8 2070 8 2071 3 Average inl 80 6 79.3 79 3 80 3 81 7 190 FLUORESCENCE OF THE URANYL SALTS. -± UR. POTASSIUM SULPHATE IN SULPHURIC ACID. 1:20 .^•r-v S^*\ -X" 1 "v -38° -90 -120" /TV -ISO" -180° ^ I6|00 ie|oo 20JOO FIG. 94. hence ii is clear that the presence of sulphuric acid in excess is essential to resolution of this type. The homologous components formed frequency intervals which were constant and of the same length as those of the parent bands; i. e., the homologous components form separate series. TABLE 114. — Uranyl potassium sulphate in sulphuric acid. Temp., etc. Band 2. Band 3. Band 4. Band 5. Band 6. Band 7. 6 c.c. of 4/15 normal aque- ous solution with 1 c.c. of acid. 1 c.c. of 4/15 normal aque- ous solution with 1 c.c. of acid. '+ 20° 0° - 30° - 60° - 90° -120° -150° -180° + 20° - 2° - 35° - 63° - 90° -120° -150° -180° 0.5392 .5388 .5382 .5379 .5377 .5371 .5373 .5377 .5385 .5378 .5374 .5364 .5362 .5350 .5347 .5343 0.5150 .5150 .5145 .5142 .6142 .5136 .5139 .5141 .5141 .5142 .5136 .5131 .5127 .5117 .5113 .5107 0.4931 .4933 .4927 .4928 .4926 .4921 .4926 .4928 .4921 .4918 .4916 .4911 .4908 .4898 .4896 .4892 0.5650 .5642 .5638 .5635 .5625 .5631 .5633 .5650 .5647 .5635 .5629 .5621 .5611 .5603 .5600 0.5917 .5917 .5911 .5914 .5917 0.6219 .6220 .6226 .5946 .5938 .5921 .5915 .5903 .5887 .5885 .6230 .6227 .6211 .6203 .6194 Temp., etc. Band 3. Band 4o. Band 5a. Band 56. Band 6a. Band 6b. Band la. Band 76. 1 c.c. of 4/15 normal aque- ous solution with 50 c.c/ acid. 0° - 30° - 60° ( - 90° -120° —150° -180° 0.5662 .5666 .5662 .5661 .5659 .5663 .5634 0.5388 .5394 .5388 .5387 .5385 .5380 .5376 0.5138 .5146 .5141 .5140 .5139 .5119 .5118 0.4914 .4916 .4911 .4905 .4902 .4897 .4895 ).5966 .5966 .5968 .5979 .5942 0.5158 .5151 0.4930 .4928 0.5401 FROZEN SOLUTIONS. 191 .52'^ .481^ 10 :o 10 :i S\ A\ 1:1 /T\ its 1:20 i:so A AA 1600 1800 FIG. 95. 2000 TABLE 115. — Uranyl potassium sulphate in sulphuric acid. — Frequencies and average intervals of fluorescence bands. Band, etc. +20° 0° -30° -60° -90° -120° -150° -180° '?, 1608.0 1607.7 1606 2 Frequencies ,3 1690.0 1690.0 1691.8 1690 9 1690 0 of 5 c.c. 4 1769.9 1772.4 1773.7 1774.6 1777.8 1775 9 1775 3 4/15 normal < aqueous with 1 c.c. acid. 5 6 7 1854.6 1941.7 2028.0 1856.0 1941.7 2027.2 1858.0 1943.6 2029.6 1859.1 1944.8 2029.2 1859.8 1944 . 8 2030.0 1861.9 1947.0 2032 . 1 1861.2 1945.9 2030.0 1859.8 1945.1 2029.2 Av. int. . . 86.7 85.8 85.7 84.8 84.8 84.8 84 5 84 6 Band, etc. +20° -2° -35° -63° -90° -120° -150° -183° '?, 1605 . 1 1605.9 1610 0 1612 1 1614 5 Frequencies 3 1681.8 1684.1 1688.9 1690.6 1694.1 1698.7 1699 2 of 1 c.c. 4/15 normal < aqueous with 1 c.c. acid. 4 5 6 J 1769.9 1857.0 1945.1 2032.1 1770.9 1859.4 1944.8 2033.3 1774.6 1860.8 1947.0 2034.2 1776.5 1864.3 1948.9 2036.2 1779.0 1865.0 1950.5 2037.5 1782.2 1869.2 1954.3 2041.7 1784.8 1870.2 1955.8 2042.5 1785.7 1871.6 1958.1 2044.2 Av. int. . . 87.4 87.9 87.5 86.2 86.3 86.3 86.1 85.9 Band, etc. +20° 0° -30° -60° -90° -120° -150° -180° f3 1676.2 1676.2 1675.6 1672.5 1682.9 4a 1766.2 1764.9 1766.2 1766.5 1767.1 1765.8 1774.9 Frequencies Ftn 1851.5 of 1 c.c. fib 1856.0 1853.9 1856.0 1856.3 1857.0 1858.7 1860.1 4/15 normal < fin 1938.7 1941.4 aqueous with fib 1946.3 1943.3 1945 . 1 1945.5 1945.9 1953.5 1953.9 50 c.c. acid. In 2028.4 2029 . 2 [7b 2035 0 2034 2 2036 2 2038.7 2040 0 2042 . 1 2042 . 9 Av. int. . . 89 6 89 8 90 0 90 6 91 1 92 4 90.0 192 FLUORESCENCE OF THE URANYL SALTS. The effect of dilution with acid at one temperature is given in table 115. The —180° vspectra of the 4/15 normal aqueous solution with varying proportions of acid are shown in figure 95. With the addition of acid the bands at first move toward the violet without resolving, then become stationary in position, and finally resolve. The ratio by volume of aqueous solution to sulphuric acid is given for each spectrum. The shift is not the same for the different bands, because the frequency interval, beginning with about 85 units for the aqueous solution, increases with increase of acid component to about 90 units in the 50 parts acid to 1 part water solution. With the exception of the two resolved spectra, the bands are too diffuse to permit of satisfactory intermediate measurements on the frequency intervals. Dilution with acid has undoubtedly increased the interval by 5 units, whereas dilu- tion with water decreased the interval by 8 units. URANYL CHLORIDE IN WATER. The absorption spectrum of the chloride is of particular interest, since Jones and Strong first located absorption bands1 in the fluores- cence region in an aqueous solution of this salt. Observations by the authors on the transmission spectrum of several crystals of the uranyl double chlorides of potassium, ammonium, rubidium, and caesium have TABLE 116. — Uranyl chloride in water. Solution and temp. Band 2. Band 3. Band 4. Band 5. Band 6. Band 7. !Q7° — y / 0.6247 0.5935 0.5639 0.5383 0.5142 0.4927 -120° -150° — ISO0 .6250 .6254 .6255 .5940 .5939 .5939 .5644 .5644 .5645 .5379 .5379 .5386 .5140 .5139 .5143 .4925 .4925 .4926 f- 90° .6245 .5931 .5643 .5382 .5141 .4926 I —120° .6246 .5935 .5643 .5382 .5142 .4929 normal < _15Qo .6252 .5936 .5643 .5382 .5141 .4926 solution. ^_lg()0 .6252 .5934 .5642 .5382 .5141 .4926 . f- 90° .6236 .5938 .5648 .5385 .5146 .4926 * , 1 -120° n,°rtmal -150° solution. 1cno [ — loU .6249 .6251 .6254 .5936 .5933 .5936 .5646 .5646 .5645 .5386 .5385 .5382 .5144 .5143 .5144 .4923 .4924 .4926 f- 90° .5941 .5645 .5382 .5144 .4923 0.05 _120° .6253 .5942 .5647 .5381 .5141 .4923 normal < _15QO Solution. icno (_ — loU .6254 .6252 .5939 .5935 .5649 .5646 .5381 .5382 .5142 .5144 .4923 .4924 resulted in the discovery of absorption bands in the same region. The view held by Jones that the fluorescence spectrum is a continuation of the absorption spectrum is to be gravely doubted, for while the chloride solution shows a fluorescence band at 0.4926 and Jones has established the position of an absorption band at 0.4920, none of the other bands 1 Jones and Strong. Carnegie Inst. Wash. Pub. No. 130, p. 90. FROZEN SOLUTIONS. 193 located by him at 0.6070, 0.6040, 0.6020, 0.6000. 0.5200, or 0.5185 coincide with a band of the fluorescence spectrum. Furthermore, it has previously been indicated that often the last band of a fluorescence spectrum coincides fairly well with a strong band in the absorption. It has also been shown in our study of the fluorescence and absorption spectra of the crystalline salts (see Chapters III to IX) that the interval between the absorption bands, although constant, is much smaller than that between fluorescence bands. The bands of the chloride in solution are separated by a very black background, but are so dim that cooling to —90° is necessary before measurements can be made. The bands continue to increase in brightness as the temperature is further decreased. The temperature shift between —90° and —180° is toward the red in the spectrum of the 3.0 normal solution. The measurements on the chloride, to be found in tables 116 and 117, indicate that difficulty is experienced in locating the positions of the bands. The remarkable TABLE 117. — Uranyl chloride in water — Frequencies and average intervals, fluorescence bands. Band. -97° -120° -150° -180° [2 1600.8 1600.0 1599.0 1598.7 Frequencies 3 1684.9 1683.5 1683.8 1683.8 in 3.0 4 1773.4 1771.8 1771.8 1771.5 normal 5 1857,7 1859.1 1859.1 1856.7 solution. 6 1944.8 1945.5 1945.9 1944.4 [7 2029.6 2030.5 2030.5 2030.0 Av. int. . . 85.8 86.1 86.3 86.3 [2 1601.3 1601.0 1599.5 1599.5 Frequencies 3 1686.1 1684.9 1684.6 1685.2 in 1.5 4 1772 . 1 1772.1 1772.1 1772.4 normal 5 1858.0 1858.0 1858.0 1858.0 solution. 6 1945.1 1944.8 1945.1 1945.1 J 2030.0 2028.8 2030.0 2030.0 Av. int . . . 85 7 85 6 86.1 86.1 r2 1603.6 1600.3 1599.7 1599.0 Frequencies 3 1684.1 1681.8 1685.4 1584.6 in 0.5 4 1770.5 1771.2 1771.2 1771.5 normal 5 1857.0 1856.7 1857.0 1858.0 solution. 6 1943.3 1944.0 1944.4 1944.0 7 2030.0 2031.3 2030.9 2030.0 Av. int . . . 86 3 86 2 86 2 86 2 '?, 1599.2 1599.0 1599.5 Frequencies 3 1683.2 1682.9 1683.8 1684.9 in 0.05 4 1771.5 1770.9 1770.2 1771.2 normal 5 1858.0 1858.4 1858.4 1858.0 solution. 6 1944.0 1945.1 1944.8 1944.0 7 2031.3 2031.3 2031.3 2030.9 Av. int . . . . 87 0 86 4 86 5 86 3 194 FLUORESCENCE OF THE URANYL SALTS. fact is that the bands of the 1.5, 0.5, and 0.05 normal solutions are not shifted by temperature, and that dilution from 3.0 normal to 0.05 normal produces a negligible shift. There is no tendency toward reso- lution. Clearly, the uranyl chloride in aqueous solution furnishes spectra of great stability, especially in view of the behavior of the bands of the uranyl nitrate. URANYL NITRATE IN WATER. The solutions of uranyl nitrate present at once the most interesting and most complicated spectra. In our first investigations the solutions were studied at —185° after suddenly plunging them into liquid air. Later it became of interest to study them at several intermediate NITRATE IN WATER. -180° ^rv A yi\/!\ 2. 4. /ML 5. II J I IN WATER. -180° M. XT\ NITRATE IN NITRIC ACID. -180* r.io 1:10 1:3.5 1800 20|00 FIG. 96. temperatures and the freezing and subsequent cooling was of necessity done slowly. To our surprise, the normal solution of the nitrate yielded an entirely different type of spectrum. Spectrum No. 1 at the top of figure 96 represents the old type and spectrum No. 5 the new type. It was found possible to produce intermediate degrees of resolu- tion somewhat similar to Nos. 2, 3, and 4 by intermediate rates of cooling. The pertinent fact is that the identical solution could, by FROZEN SOLUTIONS. 195 manipulation of the cooling process, be made to yield either an unre- solved or a highly resolved spectrum. The intermediate forms were not easy to reproduce at will. A comparison of the wave-lengths of the strongly resolved bands of the solution at —180° with those of the crystalline salt at the same temperature showed that they were identical. The uranyl ammonium nitrate and uranyl potassium nitrate in aqueous solution were similarly cooled and showed resolution of the same type. Resolution of this type has not been discovered in any other aqueous solutions, but in our first investigation uranyl acetate in alcohol was found to give highly resolved but quite dim bands superimposed on a continuous background. The spectra of the 1/100 normal solution were similarly affected by retarding the rate of cooling. Spectra L, M, and N of figure 96 show the results of successively slower rates of cooling. EFFECT OF TEMPERATURE ON SLOWLY COOLED SOLUTIONS OF URANYL NITRATE. Since the changes in the spectrum of the normal solution are very striking and are typical of the changes in many other more dilute solutions, a detailed account of the changes in this spectrum is given. Figure 97 gives a plot of the spectra. Some attempt at indicating the NITRATE IN WATER. 0° -35" -40° -60' -90° AA/ri ^ M -120° /TV . 1 -150° -180° 16100 .64* ^« ii JL- -il flft I 1 ZOIOO FIG. 97. form of the bands is made, but the changes in intensity are too great to be represented on such a plot. The wave-lengths are tabulated in table 118, and frequencies in table 119. At +20° only two broad bands located at 0.5323 and 0.5088 were of sufficient intensity to be measur- 196 FLUORESCENCE OF THE URANYL SALTS. able. With falling temperature these increased in brightness and two more bands came up to the threshold of vision. Bands 0.4890 at 0° corresponds with an absorption band at 0.4870 discovered by Jones and Strong. The crystalline nitrate, with 6 H2O, also has an absorp- tion band at 0.4870. While continuing the cooling process at a slow rate, a sharp rise in temperature from —25° to —18° was invariably noticed, probably due to undercooling or change in hydration. Immediately following this stage portions of the background increased greatly in brightness so as to broaden each band on the violet side. These very broad bands, which exist at temperatures between —25° and —40°, were found on subsequent cooling to be the parents of groups of resolved bands. The — 40° bands were five times the intensity of the +20° bands. TABLE 118. — Uranyl nitrate in water — normal solution. Temperature. +20° 0° -40° -60° -90° -120° -150° -180° Strong. . 0.6175 0.6174 .6039 .5855 0.6161 .6030 .5857 .5829 .5722 .5579 .5553 .5457 0.6183 .6022 .5861 .5827 .5713 .5577 .5554 .5454 0.6165 .6006 .5857 .5823 .5712 .5576 .5553 .5455 .5368 .5322 .5299 .5217 .5132 .5089 .5068 .5008 .4991 .4951 .4914 .4855 Weak. . Strong. 0.5932-0.5696 .5864 Weak. . . Dim. . . . .5723 .5584 Strong . . 0.5622 .5582 Dim. Dim .5461 Dim Strong. . Dim 0.5323 .5350 .5375-0.5182 .5323 .5331 .5299 .5219 .5322 .5300 .5216 .5322 .5299 .5214 .5129 .5090 .5068 Dim.. Dim. . . . Strong. . Dim .5088 .5110 .5129-0.4951 .5082 .5093 .5065 .5093 .5069 Dim Dim. . . . .4924-0.4831 .4999 .4997 .4997 Dim. . . . Dim. . . .4914 .4857 .4916 .4856 Strong. . .4890 .4862 .4858 At —46° the portion of each band toward the violet decreased in intensity as the part of longer wave-length became stronger, thereby tending to both narrow the band and produce a decided crest. It was found, with the aid of the spectro-photometer, that the intensity of the stronger crest at —60° was 85 times that of the homologous band at +20°. Further cooling resolved the stronger band into doublets without a real shift, but the dimmer component was not so easily resolved. At temperatures between — 120° and — 180° the strongly resolved doublets formed two series, both of a constant frequency interval of 88 units, the single band at 0.4885 being a member of one series. There was FROZEN SOLUTIONS. 197 JL NITRATE IN WATER. -30' -60* -90' A / V H20" yv -150° A -185" idoo leloo zoloo .6oU .561^. .52U FIG. 98. no shift here, but increasingly better resolution. The very dim inter- mediate bands resolved to form series of approximately the same interval. It was thought that by reversing the cooling process the spectra might go through the same forms at the same temperatures, which TABLE 119. — Uranyl nitrate in water — frequencies and average intervals of fluorescence bands. Band. -20° 0° -40° -60° -90° -120° -150° -180° J 1619.4 1619.7 1623.1 1622.6 1616.8 2\ 1655.9 1658.4 1660.6 1665 0 ( 1685.8-1755.6 1705.3 1705.0 1707 4 1706.2 1707 4 3 1715 6 1716 1 1717 3 I 1747.3 1747.6 1750.4 1750.7 ( 1778.7 1791.5 1790.8 1792.4 1793.1 1793 4 4 1800.8 1800.5 1800 8 { 1831.2 1832 5 1833 5 1833 2 f 1862 9 6 1878.6 1869.2 1860.5-1929.8 1878.6 1875.8 1887 1 1879.0 1886 8 1879.0 1887 1 1879.0 1887 1 1916.1 1917.2 1917.9 1916 8 1949.7 1948 6 1965.4 1956.9 1949.7-2019.8 1967.7 1963.5 1974 3 1963.5 1972 8 1964.6 1973 2 1965.0 1973 2 6I 1996 8 2030 9-2070 0 2000 4 2001 2 2001 2 2003 6 2019 9 _/ 2035 0 2034 1 2035 0 7l 2045 0 2056 8 2058 5 2058 9 2059 3 2059 7 Av. int 86.8 88.8 89.8 87.5 87.8 87.2 87.3 88.6 198 FLUORESCENCE OF THE URANYL SALTS. proved to be the case when the temperature was raised from —180° to —60°, but on further heating to —30° the —60° spectrum failed to change over to the very broad banded form. Finally, the temperature was raised to — 18°, the cryohydrate point. The original spectrum of the unfrozen solution then reappeared. -60° IN WATER. -90° -120° 7IV -ISO" /T\ A /TV A leloo ifloo zoloo .60U FIG. 99. The slow cooling of the 1/10 normal solution produced a series of spectra similar to the above, but dimmer and not quite as well defined. (See fig. 98.) The 1/100 normal, although too dim to be measured NITRATE IN NITRIC ACID. -180 20: 1 A A A » I I A y« A A A A A I yTV IA 10: 1 A A /\ A A I a A 2:1 J\A A A A AA\ \/\ A 1:2 I, A I » A yi\ ^i\ yrv C2.75 yr\ /TV A ino 1:75 1:5 1:1000 isloo 20100 FIG. 100. FROZEN SOLUTIONS. 199 until a temperature of —60° was reached, behaved similarly. (See fig. 99.) The more dilute aqueous solutions, e. g., the 1/200 and 1/500 normal, gave broad bands with no important shifts. The very dim, broad bands of the 1/1000 and 1/1 0,000 normal are probably due to the production of different hydrates. TJRANYL NITRATE IN NITRIC ACID. The spectra of the normal aqueous solution diluted with nitric acid in varying proportion are shown in figure 100. Data for 5 c.c. of acid are given in tables 120 and 121. TABLE 120. Uranyl nitrate in nitric acid (1 c.c. of normal aqueous solution with 5 c.c. of acid). -30° -60° -90° -120° -150° -180° 0.5958 .5818 .5673 .5527 .5399 .5280 .5163 .5045 .4935 .4825 0.5965 .5806 .5654 .5531 .5393 .5279 .5154 .5043 .4930 .4823 0.5817 0.5810 0.5808 .5681 .5528 .5412 .5274 .5165 .5042 .4941 .4824 0.5540 .5540 .5531 .5289 .5283 .5274 .5064 .5047 .5045 .4832 .4823 Uranyl nitrate in methyl alcohol (0.1 normal solution). Uranyl nitrate in ethyl alcohol (0.1 normal solution). -120° -135° -150° -180° -90° -120° -150° -180° 0.6045 .5882 .5780 .5600 .5503 .5341 .5252 .5104 .5025 .4887 .4815 0.5834 .5557 .5301 .5072 0.5836 .5552 .5300 .5072 .05788 .5521 .5270 .5040 .4889 .4828 0.5841 0.5894 .5804 .5605 .5525 .5345 .5272 .5106 .5041 .4887 0.5891 .5780 .5602 .5510 .5344 .5263 .5108 .5031 .4888 .4819 0.5537 .5287 .5069 .5569 .5318 .5091 .4887 The uppermost spectrum, denoted at the left by 20 : 1, was pro- duced by slowly cooling to — 180° and exciting to luminescence a solu- tion of 20 c.c. of the normal aqueous solution mixed with 1 c.c. of acid. The first effect of the acid was to bring out more distinctly the dimmest bands of the aqueous solution. There was no marked shift or change in resolution as the acid component was increased until the solution contained 1 c.c. of normal aqueous solution to 2 c.c. of acid. With further dilution, e. g., I c.c. of solution to 2.75 c.c. of acid, a marked change in the spectrum occurred, for only a broad-banded series of 200 FLUORESCENCE OF THE URANYL SALTS. TABLE 121. — Uranyl nitrate in nitric acid — Frequencies and average intervals of fluorescence bands. Frequencies in 1 c.c. of normal aqueous with 5 c.c. of acid. Band. -30° -60° -90° -120° -150° -180° 2 * { « { * { • { 7 Av. int . 1678.4 1718.8 1762.7 1809.3 1852.2 1893.9 1936.9 1982.2 2026.3 2072.5 1676.4 1722.4 1768.6 1808.0 1854.3 1894.3 1940.4 1982.2 2028.4 2073.4 1719.1 1721.2 1721.8 1760.3 1809.0 1847.7 1896.1 1936.1 1983.3 2023.9 2073.0 1805.1 1805 . 1 1808.0 1890.7 1892.9 1896.1 1974.7 1981.4 1982.2 2069.5 2073.4 84.8 87.6 88.1' 87.8 88.4 87.8 Uranyl nitrate in methyl alcohol — Frequencies in 0.1 c.c. normal solution in alcohol. Band. -30° -60° -90° -120° -135° -150° -180° 2 3 { * { * { • { ' { Av. int. 1654.3 1700.1 1730.1 1785.7 1817.2 1872.3 1904.0 1959.2 1990.1 2046.2 2076.8 1712.0 1966.6 1723.0 1784.1 1810.0 1870.9 1896.8 1958.5 1983.7 2046.2 1697.5 1730.1 1785.1 1814.9 1871.3 1900.1 1957.7 1987.7 2045.8 2075.1 1795.7 1880.4 1964.3 2046.2 83.6 87.4 87.1 86.8 Uranyl nitrate in ethyl alcohol — Frequencies in 0.1 c.c. normal solution in alcohol. Band. -90° -120° -150° -180° 3 4 5 6 ' ( Av. int. 1714.1 1799.5 1886.4 1971.7 1713.5 1801.2 1886.8 1971.6 1727.7 1811.3 1897.5 1984.1 2045.5 2071.3 1806.0 1891.4 1972 . 8 83.4 85.8 86.1 85.9 FROZEN SOLUTIONS. 201 doublets is present. This is probably caused by a change in the hydrate at this dilution. This type of spectrum persisted through five more dilute solutions, even when the solution contained only 1 part aqueous solution to 1,000 parts acid. The effect of slowly cooling five of the acid solutions is seen in figures 101, 102, 103, 104, and 105. Very often combinations of acid and aqueous solution proved to be unstable on freezing; consequently it was difficult to reproduce at NITRATE IN NITRIC ACID. + 20° -30° -60" A -90° -120' IV /\ A IV A -150" /T\ I'm /HA yn/i'l A f j\ -ISO' h , . , ll I I I t I I I I I I I I I I 16|00 leloo 20|00 .48J FIG. 101. NITRATE IN NITRIC ACID. -30° -45° -60° A -90° A A A -120 A -150° -180° ,/TV A. isloo 18|00 FIG. 102. i I 20|00 .1 I I I .52 202 FLUORESCENCE OF THE URANYL SALTS. NITRATE IN NITRIC ACID. 1:3.5 -30 -60' -90" -120° -150° -ieo leloo isloo 20|00 .48 | FIG. 103. -30° NITRATE IN NITRIC ACID. 1:5 -SO8 za ~I20° -|so° teloo isloo I 20JOO .521^ FIG. 104. NITRATE IN NITRIC ACID. to: I + 10° -30" - 60 -90" -120" -150° isloo 19100 zolpo FIG. 105. FROZEN SOLUTIONS. 203 will the spectra of such solutions. This appeared to be somewhat independent of the rate of cooling ; thus, it was discovered that a solu- tion might yield a spectrum consisting of a set of broad-banded doublets at one time and a narrower set of doublets differently spaced at other times. The three spectra shown at the bottom of figure 96 illustrate this phenomenon. The first two spectra, although entirely "different, were produced from a solution of 1 part normal aqueous solution with 10 parts nitric acid. The first was obtained after very slow cooling, the second after moderately slow cooling to liquid air. The second spectrum is identical with the third, obtained by slowly cooling a solution of 1 c.c. of normal solution to 3.5 c.c. of nitric acid. Such experiments lead to the view that the luminescence spectrum is determined by the particular hydrate which is formed on freezing. The frequency interval of an acid or aqueous solution was always constant. The change in interval through a wide range of dilutions was slight. The largest interval was of 87 + , the smallest of 84+ units. NITRATE IN WATER & ETHYL ALCOHOL. Kl -90° -120° -150" ^ £. 5-5 MITRATE IN ALCOHOL;ETHYL. -90' -120° -ISO* -IBS' isloo | leloo | 2o|oo .eolx* selx* .szL J- NITRATE IN ALCOHOL: METHYL. I A -120° X . -135' -185° A yiv ie|oo _ | _ 20)00 FIG. 106. 204 FLUORESCENCE OF THE URANYL SALTS. THE URANYL NITRATE IN ALCOHOL. The luminescence spectrum of the normal aqueous solution diluted with ethyl alcohol is distinctly different from that of the aqeous solu- tion. (See fig. 106.) The sharply resolved spectrum is quenched and the new bands are not in the same positions. The unfrozen solutions in a mixture of alcohol and water are not so opaque as the aqueous solutions; hence it is necessary to freeze them to produce sufficient absorption to bring out the luminescence. The first readings were taken when the temperature was —90°, and a consistent shift to the violet was effected by further reduction in temperature. The spectrum of a solution of uranyl-nitrate crystals in ethyl alcohol will also be found in figure 106. At -90°, -120°, and -150° slight change in form or wave-length occurs, but at — 185° fairly well resolved, crests protrude above the crests of the broad bands, still existent. It is probable that one series is due to the water of crystallization, the other to the alcohol. Jones and Strong1 have attributed the presence of two sets of absorption bands in the water and alcohol solutions to the presence of both a hydrate and an alcoholate, and the two lumines- cence spectra are undoubtedly caused by such a combination. ENERGY IN CRESTS OF BANDS. NITRATE AT -90. 6I80A 5870A 5586 A 5324A 5088A 4863 A FIG. 107. A solution of uranyl nitrate in methyl alcohol (fig. 106) presented bands which in the manner of development with temperature resem- bled the aqueous bands. The doublets fall into two series of constant intervals. It will be observed in figure 106 that the bands of the alco- holic solutions are in approximately the same positions. 1 Loc. cit., p. 104. FROZEN SOLUTIONS. 205 ENERGY DISTRIBUTION IN THE BANDS OF URANYL NITRATE. The normal aqueous solution at —90° was studied with the aid of the spectrophotometer and bar, the intensity of the crests of the bands being matched by the intensity of the acetylene flame at the same wave-length. These values were multiplied by the ordinates of the corresponding wave-lengths of the energy curve for acetylene.1 Figure 107 shows the manner in which the bands differed in intensity. The envelope is of the same form as that determined by Nichols and Merritt2 for the individual bands of the crystalline salts. SUMMARY OF CHANGES. The changes produced by slowly changing the temperature from +20° to -180° include: (1) An increase in intensity of the entire spectrum. (2) A shift which is more often toward the violet than toward the red, although both shifts may occur between the above temperatures. (3) A narrowing of the bands and in some solutions a resolu- tion of the bands. (4) A slight change in the frequency interval. (5) The formation of one or more definite hydrates. (6) A change in the form of the bands. The changes produced by dilution include: (1) A shift of the entire spectrum. (2) A change of interval. (3) A change in the hydrate. (4) A decrease in the resolution, excepting when small amounts of acid are added to an aqueous solution. (5) A decrease in intensity. CONCLUSIONS. (1) The constant-frequency intervals are due to the uranium oxide. (2) The small shifts are due to a change in the relative intensity of two or more components of a band. (3) The more remarkable changes in position are caused by the presence of a new hydrate. (4) The change in hydrate is probably often associated with a change in the crystal system, and when this phenomenon occurs a change in the grouping of the component bands occurs. The work on four double nitrates3 (Chapter VII) indicates that the crystal system is an important factor in the determination of the positions of the bands. (5) The invariable production of broad bands with extensive aqueous dilution is due to complete ionization. 1 Coblentz. Bureau of Standards, v. 7, No. 2, p. 259. 2 Nichols and Merritt. Physical Review (1), 32, p. 358. 3 Howes and Wilber. Physical Review (2), xi, p. 66. 1918. NICHOLS PLATE 1. (A) A double reversal in uranyl sulphate at 185° C. (B) The fluorescence of uranyl ammonium nitrate at 185°jC. (C) The polarized fluorescence and absorption of uranyl caesium chloride at 185° C. Photographic reproductions from the original spectrographs by Dr. R. C. Rodgers. APPENDIX I. CHEMISTRY OF FLUORESCING URANYL SALTS. The compounds studied in this work were those uranyl compounds which showed a bright fluorescence. These in general were salts of the stronger acids and usually double salts with the alkali metals. The further general characteristics were high solubility and much water of crystallization, i. e., the more water of crystallization the more intense the fluorescence, as in the case of the lithium manganese acetates. The nonfluorescing compounds of lower valence or those without the "uranyl" oxygen, as well as the sodium carbonate and zinconium oxide solutions of uranic oxide, which, though having peculiar and characteristic absorption, do not fluoresce, were not taken up. The particular groups taken up largely were the nitrates, chlorides, sulphates, and acetates, with potassium, rubidium, csesium, ammonium, and sodium in double salts. The phosphates, fluorides, oxalates, and tartrates and some double salts with the bivalent elements were studied in some cases. The material for use in this investigation was obtained at first from Kahl- baum. Later a number (25) of compounds were prepared by G. O. Cragwall in the Chemical Laboratory of Cornell University. The remainder were pre- pared by the authors. Cragwall's material was a large quantity of uranium residue originally from Kahlbaum, during the purification of which by con- version to ammonium diuranate and hen to the chloride the first ammo- nium uranyl chloride crystals with the resolved spectrum were observed. The material used by the author was chiefly uranyl nitrate hexahydrate purchased as chemically pure, but which was found to contain noticeable quantities of sodium nitrate crystals. This was dissolved in water, precipi- tated with ammonium hydroxide, washed by decantation to incipient suspen- sion, to which HC1 was added until nearly all the precipitate was dissolved. This leaves most of the iron in suspension if present in small quantities and was used to separate out iron in reworking material contaminated from spatulas, etc. On boiling this solution, if much iron is present it further coagulates and can be filtered, but if the acid concentration is low enough, quite a portion of the uranium separates as H2U04. The chloride solution was precipitated again with ammonia, washed, and redissolved in nitric acid. The nitrate was evaporated until the salt (trihydrate) began to crystallize out, and was then carefully heated until decomposition took place, with the formation of the red uranic oxide. Care was taken not to form the black uranous uranic oxide U308 by overheating. The red oxide containing some undecomposed nitrate was digested with water, which converted the oxide into the hydroxide, or acid H2U04, a bright yellow powder. This was washed free from nitrate by decantation and air-dried and formed the major part of the material used. Some stock uranyl acetate was used, but as this usually contains sodium acetate also, it is not advisable when preparing sodium-free salts to be com- pared with sodium triple salts. Some material was also precipitated as the oxalate, but this does not give complete precipitation, and as it gives the black UsOs on ignition, which is not as readily soluble, it is not of much value except for preparing oxalates. 207 208 FLUORESCENCE OF THE URANYL SALTS. For making triple sodium acetates, some material was precipitated as the sodium uranate, dissolved in sodium carbonate, and the solution treated with acetic acid, from which the sodium uranyl acetate crystallizes, leaving the sodium acetate in solution with very little waste uranyl salt. NITRATES. URANYL NITRATE. This is the commonest and best known of the uranyl salts, crystallizing ordinarily as the hexahydrate, which readily forms large, clear crystals by cooling or evaporation. It is prepared by dissolving either the uranic oxide or hydroxide H2U04 or the uranous uranic oxide U308 in nitric acid and crystal- lizing. This salt shows strikingly the property of most of the uranyl salts of strong acid, of dissolving noticeable amounts of the oxide in the neutral solution, so that a clear solution may be strongly basic. This oxide pre- cipitates on heating or evaporation. URANYL NITRATE HEXAHYDRATE. U02(N03)26H2O. The complete description1 of the crystal properties of this hydrate are given in Groth's Chemische Krystallographie, II, page 142. System rhombic; axial ratio a : b : c = 0.8737: 1: 0.6088. Forms b (010), a (100), making short rectangular prisms with pyramidal ends formed by b (111), usually cut also by q (Oil), making a a six-sided face and b eight- sided. The prism (110) was observed on one crystal which was deformed by growing near another. Specific gravity, according to Boedeker (1860), is 2.807. No statement is made as to cleavage, but it was found that very slight temperature changes produce a spontaneous cleavage, generally along q (Oil), so that the crystals can not be handled on a cold day and immersion in liquid air completely powders them. The optical properties are double refraction +, plane of axes 6 (010), acute bisectrix the c axis, apparent angle of optic axes 67° to 69°, mean index /3 1.495 to 1.502. The pleochroism, according to Schabus, gives bright yel- low-green parallel to a, b greenish yellow, c deep citron yellow. The fluorescence and absorption were investigated by Stokes2, E. Becquerel, Hagenback,3 and H. Becquerel.4 The tribo-luminescence was noticed by Herschel.5 Wasiljew6 gives the melting-point of the hexahydrate as 60.2° C. and gives the solubility curve for the hexahydrate in water. Silliman7 gave 1 de la Provostage, Ann. der Chim. Phys. (3), 5, 48. 1842. Schabus, Preischr. Wien (1855), 40. Sitz. berichte d. A. d. W. Wien, 27, 41. 1857. Rammelsberg, Neust. Fortsch. in de Kryst. Chem. Leipzig, 58. 1857. Lang, Sitz. ber. Wien, 31, 120. 1858. des Cloiseau, Annales des Mines (5), 14, 348. 1858. Quercigh, Riv. Min. crist. Ital., 4, 6-14. 1915. 2 Stokes, Phil. Trans., 142, 517, 520. 1852. 3 Hagenback, Poggendorff's Annalen, 146, 395. 1872. 4 H. Becquerel, Ann. Chim. Phys. (6), 14, 230. 1888. 6 Herschel, Nature, 60, 29. 1899. 6 Wasiljew, Chem. Zentralblatt 14, 2, n, 1527. 1910. Jour. Russ. Phys. Chem. Ges. 42, 577. 1910. 7 Silliman, Amer. Jour. Science (2), 27, 14. 1859. CHEMISTRY OF FLUORESCING URANYL SALTS. 209 59.5° C. which is probably not as accurate. Lowenstein1 gives the vapor- pressure of the saturated solution as 18 mm. approximately at 25° and the pressure of the equilibrium between hexahydrate and trihydrate as over 4 mm. The author found the two hydrates to be stable together at 5 mm. at 20°. The result is that the crystals always effloresce and fall to a yellow powder if left in the air, in the winter especially if the sun falls on them, and may deliquesce in the summer. Lescouer2 gives the vapor-pressure of the solution at 6° as 12 mm. and for the trihydrate below 3 mm. The author found that the hexahydrate dissolved in various concentrations of nitric acid at 20° C. in the following ratio: 1.6 grams of hexahydrate in 1 gram of 10 per cent HN03, 1.15 grams in 20 per cent, 0.8 gram in 30 per cent, 0.65 gram in 40 per cent to 70 per cent HN03. The values have not been determined accurately above 40 per cent on account of the complications due to the occasional formation of the trihydrate. These crystals were usually grown by evaporation in the room. For work on the polarization they were grown in thin plates tabular on a or b by putting small seed crystal in a solution of the depth desired for the thickness of the crystal, in the position desired. URANYL NITRATE TRIHYDRATE. UO2(NO3)23H2O. This hydrate is mentioned by Lescouer3 and by Ditte4 as being formed when the hexahydrate is heated to boiling. Drenkman5 and Schultz-Sellack6 found that on adding the hexahydrate to strong nitric acid and crystallizing by cool- ing or evaporation the trihydrate was obtained. Lebeau7 also obtained it by heating the hexahydrate on the water-bath or by evaporating the nitric acid solution in a dessicator over H2SO4 or KOH. Marketos8 mentions it as formed directly from the hexahydrate over sulphuric acid in a dessicator, as does also Forcrand.9 As can be deduced from the vapor-pressure data of Lowenstein and the author, this air-drying takes place as soon as the vapor- pressure of the water in the atmosphere goes below 5 mm. The best crystals are obtained by slow evaporation of the solution of the hexahydrate, dried on the water-bath in concentrated nitric acid in a dissicator over sulphuric acid and caustic potash or quicklime. The crystalline form was measured by G. Wyrouboff,10 who obtained his crystals by evaporating the neutral solution at 65°. System triclinic; axial ratio, a : b : c= 1.7753: 1 : 1. 4104. a 85°35'; /3_94°12'; 7 81°44'. Forms p (001), making pjates with h' (100), a' (101), and a* (201) on the edges, and c* (111) and b* (111) oblique-angled ends. The specific gravity was found to be 3.345. No cleavage has been noticed, although the crystals are likely to form with irregular cracks across or radiat- 1 Lowenstein, Zeit. Anorg. Chem. 63, 105-107. 1909. 2 Lescouer, Ann. Chim. Phys. (7), 7, 429. 1896. ^ Lcscoucr loc* cit* 4 Ditte, Ann. Chim. Phys. (5), 18, 337. 1879. Compt. Rend. 89, 643. 1879. 6 Drenkman, Jahrsber der Fortschritt Chem., 256. 1861. 6 Schultz-Sellack, Jahrsber. Fort. Chem., 365. 1870; Zeit. fur Chem., 646. 1870. 7 Lebeau, Bull. Soc. Chim. (4), 9, 299. 1911. 8 Marketos, Comptes Rendus, 155, 210. 1912. 9 Forcrand, Comptes Rendus, 156, 1044, 1207, 1954. 1913. 10 Wyrouboff, Bull. Soc. fran. Mineral, 32. 340. 1909. 210 FLUORESCENCE OF THE URANYL SALTS. ing from the seed. Schultz-Sellack gives the melting-point as 120° and Wasiljew as 121.5° C. It is really only a partial melting-point, as the dihy- drate is not completely soluble in the resulting solution. The solubility of the trihydrate in water above 60° or in nitric acid has not been determined, although Ditte gives a solubility of 14.39 parts of the trihydrate in mono- hydrated (91 per cent) nitric acid. URANYL NITRATE DIHYDRATE. UO2(NO3)22H2O. Ordway describes the dihydrate as resulting by boiling off the fused hexahydrate, which Lowenstein confirms. The latter finds it as the product of 6 days' dihydration over sulphuric acid of over 80 per cent strength, al- though Fourand finds 6 days required in a vacuum over strong sulphuric. Lebeau finds powdered hexahydrate converted to dihydrate in a vacuum desic- cator with concentrated sulphuric acid in 72 hours. Lebeau finds that on treating the hexahydrate with ether, two layers are formed, of which the ethereal layer can be dried with anhydrous calcium nitrate, which leaves the dihydrate on evaporation. It is to be noted in this connection that the ethereal solution, which is also used for separating uranium X, can not be boiled off, as it decomposes with explosive violence after some heating, liberating copious nitrous fumes.1 Lebeau also obtains crystals with ether of crystallization at 10° and —70°. The dihydrate may also be formed by adding dry U03 to fuming nitric acid (92 per cent), from which solution it is readily recrystal- lized. Wasiljew, crystallizing the dihydrate from fuming nitric acid (s. g. 1.502), finds quadratic tables of the rhombic system with strong fluorescence. The author found yellow plates with marked fluorescence at lower tempera- tures of probably rhombical pinacoid and pyramid, with some other forms. The crystals weather so rapidly, having a vapor-pressure of 0.2 mm., accord- ing to Lowenstein, that changing from one closed vessel to another usually tarnishes them so that little can be done in the way of measuring, handling for cleavage, etc. Wasiljew gives the melting-point as 179.3°. The mixture of dihydrate and solution obtained by melting the trihydrate goes over to solu- tion at about 160° and then goes unchanged except for slight boiling to 240°. URANYL NITRATE ANHYDROUS. UO2(NO3)2 or UO3.N2OB. Marketos produced anhydrous uranyl nitrate by heating the nitrate to 170° to 180° C., since total decomposition took place at 200°, and passing over it dry carbon dioxide saturated with nitric-acid vapors by bubbling through concentrated nitric and sulphuric acids. This produced a yellow amorphous salt soluble in water which decomposed ether, with the liberation of nitrous vapors. Forerand found that long heating above 125° C. in a current of dry carbon dioxide produced basic anhydrous nitrate and below 100° only a monohydrate. Twelve hours at 165 in a current of carbon dioxide charged with nitric-acid vapors gave U02(N03)2 + 1/31H2U04. The method evolved for producing anhydrous uranyl nitrate was to place in a train of U -tubes a tube containing uranic oxide made by heating the hydroxide or acid HVUC^ until it began to turn red and distilling over it nitric 1 Muller, Chem. Ztg., 41, 439, 1917; 40, 30, 1916. CHEMISTRY OF FLUORESCING URANYL SALTS. 211 anhydride, N20s. This was accomplished by having a reaction flask fitted to the system by a ground-glass joint, in which were placed phosphorus pentoxide and fuming (92 per cent) nitric acid in calculated amounts. From this, on heating to 50°, the N2O5 distilled out and was condensed by freezing mixture in the first U-tube, which served as a reservoir. When this was filled with solid N2O5 and a two-liquid layer of N2O5 and HN03, the flask was removed, the joint covered by a cap, and the anhydride distilled over on the uranic oxide by placing the reservoir tube in a bath of hot oil. No re- action took place between the oxide and the acid until the oxide tube was in turn put in the oil-bath and the anhydride boiled off into the last tube, which served as a second reservoir for the acid. As soon as the acid began to boil the reaction took place, producing a vivid green fluorescence and a light yel- low color instead of the reddish oxide. The anhydride could be distilled off and run back over while holding the tube with the uranyl nitrate at any temperature. Also, any acid which did not solidify could be poured off and the remaining N205 run back over the nitrate, insuring absolute freedom from water. The resulting compound was found to be stable up to 180°, at which temperature it broke up into N205 and U03, which could be recombined if the temperature was lowered. Distilling the acid on and off was performed several times with one specimen, examining the spectra each time, which showed first the anhydrous salt fluorescence and then none for the oxide. DOUBLE NITRATES. Meyer and Wendel1 prepared double salts of ammonium, potassium, rubi- dium, caesium nitrates with uranyl nitrate. These crystals were described by Steinmetz.2 They were grown from a solution in nitric acid and were of the type KUO2(N03)3. Rimbach3 endeavored to determine the solubility of these salts in water at various temperatures. He found large crystals in the am- monium and potassium solutions unlike those of Meyer and Wendel which were measured by Sachs4 and assigned formulae like those of Meyer and Wen- del, but since they were alike were called isomorphous and the a forms of NH4 and KU02(NO3)3. Examination of the spectra of these forms in the labora- tory indicated and Sachs's data itself proves that the a form is simply uranyl nitrate hexahydrate. In attempting to grow crystals according to Rimbach's method which would not be uranyl nitrate, however, two new forms were discovered containing two molecules of alkali nitrate to one of uranyl nitrate. In the process of growing the potassium salt for experimental purposes, still a third was found, but so rarely that it was not studied. In order to find out the conditions under which the various salts were formed, a series of solubility determinations were undertaken, being run at constant temperature of 20° C. in a thermostat, with varying percentages of aqueous nitric acid as a solvent. From these incomplete results it will be seen that from solutions of less than 30 per cent nitric acid and less than 1 molecule of uranyl nitrate to 1 of potassium nitrate, potassium nitrate only will crystallize; that in a 1 to 1 1 Meyer and Wendel, Ber. d. d. Ch. Ges., 36, 4055. 1903. 2 Groth's Chem. Kryst, n, 150. 3 Rimbach, Ber. d. d. Ch. Ges., 37, 472. 1904. 4 Sachs, Zeit. f. Krys., 38, 497. 1904. 212 FLUORESCENCE OF THE URANYL SALTS. solution above 40 per cent the so-called 7 form or monopotassium salt will appear. In the 1 of uranyl nitrate to 2 of potassium nitrate, the double nitrate crystallizes only above 50 per cent of nitric acid, and as the d phase or the dipotassium salt and the metastable phase at this concentration is the 7 form. Presumably at higher concentrations of potassium nitrate the last- found and undetermined form would appear. Grams of solute in 100 grams of solvent. Solvent (p. ct. HN03). KNO3.UO2(NO3)2. 2KNO3.UO2(NO3)2. KN03. 0 85.5 82.0 99.8 89.0 81.3 54.0 33.9 Solid phase. KNO3 KNO3 KNO3 Hex 7 7 7 Solid phase. 62.9 52.2 45.7 51.8 67.2 52.3 Solid phase. KNO3 KNO3 KNO3 KNO3 KNO3 5 Solid phase. 31.4 19.1 14.5 11.4 15.2 18.6 19.6 29.6 34.2 48.8 10 20 30 89.5 7 40 50 57.5 7 60 70 80 90 Solvent (p. ct. HNO3). NH4NO3.UO2(NO3)2. 2NH4NO3.UO2(NO3)2. NH4.N03 o 165 128.6 80.3 68.2 60.5 61.4 38.6 Solid phase. Hex. Hex. Hex. Hex. Hex. ft ft Solid phase. 251 201 150 144 98.2 58.0 35.6 Solid phase. Hex. Hex. Hex. Hex. ft ft ft 380 380 215 150 Solid phase. ft ft ft ft 191 151 127 104.8 86.4 76.5 10 20 144.5 144 95.6 ft ft ft 30 40 50 60 Solid phases appearing are: Potassium nitrate, KNOs. Ammonium nitrate, NH4NOs. Uranyl nitrate hexahydrate, UCMNOs^ 6H2O, Hex. Monopotassium uranyl nitrate, KUO2(NO3)3, y. Dipotassium uranyl nitrate, K2UO2(NOs)4, 5. Monoammonium uranyl nitrate, NH4UO2(NC>3)3, /3. From solutions 1 molecule of uranyl nitrate to 1 of ammonium nitrate, uranyl nitrate hexahydrate crystallizes unless the per cent of nitric acid is at least 50, above which the /3 or monoammonium form crystallizes, which is metastable practically to water solution. From 2 molecules of ammonium nitrate to 1 of uranyl the hexahydrate crystallizes up to 40 per cent, above which the /3 form appears, which is also metastable to pure aqueous solution. It will be noted that while potasisum nitrate is about as soluble as uranyl nitrate and the phase in equilibrium with the more acid solutions corresponds to the composition of the solution, the ammonium nitrate is much more solu- ble and not only does not form the solid phase in case uranium is present, but does not form the diammonium salt from solutions of that composition. Laboratory experience showed that a large excess of ammonium nitrate and CHEMISTRY OF FLUORESCING URANYL SALTS. 213 rather low acid concentration was necessary to produce this form. These higher ratios of ammonium nitrate should be investigated. These results do not check well with those of Rimbach, who presumably crystallized considerable portions of the salt, instead of determining the phase with which the solution was in equilibrium by the addition of seeds of known phases. They do not, however, materially conflict with those of Engel1 in the case of the solubility of potassium nitrate in nitric acid, where the solubility is found greater at 20° than at 0° in dilute solutions and less in highly acid solution. The mono or acid forms of the double potassium ammonium salts and the corresponding rubidium and caesium salts were found to be as described by Sacks from the preparation of Meyer and Wendel. There is no indication that the corresponding double salts containing 2 atoms of rubidium or caesium could not be produced by using solutions similar to that used for the dipotas- sium salt. Other double uranyl nitrates with other bases than the four alkalies dis- cussed do not seem to form, with the exception of thallium, which is reported by Meyer and Wendel as forming but being non-fluorescent, as the double thallous sulphate is. Sodium nitrate crystallizes side by side with the hexa- hydrate or trihydrate, according to the acidity of the solution, but no con- ditions were found under which the two salts would crystallize together. Silver, cadmium, zinc, calcium, barium, and magnesium were also tried with- out success, although a modification of the hexahydrate spectrum was pro- duced by the calcium and magnesium. Meyer and Wendel also tried lithium, sodium, and the bivalent metals without formation of double salts. MONOPOTASSIUM URANYL NlTRATE. (y form) KUO2(NO3)3. These crystals were prepared by Meyer and Wendel by crystallizing potas- sium nitrate and uranyl nitrate in equal proportions from a nitric-acid solu- tion. The crystals were examined by Steinmetz. System rhombic; axial ratio 0.8541: 1: 0.6792. Thick tabular combinations of c (001), m (110), with subordinate forms of b (010), s (102), o (111), sometimes a (100), and rarely a (Oil) and 122. Steinmetz and Sykes report good cleavage on 6, and good cleavage was also observed on a. The specific gravity was found to be 3.503. Crystals of this form are stable at 20° if the partial pressure of the water-vapor is not over 9 mm. Hg, but at that point begin to deliquesce, changing to a whitish yel- low chalky mass. On heating the crystals, yellow crusts begin to form on the crystals at 150°, violent decrepitation begins at 200°, and decomposition with liberation of nitrous fumes at 270° C. According to Steinmetz, the plane of the optical axes is c (001), acute bisectric a. Axes visible through (110). The best crystals were obtained by cooling of hot solutions supersaturated 2 grams in 50 c.c. in glass-stoppered bottles. The composition as determined by Meyer and Wendel was KU02(N03)3. An ignition run to check this gave 65.13 and 64.82 per cent, the theoretical form K2U207 being 67.29, the low values being due to loss by decrepitation. 1 Engel, Comptes Rendus, 104, 913. 1887. 214 FLUORESCENCE OF THE URANYL SALTS. DIPOTASSIUM URANYL NITRATE. (5 form) K2U02(NO3)4. These crystals appeared, after a year of effort to obtain crystals from neutral solution which were not hexahydrate, in a slightly acid solution containing an excess of potassium nitrate. It shows marked reluctance to appear and does not grow well if the room temperature is below 20° C., the hexahydrate forming instead, but above that temperature gives fine crystals, especially if seeded, although it does not grow as rapidly as the other members of the group. System monoclinic; axial ratio a : b : c = 0.6394 : 1 : 0.6190; /3 = 90±. calc. obs. calc. obs. c: a =001 100 = 90° 0' o:ir =133 232=19° 19' 20° 44' p:p'=331 331 = 63° 30' o: q =133 131 =52° 3' 51° 50' c:p =001 331 = 77° 38' o:d =133 101 =45° 38' 47° 35' c:d =001 101 =50° 1' 52° 52' p:ir =331 232 =23° 39' 22° 49' c: o =001 133 =42° 43' 43° 10' p:d =331 101 =38° 42' 38° 25' c: q =001 131 =59° 22' 60° 39' p: q =331 131 =43° 37' 42° 12' o: o =133 133 =73° 58' 73° 15' d:q =101 131 =56° 31' 56° 17' o:p =133 331 =42° 58' 42° 48' d:ir =101 232=37° 6' 38° 56' o:p'=133 331 =84° 19' 84° 0' d:m = 101 230=61° 7' 62° 48' These axes are probably not those of the space lattice, being taken from the first habitus observed, which formed in a solution having barely enough potassium nitrate to produce this phase, and consisted of e (001), o (133), and p (331) meeting in a point in front which was sometimes cut off by a (100), usually accompanied by d (101). TT sometimes occurred between o (133) and p (331) and q (131) between o 133 and p 311; b (010) and m (230) were found in measurement. One crystal showed c, o, p, d, and d' (101), a and probably q and (111). Most of the crystals grown later had a prismatic or needle habitus in which p (331) was the predominant form, with small e faces on the ends and occasionally some of the other forms. All faces on these crystals gave reflections which appeared in the goniometer as a flattened figure 8, and best agreements were found in the angles taken from the outside of every pair of readings. In case the whole figure did not appear, results were unsatisfactory. On dissolving a large crystal in the mother-liquor by heat, c (001) was un- touched, d (101) was left even and a little pitted, and the edge between p (331) and o (133) was rapidly dissolved, leaving the deepest etching where these met at d (101). No conspicuous cleavage was noticed. The specific gravity was found to be 3.359. On heating crystals of this phase, they first decrepitate to an opaque yellow powder at 200° C., which, at 260° C., flows together. Above this temperature decomposition sets in, with the evolution of nitric fumes. Various colored masses result, deep red U3O8, bright red U03, bright yellow K2UO4, and on cooling a beautiful rose pink pervades the mass. This phase does not change over concentrated sulphuric acid, i. e., has zero vapor-pressure at 20° C., but over acid corresponding to 11 mm. of mercury of partial pressure of water-vapor it turns whitish without becoming moist, probably due to the formation of KN03 and UO2(N03)2 6H20. This is the same pressure at which the diammonium salt deliquesces. The refractive index of dipotassium uranyl nitrate given by the faces (001) and d (101) were found to be 1.5422 for light vibrating parallel to the 6 axis CHEMISTRY OF FLUORESCING URANYL SALTS. 215 and 1.5349 for light vibrating in the ac plane 26° 34|' from a toward c in the "acute" angle /3. The composition was investigated by igniting to K2U04 and by washing out the K2S04 from sulphuric-acid solution precipitated with NH4OH and weighing the resulting U308 and K2SO4. Theoretical. First. Second. p. ct. p. ct. p. ct. K2U04 . . 63.78 64.61 63.52 K2SO4. . . 47.09 46.85 47.03 U308.... 29.19 30.36 29.01 The best crystals were obtained by cooling solutions supersaturated 1 gram in 100 c.c. MONOAMMONIUM URANYL NlTRATE. 0 form NH4UO2(NO3)3. This salt forms from solutions containing uranyl nitrate and ammonium nitrate at room temperature if the acid-content is high and from water at higher temperatures. System trigonal; axial ratio a :c = 1: 1.0027 (a 97°6/). The forms are prismatic combinations of prism a (1120) with the rhombohedron r (1101) on the end, on the edges of which,occur s (1012). Theory. Steinmetz. W. r:r=1101 : 1011 ... 81° 47' 81° 54' 82° 8' s:s=0112 : 1102 ... 51° 19' 51° 30' 51° 36' The column headed Theory gives the values for a substance having an axial ratio a :c = 1:1, the close approximation to which makes this a remarkable case and probably indicates something concerning the structure. Twins were observed in which the contact plane was s 1102 and the twin- ning axis, the axis of reference to which s was parallel, making the angle be- tween the two unique axes 119°54' and giving the crystal the appearance _pf a flat hemimorphic orthorhombic crystal. The angle between the two r (lIOl) faces was calculated to be 21°32' and found to be 21° =±= . Crystals of this form are stable in dry air, but begin to deliquesce and change to a light yellow chalky mass if the vapor-pressure of water is above 9 mm. Hg. Intense pleochroism was observed in crystals about 0.01 mm. thick on a microscope slide, when they occurred lying on a prism face, the light vibrat- ing parallel to the unique axis appearing white, i. e., less yellow than the mother-liquor in which the crystal lay, while that ordinary ray appeared a deep yellow. The monopotassium and monoammonium crystals separate from the same solution, there being no tendency to form mixed crystals. The composition of the crystals was checked as being the same as that given by Meyer and Wendel by igniting to the oxide. Theory, 59.22 per cent found, 58.97, and 58.86. 216 FLUORESCENCE OF THE URANYL SALTS. DIAMMONIUM URANYL NITRATE. a form (NH4)2U02(N03)42H2O. This phase crystallizes from slightly acid solutions of uranyl nitrate con- taining a large excess of ammonium nitrate. Its solubility increases very rapidly with rising temperature, until at about 60° C. the uranyl nitrate dis- solves out, leaving a residue of ammonium nitrate. This salt crystallizes very readily in large sulphur-yellow perfect crystals of 5 or 10 grams from a volume of solution that only gives 1 or 2 grams of monoammonium salt and crystallizes best if the room temperature is 10° to 15°. The fluorescence is very faint at 20° C., but increases rapidly below 0°, becoming stronger than that of the monoammonium salt at liquid-air temperatures. System monoclinic axial; ratio a : b : c = 0.8419: 1 : 0.5594; j8 94°55'. The habitus resembles that of a cube with octahedron, the forms being c (001), a (100), b (010), and o (111). The faces p (110) were also occasionally observed, and possibly (211). No cleavage was observed, thus increasing the resemblance to sulphur crystals. However, on heating rapidly, the crystals fill with cracks, and con- sequently seeded crystals often have a large single crack more or less parallel to a (100). Etch figures produced by resolution in a crystal in the mother-liquor were found once, the distinct forms being on b (010), with two rounded faces meet- ing in a line in the bottom as though a lens had been pressed in. The bottom edges were all parallel and about halfway between the edges of b (010) and o (111), i. e., parallel to (102). The specific gravity was found to be 2.777. When these crystals are heated slowly they break down at about 140°, giving a pasty mass full of bubbles, which clears up slightly at 220°, but does not give a clear solution below 240°. On cooling, bright green crystals of the monoammonium salt form in a background of white ammonium nitrate. Crystals of this phase placed over sulphuric acid with a pressure of water- vapor of 4 mm. started to lose water, turning to a whitish powder, and con- tinued to do so slowly at 5 mm., although then they do not start. Placed over sulphuric acid with a vapor-tension of 11 mm., they deliquesce rapidly, soon going completely into solution. The refractive index was observed in two different directions, using natural faces. First, between 111 and Til giving the light vibrating in the ac plane nearly parallel to the edge olllrolll, with an index of 1.546, and that at right angles to the ac plane as 1.639; between the faces b 010 and o 111, giving for light appearing on b to vibrate nearly parallel to edge between 6 (010) and o' ill as 1.508, and at right angles to that 1.619. The composition was determined by ignition to U308, which gave 47.68 and 47.62 per cent against theoretical 47.58. URANYL CHLORIDE. Neither the monohydrate U02C12H2O described by de Coninck1 as being formed by evaporating the solution prepared by precipitating the sulphate solution nor the trihydrate, which, according to Myh'us and Dietz,2 forms from 1 de Coninck, Comptes Rendus, 148, 1769. 1909. 2 Mylius and Dietz, Ber. d. d. Ch. Ges., 34, 2774. 1901. CHEMISTRY OF FLUORESCING URANYL SALTS. 217 the evaporation of the solution of the oxide in hydrochloric acid, were suc- cessfully prepared and freed from the sirupy mother-liquor so as to give good fluorescence spectra. DOUBLE CHLORIDES. The alkali double chlorides, as was discovered early in this investigation in the case of the ammonium salt, give resolved spectra at room temperature. This makes the group quite important. The four double salts of ammonium, potassium, rubidium, and caesium were prepared. Attempts were made to prepare the double salts with silver, cadmium, zinc, and calcium, and also hydrazine and hydroxylamine, but resulted in each case in the formation of the crystals of the chloride added, in a sirup or mat of the uranyl chloride. The silver was sealed in a tube with a strong HC1 solution as solvent, but although remaining white did not dissolve and recrystallize. The tetramethyl and tetraethyl ammonium chlorides described by Rimbach were not made. The alkali double chlorides in general were grown by evaporation in a des- iccator in presence of an excess of HC1, which is necessary to prevent hydrol- ysis of the uranyl chloride. This forces back the solubility of the alkali salt, so that the solutions usually contain an excess of uranyl chloride. The crystals, if allowed to stand in the open air, readily give off acid, turning the color of any indicator paper on which they are placed and in moist atmosphere deliquesce readily, the sirupy uranyl chloride running away from a skeleton of alkali chloride. POTASSIUM URANTL CHLORIDE. K2-UCvCl4-2H2O. This salt was described by de la Projvpstage1 as occurring in hexagonal tables on c (001) founded by o (111), w (ill), m (110), /x (110), 6 (010), q (021). The crystals measured by Rammelsberg were mostly prismatic along the a axis. The first type were those used in this work, although crystals tabular on b (010) were fairly frequent. System triclinic; axial ratio, a: b: c: = 0.607 : 1 : 560. a = 80°41'; 0 = 77°42'; K2UO2Cl4.2H2C ). (NH4)2UO; C14.2H2O. Calculated. de la Provostage. Rammelsberg. Grailich. W. m b = (110): (010). . H b= (1TO) :(OI"0). . 60° 28' 61° 33' 60° 30' 61° 10' 60° 52' 60°30'=t 61° 0' 60° 48' H c = (110): (001). . 83° 48' 83° 55' 83° 0' 85° 26' b c = (010) :(001) 99° 15' 98° 53' 99° 34' a c = (012): (001) . 55° 21' 55° 30' 55° 45' 56° 22' o c = (111) :(001) .. . 60° 15' 59° 40' w' c = (ill) :(001). . . 46° 55' 47° 4' o b = (Til) :(010) . . 81° 0' 80° 0' w' b= (Til): (010) .. . 75° 20' 75° 30' w' q= (111):(012) .. . 46° 17' 46° 5' o M' = (111) :(T10). . . 66° 33' 66° 45' m : c = (110) :(001). . . 76° 8' The properties of the crystals were similar to those of the ammonium salt, although the crystals seemed to grow larger more readily. 1 de la Provostage, Ann. Chim. Phys. (3), 6, 165. 1842. 218 FLUORESCENCE OF THE URANYL SALTS. In the work of Jones and Strong,1 it was found that the absorption bands persist far out into the red, only the intensity decreases with such rapidity that great depths of solution were required to show them. To try this out in the case of the resolved spectra of the chlorides, thick layers were built up of several crystals and found out to hold. A very deep crystal was grown in a glass tube ground into the bottom of an inverted bottle-neck which held the solution. This crystal, which was 3 cm. thick, was never tried. Another investigation which was never finished was that of the char- acter of the spectra of mixed crystals, of which potassium ammonium salt K'NH4-U02'Cl4'2H2O was prepared as an example. AMMONIUM URANYL CHLORIDE. (NH4)2-U02-Clr2H2O. The crystal forms of this salt are practically identical with that of the potassium salt, as shown by the table of angles under that salt. Intense pleochroism is noticed in this crystal when viewed through the c (001) face if the crystal is about 1 mm. thick. The light vibrating parallel to the b (010) edge of the face, i. e., parallel to the axis, is so little absorbed as to appear white and is also least refracted. The light vibrating nearly parallel to the 6 axis is strongly absorbed in the blue-violet and appears deep yellow even in these crystals. The refractive indices parallel to a and nearly parallel to b and c were measured on prisms cut so that light traveled parallel to the c face and at right angles to it. It happens that the letters of the refractive indices correspond to the axes to which they are nearest. a 6 c X720. 1.564-1.566 1.619 1.622 X580. 1.566-1.574 1.633 1.637 X500 1 576-1 581 1 650 The absorption is so great parallel to 6 that the value for X 500 could not be obtained. RUBIDIUM URANYL CHLORIDE. Rb2-UO2-CU-2H2O. This is similar to the potassium and ammonium salts; although no measure- ments w'ere taken, the crystals could not be distinguished, except by knowing the individual crystals. CESIUM URANYL CHLORIDE. Cs2U02Cl4. The salt was crystallized as above from a solution containing caesium chlo- ride and uranyl chloride and presented a distinctly different appearance from the other members of the group. This is accounted for by the composition, which, according to Rimback, Wells and Boltwood,2 is the anhydrous chlo- ride instead of containing 2 molecules of water, as with the other salts. The crystals were elongated rhombs of yellow color, showing less fluorescence than the other salts. Under the polarizing microscope they showed a striking 1 Jones and Strong, Carnegie Inst. Wash. Pub. No. 130, 90. 2 Wells and Boltwood, Zeit. Anorg. Chem., 10, 181. 1895. CHEMISTRY OF FLUORESCING URANYL SALTS. 219 resemblance to gypsum, possessing the "fish-tail" twins and approximately the same angles and appearing different only in the yellow absorption. The crystals were so universally twinned that the interfacial angles could not be determined certainly. The largest face was 6 (010), with the two prism faces m (110) and /x (iTO), and, as determined by measurement, practically all the end faces were d (Oil), r (031), s (031), although these usually appeared twice on a crystal, and other faces indicated by the measurement were gill, x ( ill). System triclinic. fc:m = 010:110 = 49° 7' m: g = 110:lll =43° 41' 6:^ = 010:110 = 50° 50' m: r = 110:031 =86° 49' 6:d = 010:011=40°31' M: r = 110:031 =44° 0' 6:r = 010:031=67° 48' fj.:x = 110: 111 =75° 24' b: s = 010:021 =57° 41' The refractive index was determined through the faces 6 (010) and /z (ll0), the more deviated ray vibrating at an angle of about 15° from the prism edge toward the a axis. X720 1.618 1.692 X580 1.625 1.695 X500 1.634 1.714 The index was also determined through the faces 6 (010) 'and d (Oil), the less-deviated ray vibrating parallel to this prism edge. X580 1.614 1.691 X580 1.622 1.698 URANYL SULPHATE. UO2SO«-3H2O. This salt was prepared by Cragwall, presumably as the trihydrate. On recrystallization of Kahlbaum material two forms appeared, a yellow opaque needle-mass tending to replace the bright green fluorescent grains. The yellow needles became more numerous on adding sulphuric acid, so uranic oxide was added to saturation, which proved to be in excess on evaporation. The bright green fluorescent crystals were difficult to keep, as they dried out readily in the air. Microscopic examination of the Cragwall preparation showed needles with parallel extinction and greater absorption and greater index the long way of the crystals, while the angle of the optical axes was very large and the sign positive. The acid sulphate, H2U02(S04)25420, was reported by Wyrouboff.1 DOUBLE URANYL SULPHATES. The double sulphates differ from the previous double salts in the occurrence of the sodium salt. The potassium, ammonium, rubidium, caesium, and thallous salts were also prepared by Cragwall in the form of powders result- ing from rapid precipitation by cooling. The potassium and rubidium salts especially showed very strong fluorescence, the sodium and ammonium good fluorescence, the caesium less, and the thallium practically none, and that was not resolved. Rimbach2 describes a dipotassium salt which was not pre- pared and one of hydroxylamine also. 1 Wyrouboff, Bull. Soc. fran. min. No. 32,351, 1909. 2 Rimbach, /. c., 479. 220 FLUORESCENCE OF THE URANYL SALTS. These salts were all prepared by crystallization from water of the calculated quantities of the two single salts. Rimbach1 describes the potassium ammo- nium and rubidium salts as having each 2 molecules of water of crystallization. On the other hand, de Coninck2 describes the sodium potassium and Cfiesium salts as having 3 molecules of water, while the ammonium commonly has 2, but can be made by special conditions with 3 molecules. POTASSIUM URANTL SULPHATE. K2UO2(S04)22H2O. This salt separates readily on cooling a hot saturated solution of the two salts in equimolecular proportion. This, according to Rimbach, gives the dihydrate, according to de Coninck3 the trihydrate. The salt prepared in this way is a fine crystalline powder, larger masses being clusters of crystals. A good crystal was discovered in an old solution of known strength. They were then obtained by supersaturating 0.1 to 0.5 gram of salt in 50 to 200 c.c. of solution, seeding, and allowing the tightly stoppered solution to stand from 3 to 6 months, especially in the fall, when the room temperature gradually decreases. The crystals tend to form rosettes, clusters of crystals arising from the middle of the basal pinacoid. The smaller crystals are tabular on the base; the larger ones have large prism faces made up really of repeated pyramids. These are capped by the unit pyramid with brachydome and basal pinacoid. System rhombic; axial ratio a : b : c = 0.5889: 1: 0.6253. cole. 6:m = 010:110 = 32° 1' c: n = 001:101=43° 18' c: 0 = 001:201= c: fc = 001:011=30°30' c: 1 = 001: 021= c: 3 = 001:112 = 29° 4' obs. 32° 13' 43° 0' 62° 3' 30° 19' 49° 20' 30° 0' calc. c: p = 001:lll=48° 2' c: r = 001:332=59° 3' c: s = 001:221 =65° 47' c: t =001:331 =73° 18' c:w = 001:441 = 77°20' obs. 48° 6' 59° 30' 64° 45' 71° 52' 77° 25' SI & C || to b. || to a. || to c. X720... 1.5610-1.5627 1.5220 1.5096 X580. . . 1.5670-1.5705 1.5266 1.5144 X500. .. 1.5785-1.5847 1.5350 1 . 5202 No pleochroism was observed. Plane of axes a (100), obtuse bisectrix normal to base. Double refraction +. Cleavage was observed on the base. RUBIDIUM URANYL, SULPHATE. Rb2UO2(SO4)42H2O. The rubidium salt was more fluorescent than the potassium salt and mor difficult to crystallize, so that measurements of it were not obtained. It is, however, completely isomorphous with the potassium salt. The composition, according to Rimbach,4 is as above, with 2 molecules of water. 1 Rimbach, I. c., 478. 2de Coninck, Bull. Acad. Roy. Belg., 1904, 1171; 1905, 50, 94. 3 Ibid., 1905, 50. 4 Rimbach, I. c., 479. CHEMISTRY OF FLUORESCING URANYL SALTS. 221 CAESIUM URANYL SULPHATE. Cs2UO2(SO4)23H2O. This salt is so insoluble that no crystals could be produced. The Crag wall product recrystallized showed on the microscope-slide square plates about 10 jit in length, which showed a negative uniaxial figure. This salt, according to de Coninck,1 has the same composition which he finds for the potassium salt, that is, 3H20. AMMONIUM URANYL SULPHATE. (NH4)2U02(S04)22H2O. This salt was not recrystallized, but showed similar characteristics to the potassium salt. When recrystallized it gave bundles of needles, the vibra- tions across which were most absorbed and most refracted. Rimbach de- scribes the salt as having 2 molecules of water. The crystals were found to be monoclinic by de la Provostaye.2 SODIUM URANYL SULPHATE. Na2UO2(SO4)23H2O. This salt is described by de Coninck as having 3 molecules of water, which he finds also in the potassium salt. This salt as prepared by Cragwall was not recrystallized, but showed under the microscope one optical axis and the acute bisectrix with positive double refraction. It is, therefore, presumably triclinic, as the acute bisectrix was off normal in both directions. THALLOUS URANYL SULPHATE. T12UO2(SO4)23H2O. This salt was prepared by Cragwall from weighed amounts of the two salts according to Kohn,3 who found the salt to be of the above composition with probably 3H20. The crystal description by Himmelbauer in the same article gives the system as rhombic, the symmetry from etch figures pyramidal. The forms are the three pinacoids with pyramid faces at the corners which were too small to measure. He observed through (100) in converged polarized light the plane of the axes for red and blue, at right angles the plane of the blue being that of the a and c axes; for green nearly uniaxial; for red light a is the acute bisectrix, the pleochroism on (100) distinct, parallel to c, deep yellow; parallel b yellowish white, but no noticeable pleochroism on (010). Crystals up to 3 mm. in diameter and 1 mm. thick, produced by slow cooling of the Cragwall salt, showed the axial figure, but it could not be surely seen to agree with the description, due to the intense absorption in the blue and green. The figure might be explained by anomalous dispersion due to the absorption band. PHOSPHATES. Uranyl phosphate (HUO2PO4.3|H20), which precipitates from uranyl solu- tions on adding phosphates, possesses no fluorescence. If it is dissolved in an excess of acid it gives a glass or sirup with a brilliant fluorescence which can not be resolved beyond the bands. The sodium double salt was made by adding sodium phosphate to produce H2Na2C02(P04)2 to the uranyl phosphate with an excess of water, which on standing and evaporating gave 1 de Coninck, Bull. Acad. Roy. Belg., 1905, 94. 2 de la Provostaye, Ann. Chim. Phys. (3), 5, 51. 1842. 3 Kohn, Z. Anorg. Chem., 59, 111. 1908. 222 FLUORESCENCE OF THE URANYL SALTS. a fine crystalline mass which was very fluorescent. The spectrum of this was studied. The potassium, ammonium, lithium, and calcium salts were also prepared and seen to have characteristic line spectra, but were not studied further. The mineral autunite is a basic calcium uranyl phosphate which Stokes1 says shows brilliant fluorescence, while chalcolite, the analagous copper compound, has none, but shows the same absorption bands characteristic of uranyl compounds. CHROMATES. An attempt was made to prepare the sodium uranyl chromate described by Rimbach, which resulted in a brown mass. The uranyl chromate UO2CrO4- 3H20 (Orloff)2 from TJ03 and Cr03 gave yellow needles with no fluorescence. The potassium salt from K2Cr207 and UO3 was also without fluoresecence. FLUORIDES. Cragwall prepared the uranous and uranyl fluoride fromUsOgand HF, which showed practically no fluorescence. He also prepared the double potassium salt K3U02F5 by adding KF to uranyl nitrate and (NH4)3 U02F5 by dissolving (NH4)2 U207 in HF.3 The double salts showed characteristic spectra, but the fluorescence was very weak. URANYL IODATE. This salt was prepared from sodium iodate and uranyl nitrate by Cragwall by a method which, according to Artmann,4 would result in U02(I03)2H20 of the rhombic form. This showed little fluorescence. MISCELLANEOUS INORGANIC COMPOUNDS. An attempt was made to produce bromides and iodides analogous to the chloride salts without results, due to decomposition with the liberation of bromine and iodine. An attempt was also made to produce molybdyl and tungstyl ammonium chloride double salts analogous to the uranyl salts by heating the oxides with ammonium chloride and hydrochloric acid in sealed tubes, which in some cases resulted in crystals, which, however, showed no fluorescence. Uranic acid or H2UO4 was also sealed up in tubes with anhy- drous liquid NH3, C02, S02, and HC1. None of the resulting compounds were soluble or fluorescent, although changes took place, the carbonate being nearly white, the sulphur-dioxide tube greenish, due to reduction, and the ammonia tube reddish like the diuranate. Of the uranates, the sodium potassium calcium and barium were made, none of which showed fluorescence, the first two being golden yellow plates, the latter two an amorphous greenish mass. URANYL ACETATES. The anhydrous uranyl acetate U02(C2H302)2 was. prepared by Cragwall according to Spath5 by adding acetic anhydride to uranic oxide. This latter took up some water and became partially the dihydrate. On recrystallizing some of the material from acetic-acid solution, small clear cubes were obtained which appeared to contain acetic acid of crystallization. 1 Stokes, Phil. Trans. Roy. Soc. London, 142, 518. 1852. 2 Orloff, Chem. Ztg., 31, 375. 1907. 3 H. F. Baker, Chem. Soc. Jour., 35, 763-769. 1879. 4 Artmann, Z. Anorg. Chem., 79, 327, 1913. 6 Spath, Monatsh. J. Ch. 33, 248. 1912. CHEMISTRY OF FLUORESCING URANYL SALTS. 223 URANYL ACETATE DIHYDRATE. This salt was prepared by Crag wall by recrystallizing the anhydride from water solution. There was also a stock of material from Kahlbaum and un- known sources. In an attempt to recrystallize this salt in large, clear crystals much difficulty was met, as it usually fills with cracks as it grows. The best material, having as much as 4 mm. cube of clear material, was obtained by supersaturating 2 grams in 200 c.c. and allowing a month or two to crystallize. The crystal properties were found to be similar to those described by Schabus,1 the system being rhombic, with an axial ratio of 0.7817: 1: 0.3551, with the forms TO (110), a (100), r (101), n (120), and b (010). The prism zone is very much striated, affording a continuous procession of reflections in the gonio- meter. Schabus finds cleavage on m, a, b, c, which accounts for their extreme friability. Zehenter2 finds the specific gravity to be 2.893. The refractive index was determined through the dome as being 1.490 for light vibrating parallel to the c axis and 1.521 parallel to b. The uranyl acetate trihydrate, which, according to Schabus, forms below 10° C., was not prepared. It crystallizes in the tetragonal system with an axial ratio of a : c = 1: 1.4054. DOUBLE URANYL ACETATES. The sodium salt occurs in the acetates as well as the sulphates and is the only uranyl salt crystallizing in the regular system. The ammonium and potassium salts seem to be isomorphous, in spite of the fact that the potassium salt is said to contain 1 molecule of water and the ammonium salt to be anhy- drous. The silver salt, which contains 1 molecule of water, is apparently isomorphous and the rubidium salt was found to have a similar axial ratio, as usual very near that of the ammonium salt. The similarity of axial ratio to that of the uranyl nitrate trihydrate suggests that these salts form a group that might well be studied further. The caesium salt could not be obtained, the uranyl dihydrate crystallizing out and leaving the caesium acetate in solution. The two hydrates of lithium uranyl acetate fully described by Wyrouboff3 as being monoclinic were attempted, but only the room-tempera- ture form was obtained. The double salts of uranyl acetate with bivalent acetates were in general prepared by dissolving the oxide or carbonate of the second metal in acetic acid in excess, adding uranyl acetate in calculated amount, with water enough for complete solution, and allowing to crystallize by slow evaporation. Difficulties were encountered in the preparation of some of the salts, such as the calcium salt, which Rammelsberg also could not obtain, as described by Weselsky,4 which was finally prepared by Weselsky method of precipitation with calcium carbonate and solution of the precipitate in acetic acid. The barium salt was finally prepared by this method. The cadmium was never prepared at all; at least, no specimen that gave anything but the uranyl-acetate spectrum. An attempt to produce a mercuric acetate also failed. 1 Schabus, Prieschr. Wien, 207. 1855. 2 Zehenter, Monats f. Ch., 21, 235. 1900. 3 Wyrouboff, Bull. Soc. fran. min., 8, 115-122. 1885. 4 Weselsky, J. Prakt. Chem., 75, 55. 1858. 224 FLUORESCENCE OF THE URANYL SALTS. The double acetates as a group were studied by Wertheim,1 Schabus,2 Grailich,3 Weselsky,4 and Rammelsberg.5 The acetate group in general show much less intense fluorescence, tending to be of a dull yellow color. The uranyl double acetates with bivalent metals may be divided into two classes — the normal and the abnormal. The group HUC^^HsC^sSH^O appears to act as a unit in forming crystals. In the alkali double salts the water of crystallization seems to be lacking, at least in the well-confirmed cases of the sodium and ammonium salts. The case 'of the manganese, cad- mium, and lead double salts seem also to be an exception, but with the other double acetates the ratio of uranium acetate to bivalent acetate seems to be 2 to 1. The water of crystallization is variously given from 7, which was found uniformly by Rammelsberg, to 8 by Wertheim and 10 by Grailich. Since the water is likely to run high, due to occluded mother-liquor, and is such a small per cent of the total weight, it is not unreasonable to assume that these really are all hexahydrates. The manganese, when satisfying this valence ratio, and magnesium salts seem also to have 2 molecules of water to each uranyl radical. In the case of the triple salts this requirement is exactly fulfilled, each valence of base having a UC^^HsC^SE^O group attached to it. In the case of the manganese, cadmium, and lead salts, this radical does not seem to act, but simply the two acetates are present in a 1 to 1 ratio. The spectra of the manganese salt was like that of the other double acetates; the cadmium was not formed or else gave a spectrum like that of the single ace- tate, and the lead was one of the salts which showed fluorescence lines coinci- dent with spark lines, so that no generalization can be made. SODIUM URANYL ACETATE. NaUO2(C2H3O2)3. This well-known salt described by Grailich6 crystallizes in the regular system with the least or pentagonal dodecahedral symmetry. It is usually in the form of tetrahedra, yellow, with light green fluorescence. Johnsen7 gives the specific gravity as 2.562 and the refractive index as 1.5014. Marback8 and Traube9 give the optical rotation as 1.48°. The best crystals, up to 3 mm. in thickness by 8 mm. diameter, were obtained on long standing of slightly supersaturated solutions. Dr. Nishikawa tried to obtain X-ray diffraction patterns with these crystals, but could obtain nothing. POTASSIUM URANYL ACETATE. KUO2(C2H3O2)3H2O. This salt was described by Wertheim10 as having 1 molecule of water, which was also found by Schabus11 and Rammelsberg.12 A recent determination by 1 Wertheim, Jour, of Prakt. Chem. 29, 207-231. 1843. 2 Schabus, Best. d. Kystall gest. i. chem. Lab. Erz Prod. Preischr. Wien. 1855. 3 Grailich, Kryst Opt. Untersuchung. Preisch. Wien, pp. 151-175. 1858. 4 Weselsky, Jour. f. Prakt. Chem., 75, 55-62. 1858. 5 Rammelsberg, Sitz. ber. Acad. Wiss. Berl., 857-887, 1884; Wied. Ann., 24, 293-318, 1885. 6 Grailich, Preisschr. Wien, 151. 1858. 7 Johnsen, N. Jahrbuch of Min. B. B., 23, 259. 1907. 8 Marback, Pogg. Ann. d. Phys., 94, 422. 1855. 9 Traube, Liebisch Grundries der Phys. Kryst., 327. 1896. 10 Wertheim, J. Prakt. Chem. 29, 223. 1842. 11 Schabus, Sitz. ber. k. Akad. Wiss Wien, 857. 1884. 12 Rammelsberg, Wied. Ann., 24, 293. 1885. CHEMISTRY OF FLUORESCING URANYL SALTS. 225 Zehenter1 gives 1/2 H20. Since the isomorphous ammonium salt is without water, and since both potassium acetate and uranyl acetate are hygroscopic and the salt occurs in needles, it seems likely that the water is not in the crystals. The crystals are mostly prism and pyramid of the tetragonal system, with the axial ratio a:c = 1: 1.2831, according to Schabus. The specific gravity is given by Zehenter as 2.396. The best crystals were obtained by slow cooling, the tendency being to form needles. It was prepared by dis- solving weighed potassium carbonate in an excess of acetic acid and adding the required amount of uranyl acetate. AMMONIUM URANYL ACETATE. (NH4)U02(C2H302)3. This salt was prepared and measured by Rammelsberg,2 who found the axial ratio 1: 1.4124 in the tetragonal system apparently isomorphous with the potassium salt. Grailich and Schrauf3 assigned 1 molecule of water to this salt, but Rammelsberg denies this. Zehenter gives the specific gravity as 2.219. This was prepared by Cragwall by crystallizing equal molecular quantities of the two salts together. RUBIDIUM URANYL ACETATE. RbUO2(C2H302)3.?H2O. This salt was prepared by crystallizing uranyl acetate in calculated amount with rubidium acetate produced by evaporating off rubidium chloride several times with acetic acid. On cooling, tetragonal needles separated out, but on further evaporation the uranyl acetate separated, leaving the rubidium acetate in solution. The crystals were measured, giving an angle o:m = (111): (110) = 26°33', which corresponds to an axial ratio of a: c = 1: 1.4151. SILVER URANYL ACETATE. AgUO2(C2H3O2)3H2O. This salt was prepared and measured by Wertheim, who found it tetra- gonal, with an axial ratio of 1.5385. He assigns 1 molecule of water of crystallization, which seems doubtful. This was prepared from calculated quantities of uranyl acetate and silver acetate dissolved in aqueous acetic acid. If left in the light the solution decomposes, which resulted in many of the crystals being covered with a black coating of silver. These crystals are more inclined to be granular. LITHIUM URANYL ACETATES. LiUO2(C2H3O2)33H2O. This is given by Wyrouboff4 as crystallizing in the monoclinic system with the axial ratio a: b: c = 1.2647: 1: 1.5849; /3 = 99°53'. It forms readily on crystallization of water solution of the two acetates, but does not give very good crystals. The other hydrate, LiUC^^HsC^sSH^O, with forms below 15° was not obtained. The solution was made by adding to weighed lithium carbonate which had been treated with an excess of acetic acid a calculated amount of uranyl acetate. This was put in a desiccator over dehydrated potassium acetate in an unheated room in winter, but the SH^O phase was not found. 1 Zehenter, Monatsh. f. Chem., 21, 235. 1900. 2 Rammelsberg, Sitz. ber. Acad. Wiss. Beri. 1859. 3 Schrauf, Sitz. ber. d. Acad. Wiss. Wien, 41, 779. 1860. 4 Wyrouboff, Bull. Soc. fran. Min., 8, 115. 1885. 226 FLUORESCENCE OF THE URANYL SALTS. MAGNESIUM URANTL ACETATES. Mg(UO2. (C2H3O2)3)27H2O. This salt was found by Wertheim,1 who assigned it 8 H2O, examined by Grailich,2 who gave 1OH2O, and finally given 7H2O by Rammelsberg.3 It was found to crystallize in the rhombic system with the axial ratio 0.8944 : 1 : 0.9923, one of the series of five isomorphous salts Mg, Fe, Co, Ni, and Zn. Lang4 gives negative double refraction, axes in (001) plane, bisectrix a 2E = 100°, and p < v. This salt was prepared by evaporation of a solution made by adding to a weighed sample of MgO an excess of acetic acid and the cal- culated amount of uranyl acetate. The crystals grow fairly large, up to 5 mm. with some readiness. Mg(UOz(C2H3O2)3)212H2O. This hydrate, according to Rammelsberg,5 forms at low temperatures in large, rapidly weathering crystals with an axial ratio of 0.7667:1:0.5082. According to Grailich and Lang,6 the double refraction is negative, the axes in a (100), bisectric c, 2E=13° for red and 10.5° for blue. The fluorescence is given as very strong emerald green. This hydrate was not obtained for examination. CALCIUM URANYL ACETATE. The salt was prepared by Weselsky and examined by Grailich.7 Rammels- berg attempted to repeat this determination and could not obtain good crystals. Grailich found them to be of the rhombic system, with an axial ratio of 0.9798:1:0.3865, with many faces. Lang8 found interior twins. Grailich found no pleochroism, but greenish-blue fluorescence. This salt was produced according to Weselsky's precipitation method after the addition method had failed. STRONTIUM URANYL ACETATE. Sr(UO2(C2H3O2)3)26?H2O. This salt was also prepared by Weselsky and could not be obtained by Rammelsberg. It was measured by Grailich, who reports it as being in the tetragonal system, with an axial ratio of 1 : 0.3887. No trouble was found in producing it from strontium carbonate, acetic acid, and uranyl acetate. BARIUM URANYL ACETATE. This salt was first produced by Wertheim and9 later described by Rammels- berg,10 both of whom found 6 molecules of water, but neither of whom obtained crystals good enough to measure. The Weselsky method was also necessary to produce this compound, the first attempt giving only the uranyl acetate crystals. 1 Wertheim, 1. c., 225. 6 Lang, Sitz. her d. Akad. Wiss Wien, 108, no, 562. 1899. 2 Grailich, 1. c., 152. 'Grailich, I. c., 159. 3 Rammelsberg, Wied. Ann., 24, 303. 8 Lang, I. c., 107. 4 Lang, 1. c., 107. 9 Wertheim, I. c., 230. 6 Rammelsberg, Sitz. ber., 869. 10 Rammelsberg, I. c., 300. CHEMISTRY OF FLUORESCING URANYL SALTS. 227 ZINC URANYL ACETATE. Zn (U02(C2H302)3)2 7H20. This sal£ was prepared by Weselsky1 and examined by Grailich.2 Rammels- berg found the axial ratio to be 0.8749 : 1 : 0.9493 in the rhombic system. Grailich and Lang3 found negative double refraction with the axes in c (001), bisectrix 6. The color, according to Grailich, is yellow with weak pleochroism; fluorescence is faint greenish. This salt from zinc oxide, acetic acid, and uranyl acetate crystallized after some trouble in rhombic plates with little fluorescence. The uranyl acetate sometimes separated without the formation of the double salt. This belongs to the isomorphous magnesium group. CADMIUM URANYL ACETATE. Cd.UO2.(C2H3O2)46H2O. These crystals were made by Weselsky4 and examined by Grailich, who found an axial ratio of 0.6289 : 1 : 0.3904 in the rhombic system. These crystals were also analyzed by Rammelsberg,5 who found that the composition did not correspond to that of the zinc salt, but had a 1 to 1 ratio of bivalent cadmium to uranium, thus making it like manganese and lead. These crystals could not be prepared, nothing but the uranyl acetate spectra being obtained. MANGANESE URANYL ACETATE. MnUO2(C2H3O2)46H2O. These crystals, prepared by Weselsky6 and examined by Grailich,7 were found to have the axial ratio of 0.6330 : 1 : 0.3942 in the rhombic system by Rammelsberg. The optical properties, according to Lang, are double re- fraction, negative, plane of the axes a (100), bisectric c, 2E = 31°, with a yellow color. When prepared by evaporation of an acetic-acid solution of manganese carbonate and uranyl acetate, small yellow crystals were obtained. Mn [(U02) (C2H3O2)3]2 12H2O. These efflorescing crystals, obtained by Rammelsberg8 from the 1 : 1 solu- tion of the acetates while warm, which solution later deposited the other hydrate, is isomorphous with the magnesium dodecahydrate. Rammelsberg found it to be of the rhombic system, with the axial ratio of 0.7536:1:0.4957. LEAD URANYL ACETATE. PbU02(C2H3O2)44H2O. This salt was obtained by Wertheim9 and later by Rammelsberg,10 who gave it the above formula, which puts it in the 1 : 1 class with cadmium and man- ganese. The crystals are very readily formed as needles of any length — the greater the supersaturation the longer the needles. Some fairly compact plates were obtained as a second crop from a highly supersaturated solution. These were made from lead acetate and uranyl acetate in weighed proportions. TRIPLE ACETATES. These salts were discovered by Rammelsberg11 in an attempt to get a copper uranyl double acetate and are found when sodium acetate is present with the bivalent metals, magnesium, manganese, iron, nickel, cobalt, copper, zinc, 1 Weselsky, . lc., 58. 6 Rammelsberg, 1. c., 887. « Wertheim, I. c., 227. 2 Grailich, 1. c., 171. 6 Weselsky, I. c., 59. l° Rammelsberg, Wied. Ann., 314. 3 Lang, 1. c., 51. 7 Grailich, I. c., 175. n Ibid., 315. 4 Weselsky, 1. c.,61. 8 Rammelsberg, I. c., 872. 228 FLUORESCENCE OF THE URANYL SALTS. and cadmium salts being reported. They are characterized by a temperature dimorphism being above a given temperature, trigonal, and below that mono- clinic twins and pseudotrigonal. They all have the same type formula de- rived from the monovalent uraniacetic acid HU02(C2H3O2)33H20, which takes on in this case 1 atom each of monovalent metal (sodium in all cases studied) and 1 bivalent metal. These formula may also be written NaOH-fR (OH)2+3UO2(OH)2 + 9HC2H302, which would explain the exact relation between the number of acetic radicals and molecules of water of crystallization. This series was studied by Erb1 and by Wyrouboff .2 The temperature change was studied by Schwarz.3 This group is characterized by the pseudotrigonal appearance of a large basal pinacoid, six-sided, due to rhombohedron faces which are uneven, due to twinning. They are readily distinguished from uranyl acetate or the metallic acetate or uranyl double acetates by this char- acteristic shape. They do, however, resemble the sodium uranyl acetate considerably if it has uneven faces, due to the varying composition of the solution as it crystallizes. Under the crossed nicols, however, there is no question, the very characteristic twinning surfaces showing unmistakably. SODIUM MAGNESIUM URANYL ACETATE. NaMg[(UO2(C2H3O2)3 3H2O2]3. These crystals, according to Wyrouboff,4 are simply monoclinic if grown below 15° C., but as the temperature rises the twins become more numerous, until at 50° the whole crystal becomes trigonal. This process then reverses on cooling. Erb5 describes them as sulphur-yellow crystals which weather readily. The axial ratio has not been determined closely because of the twinning. These crystals were grown better by slow evaporation than by slow or rapid cooling of supersaturated solutions. SODIUM ZINC URANYL ACETATE. NaZn[(UO2(C2H3O2) 3H2O]3. These crystals were produced by Erb, who did not obtain sufficient measure- ments to calculate the axial ratio. The temperature of conversion was found by Schwarz to be 95°. These crystals were made by adding to zinc oxide an excess of acetic acid, sodium uranyl acetate, and uranyl acetate in calculated proportions. These crystals were identified by the twins, as shown by polarization. They were found to weather fairly rapidly, even in moist summer weather. SODIUM CADMIUM URANYL ACETATE. NaCa[UO2(C2H3O2)3 3H2O]3. These crystals, prepared and measured by Wyrouboff,6 have the axial ratio a: b: c = 0.5162: 1:0.9798; /3 = 90°9'; plane of the axes normal to b (010) and nearly parallel to a (100) ; positive bisectrix parallel to 6, with large axial angle. The change to a uniaxial figure does not take place until nearly 200°, at which temperature the crystals effloresce. 1 Erb, N. Jahr. of Min. B. B., 6, 121-147. 1888-89. 2 Wyrouboff, Bull. Soc. fran. Min. 24, 93-104. 1901. 3 Schwarz, Beitr. Z. Kenntn. d. umkehrbaren Umwandlungen polymorpher Korper. Preisschr d. Univ. Gottingen. 1892. 4 Wyrouboff, I. c., 104. 5 Erb, I. c., 126. 6 Wyrouboff. 1. c.. 103. CHEMISTRY OF FLUORESCING URANYL SALTS. 229 SODIUM COPPER URANYL ACETATE. Na Cu[UO2(C2H3O2)3 3H2O]3. This was the first salt of this series to be discovered by Rammelsberg. The axial ratio is given by Wyrouboff as a : b : c = 0.5354:1 : 0.9950; 0 = 89° 55'. The double refraction is weakly positive, plane of the axes normal to b (010) ; the bisectrix 50° from c axis in the acute angle through the face c (001); 2 E = 90° 50' ; dispersion weak, r < v. According to Wyrouboff, it only becomes uniaxial at 140°, while Schwarz finds, the conversion-point at 93.8°. This salt was prepared from cupric hydroxide, acetic acid, uranyl acetate, and sodium acetate in weighed amounts. On slow evaporation, large, clear crystals, though not free from twins, were obtained, some being 1 cm. in dia- meter and 2 mm. thick. These were sealed in glass to prevent efflorescence. An attempt was made to produce the cobalt salt of this group, but it was not very successful. The manganese, iron, and nickel were not tried. The last salt was studied rather completely by Johnsen1 in an investigation of twinning. URANYL OXALATE. 3H2O. This salt was described by Peligot and Ebelmen2 and later by Zimmerman.3 This is an apparently amorphous powder produced by adding oxalic acid to the neutral nitrate, which shows under the microscope small grains with brilliant polarization colors and an extinction angle with the long side of the crystals of 13°. A specimen of this was prepared by Cragwall and some material was also purified this way. DOUBLE URANYL OXALATE. The double salts seem to be formed fairly readily, the potassium salt being described by Ebelmen4 and the ammonium salt by de la Provostage5 and by Rammelsberg.6 Wyrouboff7 makes a comprehensive review, giving the following : K2 UO2(C2O4)2 3H2O monoclinic. (NH4)2 UO2(C2O4)2 2H2O rhombic. Cs2 UO2(C2O4)2 2H2O rhombic isomorphous with above. T12 U02(C2O4)2 2H2O rhombic isomorphous with above. Na2 UO2 (C2O4)2 6H2O triclinic. (NH4)4 UO2 (C2O4)3 monoclinic. TLj UO2 (C2O4)s monoclinic isomorphous with above. K« UO2 (C2O4)4 10H2O triclinic. None of these salts were tried. URANYL TARTRATE. UO2C4H406.4H2O. This salt was prepared by Cragwall by the method of Peligot8 from uranyl hydroxide U02(OH)2 from the ignition of uranyl nitrate and tartaric acid. 1 Johnsen, N. Jahrb. Min. B. B., 23, 259. 1907. 2 Peligot and Ebelmen, Liebig. Ann. Chem. 43, 282, 287. 1842. 3 Zimmerman, Liebig. Ann. Chem., 232, 300. 1886. 4 Ebelmen, Ann. Chim. Phys. (3), 5, 200. 1842. 6 de la Provostage, ibid., 49. 6 Rammelsberg, Handbuch d. Krystall. Chem., 264. 1855. 7 Wyrouboff, Bull. Soc. Fran. Min., 32, 352. 1909. 8 Peligot, Liebig Ann. Chem., 56, 231. 1845. 230 FLUORESCENCE OF THE URANYL SALTS. This was recrystallized from water and left microscopic plates with wide black edges, as on a crystal lying on 6 (010) and bounded by m (110) and c (001), or else having a very high index, a low double refraction, and straight hyper- bolas, as from a tipped uniaxial crystal or near an optic axis. DOUBLE URANYL TARTRATES. The tartrates are exceedingly soluble, and likely to result in gums on drying, which do not crystallize. The existence of a sodium double salt in solution is indicated by the work of Grossman1 and Loeb on the effect of heavy metals on the rotation of tartaric acid. The potassium double salt is de- scribed by Frisch2 and the antimonyl salt by Peligot (see uranyl tartrate), but none of the salts show fluorescence, so they were not taken up further. The corresponding molybdyl and tungstyl compounds were attempted, since the oxides are soluble in tartaric acid, but only thick gums which did not crystallize or fluoresce were obtained. POTASSIUM URANYL TARTRATE. This salt, described by Frisch, was prepared by Cragwall by adding uranyl nitrate to potassium tartrate and washing free from nitrates. Since Frisch produced his material by dissolving precipitated uranyl hydroxide in acid potassium tartrate (Weinstein) and found that it could not be crystallized by drying, but was precipitated by alcohol, it seems probable that the material which Cragwall obtained on washing was simply the acid potassium tartrate itself. Examination for fluorescence and under the microscope indicated this. ANTIMONYL URANYL TARTRATE. UOs.(SbO.C4H4O,),4H2O. This is produced, according to Peligot, by adding uranyl nitrate solution to potassium antimonyl tartrate solution. Cragwall washed the precipitate free from nitrates. The resulting material appeared to be amorphous and showed no fluorescence. In this case both basic radicals are commonly in the complex anion, so that it is difficult to decide which comes first. POTASSIUM URANYL PROPIONATE. K UO2(C2H602)3. Rimbach3 examined the potassium and ammonium double salts with pro- pionic acid and the potassium double salt with butyric and valerianic acids. These crystallize in tetrahedra, according to Sachs,4 but were not attempted for this work. 1 Grossman and Loeb, Z. Phys. Chem., 72, 93. 1910. 2 Frisch, J. Prakt. Chem. 97, 281. 1866. 3 Rimbach, Ber., 37, 484. 1904. 4 Sachs, Ber. 37, 484. 1904. APPENDIX 2. ON PHOSPHOROSCOPES. The uranium salts have exhibited under photo-excitation a type of phos- phorescence which persists but a few thousandths of a second, while under cathodo-excitation the type endures for several minutes. It was necessary to devise two phosphoroscopes of entirely different design to measure the two types of phosphorescence. The choice of a suitable phosphoroscope is a matter of great importance ; hence, a brief summary of the types of phosphoro- scopes which have been employed since the time of the great pioneer student of phosphorescence, E. Becquerel, follows. Among the considerations which present themselves when the construction of a phosphoroscope is contemplated are the quantity of phosphorescent material available, the total time of decay, the temperature at which the specimen is to be studied, the nature of the ex- citation (e. g., photo-, ultra-violet, cathode rays, etc.), the ease with which saturation is obtained, and the initial brightness of the specimen. The phosphoroscopes which have been constructed can be divided into three classes : Type 1. — The specimen is periodically excited and periodically viewed at a later phase. Type 2. — The specimen is continuously excited and continuously viewed at a later phase. Type 3. — The specimen is excited for a measured interval of time and the intensity measured at a later time. This method is applicable to the slowest types of decay. Machines which may be classified as belonging to type 1 must operate at such speeds that no flicker is noticeable; hence the weakest intensity measured must fall within a total time of decay of one-sixteenth of a second. Many natural crystals, under photo-excita- tion, present very interesting phos- phorescence processes which appar- ently cease in less than this time. The very first steps in the long-time decays of such substances as the natural and artificial sulphides may be studied with the aid of a phosphoro- scope belonging to type 1. E. Bec- querel1 devised, among other forms, a phosphoroscope of the intermittently excited type. The specimen was mounted between two parallel disks and was alternately illuminated and observed through properly adjusted FlG- 1- openings in the disks. Figure 1 shows two such disks, each having four open sectors, mounted on the same axis but in different phase. Bec- querel caused the exciting light to pass into the translucent crystal through 1 E. Becquerel, Annales de Chimie et de Physique (3), 55, p. 5. 1859. 231 232 FLUORESCENCE OF THE URANYL SALTS. the rear disk, while the opaque sector prevented the light from coming through the phosphoroscope to the eye. As the disk was revolving at a high speed, the light was quickly stopped from passing to the crystal by the interposition of an opaque sector of the rear disk. A small fraction of a second later the continued rotation brought an open sector of the front disk in line with the crystal and the eye, thereby allowing the phosphorescent light to be viewed on a dark field. The rotating parts were neatly mounted in a brass drum and driven by a crank through a system of gears. With such a type of phosphoroscope Becquerel detected the glow of the platino-cyanides only 0.003 of a second after excitation. It is evident that to view opaque specimens, the excitation can not be directly behind the specimen; hence Becquerel1 devised a phosphoroscope possessing only one rotating disk, figure 2, both diagrams with three openings, K, L, and R, arranged 120° apart. The disk revolved on a vertical axis between two fixed openings, 180° apart, the exciting light from X passing in through one of these two openings and the luminescent light passing out from the specimen P, upper diagram, through the other opening to the observer 0 a fraction of a second later. In figure 2 the sector is shown, in elevation, at such a position that the open sector R admits exciting light to P, and it is evident that an opaque sector is at that time to be found at S. On the other hand, when S is opened by the passage of an open sector, the open sector R has passed out of the line XP and an opaque sector is interposed. FIG . 2. E. Wiedmann2 devised a phosphoroscope consisting essentially of a hollow brass drum fitted with a collimator and lens and revolving disk for the ad- mission of the exciting light. The exciting light passed intermittently through the open sectors of a sectored disk and the specimen, which was mounted within the drum, was thus intermittently excited. For the purpose of view- ing opaque specimens and liquids at a later phase, the revolving disk carried on the periphery a band with open sectors for excitation; thus the path of the phosphorescent light was at right angles to that of the exciting light. Wiede- mann constructed driving-gears with a ratio of 1,000 :1 and hence obtained a rotary speed as great as 140 revolutions per second. Either the unassisted eye or a spectrometer was employed to view the phosphorescence. Stray 1 E. Becquerel, Annales de Chimie et de Physique (3), v. 55, p. 80. 1859. 2 E. Wiedmann, Wiedmann's Annalen, vol. 34, p. 446. 1888. ON PHOSPHOROSCOPES. 233 I ight, always a menace in phosphorescent studies where photo-excitation is used, was present, and Wiedemann appreciated the necessity of devising some addition to his apparatus for the purpose of eliminating it. He favored the addition of another revolving sector, similar to and coaxial with the first, having openings in phase with it, and mounted between the first sector and the lens. When it is considered that the intensity of the undispersed phosphor- escence is from 1/100 to 1/1,000,000 that of the exciting light, it is evident that the introduction of a small per cent of the exciting light into the field of the phosphorescence completely aborts any attempt at quantitative measure- ments. I 1 \/ M I FIG. 3. E. Merritt1 devised a phosphoroscope of the first type in which the phos- phorescent surface P (see fig. 3) was illuminated periodically by the passage of light from a spark S through an opening through a revolving disk 00'. The phosphorescence could be observed at the desired later time by changing the phase, while the machine is turning, of the mirror M relative to the opening 0. This was accomplished in a unique manner by means of the rod L, which engages a hollow sleeve R having a spiral slot cut in it. A pin capable of sliding in this slot and attached to the opposite end of the same inner shaft as the mirror M, is moved into different phases with the disk 00', which is mounted on the outside shaft. This outer shaft is driven by a belt and pulley and is keyed to the sleeve R; thus the drive is complete. Rod L does not rotate, but can be locked at any desired phase. Reflection of the phosphorescence, then, occurs at M , a simple photometer being employed to view the light. On the revolving shaft was mounted a worm-gear for record- ing the number of revolutions per minute. With this form of phosphoroscope the curves of decay of many substances were traced from zero time up to 0.06 second. In the preceding phosphoroscopes the source of light has not been inter- mittent, but the periodic interruption of the beam of exciting light has pro- duced the effect of intermittent excitation on the specimen. Another group of instruments belonging to this type employs an intermittent discharge from condenser, induction coil, or transformer, as in the spark phosphoroscope of 1 E. Merritt, Nichols and Merritt, Studies in Luminescence, Carnegie Inst. Wash. Pub. No. 152, p. 109. 1912. 234 FLUORESCENCE OF THE URANYL SALTS. Laborde,1 which included a rotating pattern to hide the specimen from the observer during excitation by the spark from an induction coil, and later uncover the specimen. Wm. Crookes,2 in his study of the cathode phosphorescence of yttria, noted that the color at the beginning of decay was different from that observed after the decay had continued for a short period, and accordingly devised a phos- phoroscope to study this change. Figure 4 serves to show that it was of the sectored-disk form and driven by cord and pulley. At a convenient distance was located an induction coil whose primary circuit was alternately made and broken by a commutator near the end of the revolving shaft C. The brushes could be so shifted as to cause the excitation of the phosphorescent substance P when an opaque sector passed between P and the eye. It was possible to change the period of decay by changing the speed or by changing the phase of the brushes relative to the edge of the sector. The phosphorescent substance was mounted in a convenient form of cathode-ray tube and excited by the dis- charge from the secondary of the coil. With the aid of the spectrometer, Crookes discovered that different lines appeared in the spectrum of the phosphorescent yttrium after 0.000875 second than at 0.0035 second. FIG. 4. Ph. Lenard3 devised a phosphoroscope which differs from the preceding forms, since no revolving disk is employed. To hide the specimen from view during excitation he used the motion of a screen mounted on the plunger of a Ruhmkorff mercury interrupter. The frequency of the interrupter was de- termined by that of the spring fork on which it was balanced; hence change in the period between excitation and observation was accomplished by chang- ing forks. The discharge of a condenser in parallel with secondary circuit of the coil was thus timed by the interruptions of the primary circuit to occur while the vibrating screen was in front of the specimen. De Watteville4 constructed a machine similar in principle to that of Laborde and Crookes (see fig. 5). The specimen, together with the spark, was mounted 1 Laborde, Comptes Rendus, vol. 68, p. 1576. 2 Crookes, Proceedings of the Royal Society, vol. 42, p. 111. 3 Ph. Lenard, Wiedmann's Annalen, 46, p. 637. 4 De Watteville, Comptes Rendus, 142, p. 1078. 1887. ON PHOSPHOROSCOPES. 235 in the box K and was visible to the observer, except when the two arms of the rotating disk D intercepted the phosphorescent light. At such times one of the points, P or Pr, completed the circuit from B through A to K, allowing the discharge of the condenser C to excite the specimen at K; the condenser had been previously charged by coil S. Twice a revolution, then, the specimen was excited and observed. FIG. 5. Nichols and Howes,1 to study the phosphorescence of the uranyl salts, de- vised a phosphoroscope of considerable precision. Except for the work of Nichols and Merritt, Trowbridge, Ives, and a few others, the previously mentioned students of phosphorescence have been content to measure the intensity at two or three periods of decay, but the above-mentioned investi- gators have taken many observations on one substance and established curves of decay for each substance studied. For such measurements, refinements for precluding measureable stray light and for accurately measuring the time intervals and for maintaining constant speed are a necessity. The synchrono- phosphorosope (see fig. 6) was so named because it employs the principle of a sectored disk mounted on the axle of a synchronous motor A . C. This motor was raised to synchronous speed by the direct-current motor D. C. The transformer TT was attached to the same alternating-current terminals as was the A. C. motor; hence the discharge of the condenser occurred as many times per second as the number of wave-crests, i. e., 120. There were four opaque sectors and four open sectors in the disk WW, and since the four-pole machine turned at a speed of 30 revolutions per second, there were 120 eclipses 1 Nichols and Howes, Science, n. s., vol. 43, p. 937. 1916. 236 FLUORESCENCE OF THE URANYL SALTS. per second. By means of a small set-screw the disk could be clamped at various positions on the shaft, corresponding to various times in the decay process. Without the star-wheel SS the discharge of the condenser KK produced an exciting spark at E, which, with the aid of a revolving mirror, was found to consist of a pilot-spark, followed by five or six smaller sparks; hence the zinc star-wheel SS was mounted on the shaft to reduce the discharge to one spark. By experimenting with small and large capacities it was found that resonance must be recognized, and the proper amount of capacity to produce a regular, strong spark was finally discovered. The measurements of time were read with the aid of the light yielded by the exciting spark by noting the position of the edge of the sectors on a protractor mounted rigidly in front of the machine. The range of times accurately measureable include those from 0.0001 to 0.0040 second by 0.0001-second steps. The photometer, spectro-photometer, camera, and spectrograph have been successfully used with this instrument. FIG. 6. In their preliminary study of the cathodo-phosphorescence of the rare earths, Nichols, Wick, and Wilber1 employed a phosphoroscope with a disk mounted on the shaft of a motor. The primary of an induction coil was interrupted by a plunger attached to a crank on the shaft of the motor. The plunger, once in a revolution, dipped into the mercury cistern, while the opaque portion of the disk hid the specimen from view, and as a result the specimen was excited by the discharge of the secondary coil through a cathode- ray tube, much after the manner of Crookes's device. The change in time between excitation and observation was accomplished by changing the speed of the motor, and an ammeter in the field circuit was calibrated to measure the angular velocity. It is evident that with only one excitation per revolution only one open sector could be used. The second type of phosphoroscope includes those instruments by means of which the specimen is constantly excited and constantly viewed at a later time. Such a form had its origin with E. Becquerel.2 This form was used for lecture demonstrations by Tyndall.3 In its simplest form it consists of a 1 Nichols, Wick, and Wilber, Physical Review (2), vol. 14, 1919. 2 Becquerel, E., Annales de Chimie et de Physique (3), vol. 62, p. 5. ' Tyndall, see Lewis Wright, "Light," p. 152. London, 1882. 1861. ON PHOSPHOROSCOPES. 237 drum (see fig. 7) whose periphery P is covered with a phosphorescent sub- stance, provided with a source of excitation S mounted in a box A, and driven by belt BB and pulley. The viewing collimator or photometer is indicated at E. Kester1 employed a device of this type, together with spectrometers and radiometer, to study the relation of intensity of excitation to that of phos- phorescence. B B, FIG. 7. Waggoner2 mounted on a vertical shaft an iron drum 45 cm. in diameter. This mass, being considerable, acted as a balance on the irregularities of the motor speed. The exciting spark was so mounted that it could be moved about the drum, thus enabling the observer to measure the brightness at various times after the spark ceased. The periphery was painted, as before, with the phosphorescent substance. The revolutions of the drum were auto- matically recorded on a chronograph. With this device the spectrum of early phosphorescence was studied with the aid of the spectrometer and decay curves of the total visible radiation were taken. Nichols and Howes,3 in the study of the phosphorescence of calcite, em- ployed this type with a drum of 8.0 cm. diameter (see fig. 8). The eye-piece E was arranged at 180° from the spark A, the spark thus being completely hidden from the observer by the opaque drum Z). Effective screening was added to prevent stray light from entering either face of the Lummer-Brodhun cube L. B. and a filter F was interposed between the comparison lamp C 1 Kester, Physical Review (1), vol. 9, p. 164. 2 Waggoner, Carnegie Inst. Wash. Pub. No. 152, p. 119. 3 Nichols, Howes, and Wilber, Physical Review (2), vol. 12, p. 350. 1918. 238 FLUORESCENCE OF THE URANYL SALTS. and the cube to produce a color match with the reddish color of the phos- phorescence. Although the angle between spark and observing photometer remained constantly 180°, the time of decay could be varied between 0.01 second and 3 or 4 seconds by changing the motor field or by throwing in a DISK PHOSPHOROSCOPE. 0 FIG. 8. M A H BJ I A 0 0 ! L ^ FIG. 9. worm-gear drive. Two features were added to make the device more useful. First, the phosphorescent substance was never painted on the disk, but on brass rings which fitted neatly on the disk; second, the speeds were known by reading on a galvanometer G the deflections produced by a current induced ON PHOSPHOROSCOPES. 239 by a disk below the phosphorescent disk, but on the same shaft. This disk cut the field between two electromagnets M, one brush B on the rim, the other, B', on the shaft delivering the current to the galvanometer. The con- stancy of the magnetic field was maintained by examining the readings of an ammeter in series with the electromagnets and storage cells, and precautions were taken to eliminate thermal electromotive forces at the brushes. In the use of this type of phosphoroscope, as well as the first type, readings of in- X L r ^ » FIG. 10. tensity are only comparable through a range of speeds for which saturation is obtained. With the red variety of calcite, saturation was found to exist with the iron spark 1 cm. from the disk for all speeds, which gave more than 0.02 second decay; hence measurements in which the time interval from the close of excitation to observation was not greater than 0.02 second were rejected. To use this instrument with greater precision it is necessary to take account of the variations in the spark. For this purpose an auxiliary station, with photometer P and lamp C2, was arranged (fig. 9), where simultaneous readings of intensity of phosphorescence were taken while the chief observer, with aid of the spectrophotometer H and lamp Ci, measured the intensity of the phos- phorescence throughout the spectrum. 240 FLUORESCENCE OF THE URANYL SALTS. To study the early stages of the cathodo-phosphorescence of calcite, Nichols and Howes1 devised a vacuum phosphoroscope, outlined in figure 10. The phosphorescent specimen was applied to the periphery of the disk P and excited by means of the cathode discharge from K. The vacuum-tube V was fitted to the ground plate N. The shaft was balanced in an iron tube 115 cm. in length, the mercury rising from the iron reservoir C to the barometric height. The shaft was driven by a pulley, cord, and variable-speed motor, and the revolutions were recorded by the commutating device at the bottom wired to the chronograph. Intensities of phosphorescence were measured with the aid of lamp P, photometer bar S, and Lummer-Brodhun cube T. The third type of phosphoroscope, in which the specimen is excited for a definite time and viewed at varying but definite times after excitation, in- cludes the form used by Nichols and Merritt in their extensive studies2 of the luminescence of sidot blende. FIG. 11. The specimen was mounted diagonally in a box having two openings with shutters, one to admit excitation, the other to allow the luminescence to be viewed after excitation. The eye of the observer was thus protected from the brilliant luminescence during excitation, but was able to view the phosphor- escence with no fatigue when the shutter of the luminescence window opened. The time when the phosphorescence intensity became equal to that of the comparison field was recorded on the chronograph by means of a key in the 1 Nichols, Howes, and Wilber, I. c. 2 Nichols a"nd Merritt, Carnegie Inst. Wash. Pub. No. 152, p. 41. ON PHOSPHOROSCOPES. 241 hand of the observer. A series of such comparisons, together with the initial time, formed a series of points for a decay curve. It is clear that such a device is only suitable for the study of decay which endures for several minutes. E. H. Kennard1 devised a phosphoroscope of this type with shutters actuated by the magnetic release of phosphor-bronze springs. Three shutters (Si, 82, S3, of fig. 11) were used, Si to admit exciting light from the mercury arc A to the specimen P, S2 and 83 to limit the time during which the phosphorescence was allowed to produce photo-electric action on the cell C. The latter adapta- tion to phosphorescence work is unique. In his preliminary work the times between opening and closing of shutters were determined from the known curve of a ballistic galvanometer, the passage of a shutter opening and closing shunts which allowed a definite quantity of electricity to pass into the gal- vanometer. In his later work the shutters were magnetically released by the swing of a seconds pendulum across mercury cups set at convenient positions along the path of the bob. The photo-electric current, for low intensities, is proportional to the intensity of phosphorescence and was measured by a quadrant electrometer. It is to be inferred from the preceding summary on phosphoroscopes that there may be no single machine which is well adapted to the study of a par- ticular phosphorescent specimen under investigation. As in the study of phosphorescence in the past, the investigator has often devised one or more machines for the study of the various types of phosphorescence; so in the future the machine must be adapted to the behavior of the specimen. Then, too, the precision with which the time of observation is desired makes it necessary that the modern phosphoroscope be equipped with an accurate and if possible a direct-reading speed register. The plotting of decay curves by Nichols and Merritt, Trowbridge, Ives, and others made it imperative that both times and excitations be well known. The lack of constancy of excita- tion has been recognized and observations corrected. The importance of obtaining the same degree of saturation before decay begins is paramount if results are to be considered comparable. In the use of the continuously excited type the importance of this factor becomes evident. The use of the interrupted excitation type for eclipses of 16 or less a second should be entirely avoided, because of the behavior of the eye when flicker is noticeable. The human eye should not be fatigued beyond instant recovery during the process of observing decay, neither should it be dark-adapted before beginning a set of observations. The necessity of adequate screening is of great importance when it is considered that luminescence radiation is several thousand times less than the photo-excitation of approximately the same wave-length. Stray luminescence may add to the selected portion of the luminescence beam and produce errors. These factors are fundamental considerations for the future student of phosphorescence. 1 Kennard, Physical Review (2), vol. 4, p. 278. 1914. - MBL WHOI LIBRARY •••••• • II I I | || HI UH IflLP I •o