No. 4156 June 25, 1949 NATURE 989 LETTERS TO THE EDITORS be identified as sodium nitrate. Similar crystals are also formed on exposing rock salt to the action of The Editors do not hold themselves responsible nitri? acid or to nitrogen dioxide prepared by for opinions expressed by their correspondents. heatmg lead rutrate. The crystals obtained when No notice is taken of anonymous communications exposing rock salt to o:-rays are evidently due to the formation of nitrogen dioxide by the rays in air. The Light Emitted by Europium Compounds The action of the o:-rays on the rock salt surface SOME time ago, my co-workers and I 1 found that seems, however, also to intervene; various samples the well-known blue fluorescence band of some of rock salt that show differences in the rate of colour• fluorites is due to traces of bivalent europium. In ing by the rays also show differences in the number later papers•, I showed that europium can easily be of microscopic crystals formed, while no difierence reduced to the bivalent form by heating in various is found when nitrogen dioxide alone acts on the basic materials. same samples. Sometimes the microscopic crystals F. Weger, in this laboratory, has studied the show a marked orientation on the rock salt surface. fluorescence spectrum of Eu++ in various substances. . On potassium chloride, crystals of potassium There is always a broad band in the blue, but the mtrate are formed. On potassium bromide, however, position of the maximum varies widely with the there appear isotropic triangular crystals which can• nature of the basic material and with the way the not be potassium nitrate (birefringent), their melt• sample has been heated ; heating on a platinum point also being much higher. Such isotropic wire in the flame shifts the maximum towards longer tnangular crystals are also occasionally found on wave-lengths as compared with a sample heated in sodium chloride and potassium chloride ; so far we a glass tube. have not been able to identify them. This investigation was well advanced when I saw On fluorite and on calcite we have obtained aniso• the important paper by S. Freed and S. Katcof"f3 on tropic crystals, which cannot be the known form of the absorption and fluorescence spectra of bivalent calcium nitrate, as this substance is isotropic. europium. These authors, too, note the strong A m.ore detailed account of this work appears in dependence of the maximum on the nature of the the Wwn. Ber. and the Acta Physica Austriaca. We crystalline surroundings, and give for the bands of are continuing these investigation<;. Eu++ the interpretation proposed by me• in 1936, L. vVIENTNGER namely, transitions between 4f and 5d, etc. .N. ADLER With a mixture of calcium chloride and borax II. Physics Institute, containing traces of europium, we get by suitable University of Vienna. heating a bright yellow fluorescence with a maximum Jan. 5. somewhere in the near infra-red for which we have as yet no interpretation. A borax bead with traces of europium glows, Decoloration and the Ultramicroscopic when heated on a platinum wire loop in the blow-pipe Behaviour of Natural Blue and flame, with a blue light6 nearly as intense as the blue Violet Rock Salt component of the light emitted by the glowing platinum wire ; the light resembles in colour the THE absorption spectrum of blue rock salt from fluorescence excited by ultra-violet. This might be Stassfurt was determined photo-electrically in the chemiluminescence (the candoluminescence of Nichols natural state and after heating for a certain time at and his co-workers); but it might also be an extreme various temperatures. The spectrum shows the well• case of selective temperature radiation as, according known high maximum at 653 mfL and the smaller to Freed and Katcoff, the absorption of bivalent one at 535 mfL. On increasing the time of heating, europium is very strong in the near ultra-violets. the first maximum begins to shift towards shorter A more detailed account appears in the Wien wave·lengths ; its intensity increases at first and then Anzeiger of December 2, 1948, and in the Acta Physica decreases. The second maximum does not alter its Austriaca. position ; it decrease3 immediately and disappears K. FRZIBRAM sooner than the first. The violet salt from the Grim• II. Physics Institute, berg mine in the Werra valley behaves quite differ• University of Vienna. ently. It shows only one maximum, at 583 m Jan. 5. which is .not displaced after heating, and begins decrease Immediately. 1 Haberlandt, H., Karlik, B., and Przibram, K., Wien. Ber. Ila 143 151 (1934). ' ' ' The ultramicroscopic behaviour of the two sorts 'Przibram, K., (a) Wien. Ber., IIa, 144, 141 (1935); (b) ibid., 147, 261 of salt is also quite difL'erent. The blue salt from (1938). Stassfurt shows the well-known bright red Tyndall 'Freed, S., and Katcoff, S., Physica, 14, 17 (1948). • Przibram, K., Z. Phys., 102, 331 (1936). cone, the violet one at the most a colourless cone 'Przibram, K., Wien. Anz., 98 (1946). In this paper ref. 2 ought to not brighter than that sometimes observed in colour• read: Nichols, JCrystal Y.ellow Tyndall cone; the latter, as already men• Surfaces Exposed to oc;-Rays twned, only a weak colourless one. DURING some experiments on the coloration of T_his seems to support the assertion made by K. rock salt by the oc;-rays of polonium, we noticed that Przibram1 a long trme ago, that there is an inter• the surface exposed to the rays in air became dull. mediate stage between the primary amicroscopic Examined under the microscope, it was found to be yellow colour (F-centres) and the colloidal blue that covered by minute birefringent crystals which could is, an amicroscopic violet or blue colour, due

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