for references up to 1953 to the rather extensive literature concerning this method. b. Interference by Fluorescence of Other Elements. In the carbonate- fluoride flux employed, the fluorescence of uranium excited by 3600 A light: is for all practical purposes a specific property and the method is essentially free from interference by other elements. The article of Price, Ferreti and Schwartz includes specific references and data in regard to this aspect of the method. In all steps of the procedure great care is taken to elimi - nate the possibility of error from fluorescent organic compounds. c. Interference by Quenching. Much more important for the applica- tion of the method is the fact that many ions will decrease or "quench" the fluorescence of uraninm when they are present in the flux above certain threshold amounts. Thus, for example, amounts of iron up to approximately 10 pg in a 0.3 g flux show no effect on the fluorescence, but above this amount the fluorescence is quenched and is reduced by almost 50% in intensity when the amount of iron reaches approximately 100 pg. Fortunately, no element has been found to quench significantly in amounts less than 1. to 0.1 ug and microgram amounts of many substances can be tolerated. In a general way then, it can be said that if the amount of uranium determined is 0.1 4g or less no significant quenching will be observed for any element present in amount equal to that of the uranium. To prevent quenching, by separation of the uranium from interfering elements, it is therefore only necessary to adopt procedures that bring the weight of the most critical quenchers to the same level as that of the uranium. In the procedure given below this separation is made by extracting the uranium, present as uranyl ion in saturated aluminum nitrate solution, into an approximately equal volume of ethyl acetate. As an alternative to this separation procedure, consider the amounts of the possible interfering elements expected to be associated with the ura- nium. If the amounts accompanying 0.1 pg of uranium fall in order of magnitude below the threshold quenching amounts, it is possible to transfer directly to the fusion step without separating the uranium. The reliability of this approach is somewhat uncertain due to the possible introduction of quenching elements present as impurities in the rather large amounts of on Sane r on 1 reagents required to dissolve the filter paper samples. Because of the