IN his interesting letter (NATURE, August 25) Mr. A. S. Russell describes the result of a determination of the amount of radium in a specimen of autunite from Autun, France, made by him in Prof. Marckwald's laboratory, which he found to be only 27 per cent. of the equilibrium amount. The ratio found by Mlle. Gleditsch in Mme. Curie's laboratory for the same mineral was 80 per cent., while Miss Pirret and I recently, for an autunite from Guarda, Portugal, found 44 per cent. Some results I have obtained since the paper with Miss Pirret was published appear to put a new complexion on the matter. Dual measurements of the radium ratio and of the helium content of several specimens of Portuguese autunite have shown that both vary considerably for different specimens of the same mineral. Prof. Piutti (“Helium in Recent Minerals,” Le Radium, 1910, vii., 178) found that autunite was the only radio-active mineral in which helium could not be detected.