=237. Experiments with old radium.= Since the substance radium D is produced from radium at a constant rate, the amount present mixed with the radium will increase with its age. The writer had in his possession a small quantity of impure radium chloride, kindly presented by Professors Elster and Geitel four years before. The amount of radium D present in it was tested in the following way:—The substance was dissolved in water and kept continuously boiling for a period of about six hours. Under these conditions the emanation is removed as rapidly as it is formed, and the β rays from the radium, due to the product radium C, practically disappear. A newly prepared specimen of radium bromide under these conditions retains only a fraction of 1 per cent. of its original β radiation. The old radium, however, showed (immediately after this treatment) an activity measured by the β rays of about 8 per cent. of its original amount. The activity could not be reduced any lower by further boiling or aspiration of air through the solution. This residual β ray activity was due to the product radium E stored up in the radium. The β ray activity due to radium E was thus about 9 per cent. of that due to radium C. Disregarding the differences in the absorption of the β rays, when the activity of the product E in radium reaches a maximum value, the β ray activity due to it should be the same as that due to C. Since the parent product D is half transformed in forty years, the amount present in the radium after four years should be about 7 per cent. of the maximum amount; _i.e._ it should show a β ray activity of about 7 per cent. of that due to radium C. The observed and calculated values (7 and 9 per cent. respectively) are thus of the same order of magnitude. The amount of β rays from radium E present in pure radium bromide about one year old was about 2 per cent. of the total. The amount of radium F present in old radium was measured by observations of the activity imparted to a bismuth disk left for several days in the solution, and was found to be of the same order as the theoretical value. Radium F is not deposited to an appreciable extent on the bismuth from a water solution of radium bromide. If, however, a trace of sulphuric acid is added to the solution, the radium F is readily deposited on the bismuth. The addition of sulphuric acid to the radium solution practically effected a separation of radium D, E and F from the radium proper; for the latter was precipitated as sulphate and the products D, E and F remained in solution. After filtering, the solution contained the greater proportion of the products D, E, and F and very little radium.