The curves showing the variation of the excited activity with time are very complicated, depending not only upon the time of exposure in the presence of the emanation, but also upon the type of radiation used for measurement. The greater portion of the activity of this deposit dies away in the course of 24 hours, but a very small fraction still remains, which then changes very slowly. It will be shown in this chapter that at least six successive transformations occur in the active deposit. The matter initially produced from the emanation is called radium A, and the succeeding products B, C, D, E, F. The equations expressing the quantity of A, B, C,...... present at any time are very complicated, but the comparison of theory with experiment may be much simplified by temporarily disregarding some unimportant terms: for example, the products A, B, C are transformed at a very rapid rate compared with D. The activity due to D + E + F is, in most cases, negligible compared with that of A or C, being usually less than ¹⁄₁₀₀₀₀₀ of the initial activity observed for A or C. The analysis of the active deposit of radium may thus be conveniently divided into two stages: (1) Analysis of the deposit of rapid change, which is mainly composed of radium A, B, and C; (2) Analysis of the deposit of slow change, which is composed of radium D, E, and F. =219. Analysis of the deposit of rapid change.= In the experiments described below, a radium solution was placed in a closed glass vessel. The emanation then collected in the air space above the solution. The rod, to be made active, was introduced through an opening in the stopper and exposed in the presence of the emanation for a definite interval. If the decay was to be measured by the α rays, the rod was made the central electrode in a cylindrical vessel such as is shown in Fig. 18. A saturating voltage was applied, and the current between the cylinders measured by an electrometer. If a very active rod is to be tested, a sensitive galvanometer can be employed, but, in such a case, a large voltage is required to produce saturation. A slow current of dust-free air was continuously circulated through the cylinder, in order to remove any emanation that may have adhered to the rod. For experiments on the β and γ rays, it was found advisable to use an electroscope, such as is shown in Fig. 12, instead of an electrometer. For measurements with the γ rays, the active rod was placed under the electroscope, and before entering the vessel the rays passed through a sheet of metal of sufficient thickness to absorb all the α rays. For measurements with the γ rays, the electroscope was placed on a lead plate 0·6 cms. thick, and the active rod placed under the lead plate. The α and β rays were completely stopped by the lead, and the discharge in the electroscope was then due to the γ rays alone. The electroscope is very advantageous for measurements of this character, and accurate observations can be