Experiments to determine accurately the rate of decay of activity of the emanation have been made by P. Curie[237], and Rutherford and Soddy[238]. In the experiments of the latter, the emanation mixed with air was stored over mercury in an ordinary gas-holder. From time to time, equal quantities of air mixed with the emanation were measured off by a gas pipette and delivered into a testing vessel. The latter consisted of an air-tight brass cylinder carrying a central insulated electrode. A saturation voltage was applied to the cylinder, and the inner electrode was connected to the electrometer with a suitable capacity in parallel. The saturation current was observed _immediately_ after the introduction of the active gas into the testing vessel, and was taken as a measure of the activity of the emanation present. The current increased rapidly with the time owing to the production of excited activity on the walls of the containing vessel. This effect is described in detail in chapter VIII. The measurements were made at suitable intervals over a period of 33 days. The following table expresses the results, the initial activity being taken as 100. Time in Relative hours Activity 0 100 20·8 85·7 187·6 24·0 354·9 6·9 521·9 1·5 786·9 0·19 The activity falls off according to an exponential law with the time, and decays to half value in 3·71 days. With the usual notation $$ \frac {I_T} {I₀} = e^{–λt} $$ . the mean value of λ deduced from the results is given by λ = 2·16 × 10⁻⁶ = ¹⁄₄₆₃₀₀₀. P. Curie determined the rate of decay of activity of the emanation by another method. The active matter was placed at one end of a sealed tube. After sufficient time had elapsed the portion of the tube containing the radium compound was removed. The loss of activity of the emanation, stored in the other part, was tested at regular intervals by observing the ionization current due to the rays which passed through the walls of the glass vessel. The testing apparatus and the connections are shown clearly in Fig. 53. The ionization current is observed between the vessels _BB_ and _CC_. The glass tube _A_ contains the emanation. [Illustration: Fig. 53.]