Fig. (1) shows the general arrangement. A long brass cylinder \(A B\) 6 cms . in diameter, 73 cms. long, was divided into two equal parts by a movable metal slide \(S\). The ends of the cylinder were closed with ebonite stoppers. Two insulated brass rods \(a\) and \(b\), each half the length of the tube, passed through the ebonite stoppers and were supported centrally in the tube. The cylinder was insulated and connected to one pole of a battery of 300 volts, the other pole of which was to earth. The central rods could be connected to a sensitive quadrant electrometer. The cylinder was covered with a thick layer of felt, and placed inside a metal box filled with cotton wool, in order to keep temperature conditions as steady as possible. [Illustration: Fig. I.] In order to carry a sufficient quantity of emanation into the half cylinder \(A\), it was necessary to slightly heat the radium. The slide \(S\) was closed and the side tubes opened. A slow current of dry air from a gas bag, passed through a platinum tube, in which a small quantity of a radium compound was placed. The emanation was carried with the air into the cylinder \(A\). When a sufficient quantity had been introduced, as tested by the electrometer, the current of air was stopped. The side tubes were closed by fine capillary tubes. These prevented any appreciable loss of gas due to diffusion, but served to keep pressure of gas inside \(A\) at pressure of outside air. The three entrance tubes into the cylinder, shown in the figure, were for the purpose of initially mixing the emanation and gas as uniformly as possible. After standing for several hours to make temperature conditions steady, the slide was opened, and the emanation began to diffuse into the tube \(B\). The current through the tubes \(A\) and \(B\) was measured by an electrometer, with suitable capacity in parallel, at regular intervals. Initially there is no current in \(B\), but after the opening of the slide, the amount in \(A\) decreased and the amount in \(B\) steadily increased. After several hours the amount in each half is nearly the same, showing that the emanation is nearly uniformly diffused throughout the cylinder. It can be readily shown that if \[ \begin{aligned} K &= \text{coefficient of diffusion of the emanation into air}.\\ f &= \text{duration of diffusion experiments in secs}.\\ a &= \text{total length of cylinder}.\\ S &= \text{amount of emanation in tube}\, A\, \text{at end of diffusion}.\\ S_{2}& = \text{amount of emanation in tube}\, B\, \text{at end of diffusion, then} \end{aligned} \] \[ \frac{S_{1}-S_{2}}{S_{1}+S_{2}}=\frac{8}{\pi^{2}}\left\{e^{\frac{-\pi^{2} K t}{a^{2}}}+\frac{1}{9} e^{\frac{-\pi^{2} K t}{a^{2}}}+\text { etc. }\right\} \] SEE STEFAN AND LOSCHMIDT, BERICHTE WIEN. AKAD., 63, 1871. From this equation \(K\) can be determined, if \(S_{1}\) and \(S_{2}\) are known.