A dry current of air or any other gas is passed at a constant rate through a long metal tube _TL_. After passing through a quantity of cotton-wool to remove dust particles, the current of air passes over a vessel _T_ containing a radio-active body such as uranium, which does not give off a radio-active emanation. By means of insulated electrodes _A_ and _B_, charged to a suitable potential, the current between the tube and one of these electrodes can be tested at various points along the tube. A gauze screen, placed over the cross-section of the tube at _D_, serves to prevent any direct action of the electric field in abstracting ions from the neighbourhood of _T_. If the electric field is sufficiently strong, all the ions travel in to the electrodes at _A_, and no current is observed at the electrode _B_. If the current is observed successively at different distances along the tube, all the electrodes except the one under consideration being connected to earth, it is found that the current diminishes with the distance from the active body. If the tube is of fairly wide bore, the loss of the ions due to diffusion is small, and the decrease in conductivity of the gas is due to recombination of the ions alone. On the ionization theory, the number _dn_ of ions per unit volume which recombine in the time _dt_ is proportional to the square of the number present. Thus _dn_ ---- = α_n²_, _dt_ where α is a constant. Integrating this equation, 1 1 --- − --- = α_t_, _n_ _N_ if _N_ is the initial number of ions, and _n_ the number after a time _t_. The experimental results obtained[51] have been shown to agree very well with this equation. In an experiment similar to that illustrated in Fig. 6, using uranium oxide as a source of ionization, it was found that half the number of ions present in the gas recombined in 2·4 seconds, and that at the end of 8 seconds one-fourth of the ions were still uncombined. Since the rate of recombination is proportional to the square of the number present, the time taken for half of the ions present in the gas to recombine decreases very rapidly with the intensity of the ionization. If radium is used, the ionization is so intense that the rate of recombination is extremely rapid. It is on account of this rapidity of recombination that large voltages are necessary to produce saturation in the gases exposed to very active preparations of radium.