Several methods have been used to measure [alpha]. In one method air, exposed to some ionizing agent at one end of a long tube, is slowly sucked through the tube and the saturation current measured at different points along the tube. These currents are proportional to the values of n at the place of observation: if we know the distance of this place from the end of the tube when the gas was ionized and the velocity of the stream of gas, we can find t in equation (3), and knowing the value of n we can deduce the value of [alpha] from the equation 1/n1 - 1/n2 = [alpha](t1 - t2), where n1, n2 are the values of n at the times t1, t2 respectively. In this method the tubes ought to be so wide that the loss of ions by diffusion to the sides of the tube is negligible. There are other methods which involve the knowledge of the speed with which the ions move under the action of known electric forces; we shall defer the consideration of these methods until we have discussed the question of these speeds. In measuring the value of [alpha] it should be remembered that the theory of the methods supposes that the ionization is uniform throughout the gas. If the total ionization throughout a gas remains constant, but instead of being uniformly distributed is concentrated in patches, it is evident that the ions will recombine more quickly in the second case than in the first, and that the value of [alpha] will be different in the two cases. This probably explains the large values of [alpha] obtained by Retschinsky, who ionized the gas by the [alpha] rays from radium, a method which produces very patchy ionization. _Variation of [alpha] with the Pressure of the Gas._--All observers agree that there is little variation in [alpha] with the pressures for pressures of between 5 and 1 atmospheres; at lower pressures, however, the value of [alpha] seems to diminish with the pressure: thus Langevin (_Ann. chim. phys._, 1903, 28, p. 287) found that at a pressure of 1/5 of an atmosphere the value of [alpha] was about 1/5 of its value at atmospheric pressure. _Variation of [alpha] with the Temperature._--Erikson (_Phil. Mag._, Aug. 1909) has shown that the value of [alpha] for air increases as the temperature diminishes, and that at the temperature of liquid air -180° C., it is more than twice as great as at +12° C.