IN addition to the well-known small ions, which are of a type common to all gases, two classes of larger ions exist in the air under ordinary conditions. One of these consists of the large ions of Langevinr which have a mobility of about 1/3000, while the other contains ions with a mobility of about 1/50, As the latter value lies between those of the mobilities, of the small and large ions, the members of this latter class may be called the ions of intermediate mobility, or, shortly, the intermediate ions.