The normal law connecting the mobility ( k ) of a gas ion with the pressure ( p ) of the gas, is that the product pk is constant. More than 20 years ago Kovarik found that this product showed an abnormal increase in the case of the negative ions formed in air and carbon dioxide when the pressure was lowered below 20 cm. approximately; this result was interpreted as denoting a progressive diminution in the size and mass of the ion as the pressure is reduced. Some years later the author conducted an experimental investigation, from the results of which he concluded that at the lower pressures the carriers of negative electricity consist of two kinds, electrons and ions; the electrons come more and more into evidence as the pressure of the gas is reduced and travel freely under the influence of an applied field without becoming attached to molecules. From this point of view anomalous mobilities will be obtained at lower pressures unless the electrons are separated experimentally from the negative ions. The author found that, when this separation was effected, the mobility law, pk = const., remained valid down to the lowest pressure employed, viz., 0·15 mm.