In a paper of 1928 the author dealt with the factors which interfere with the flow of mixtures of gas and liquid in vertical channels, and again in English in 1929. In these two papers a cause of intermittency was already mentioned,viz., that although there may be sufficient gas to establish the mist condition as described below, when the rate of speed of this fluid is smaller than the differential velocity of gas and liquid, drops of liquid cannot be lifted by the gas. Liquid then, if continuously supplied, collects at the bottom and can only be lifted when enough of it has been gathered so that for some time a foam is formed.