There the volcanic phenomenon, when taken in its wider implications, made us realize that the upward movement of physical masses, in itself part of the total phenomenon, is due to a dynamic cause which we had to describe, in contrast to centripetally working pressure, as peripherally working suction. Of this concept of suction we must now observe that we may apply it with justification only if we realize that suction can be caused in two different ways. In the sense in which we are wont to use the term, suction is the result of a difference of pressure in adjacent parts of space, the action taking place in the direction of the minor pressure. Apart from this, however, suction can occur also as a result of the outward-bound increase of the strength of a levity-field.