about their central axes and separately driven the first conveying element possesses a substantially smooth circumferential surface i e its cross section is essentially circular or straightlined without elevations the circumferential surface of the second conveying element is provided with a number of elevations in the form of freely rotatable rollers the circumferences of which are small compared with the circumferences of the conveying elements the rollers are fastened to the circumference of the second conveying element by their axes of rotation as a result of the rollers of the second conveying element sections of the tubing are pressed against the smooth circumferential surface of the first conveying element over the entire width of the tubing and perpendicularly to its longitudinal axis so that the tubing is laid completely flat in those areas where pressure is exerted between two compressed portions of the tubing a portion of the total quantity of liquid is enclosed which by the rotation of the rollers of the second conveying element can be moved in the direction in which the tubing is conveyed or alternatively in the opposite direction the speed of the rollers and thus the quantity of liquid conveyed depends on the speed of rotation of the second conveying element the second conveying element is a rotatably driven unit rotating about its axis of rotation in the case where the second conveying element is wheel or roller shaped and about a driven guide roller in the case where the second conveying element is in the form of an endless belt in general the two conveying elements are driven at different speeds with the first conveying element being used merely to.