These Pyrex® Kuderna-Danish concentrator tubes do not have spring hooks but instead use Keck clamps to attach to flasks. The insulated tip Kuderna-Danish tubes have an air jacket around the tip. Due to this insulation less heat reaches the tip, reducing the chance that the sample will boil to dryness. These tubes are compatible with all standard Kuderna-Danish apparatus and come with one Keck clamp. 19/22 ST joint size.