Mercury bubblers are avoided where possible, due to the difficulty and expense in dealing with mercury spills. In addition, mercury can react with various chemicals, such as ammonia and acetylenes, with potentially hazardous results. In the context of cannula transfer, mercury bubblers may be avoided by pulling (instead of pushing) liquids through the cannula by vacuum. Alternatively, the oil bubbler may be closed off from the inert gas line with a stopcock, allowing inert gas to push the reagent through the cannula. Excess pressure is bled off from a bubbler fitted directly over the receiving vessel.