In a mercury‐vapor trap of appropriate design an alloy of sodium with 5 to 10 percent potassium is more satisfactory than either of the metals used alone. The trap does not need reactivation at intervals and, therefore, during its life affords continuous protection from mercury vapor. Over a period of twenty‐two days a vacuum system protected by this type of trap was pumped repeatedly from atmospheric pressure to 5×10−7mm of Hg; during this time the trap required no attention.