Phaser coolant
From Memory Alpha, the free Star Trek reference.
Phaser coolant is a purple gaseous substance which prevents phasers from overheating and causing damage to the unit or surrounding structures or circuitry. The gas is harmful to anyone who aspirates it. Effects of inhaling the gas range from simple irritation and respiratory distress to death.
The phaser room became engulfed with phaser coolant when battling a Romulan Bird-of-Prey in 2266. Lieutenant Robert Tomlinson became overwhelmed by the gas and later died from the gas' exposure shortly before he was due to marry his fiancée, Crewman Specialist Angela Martine. (TOS: "Balance of Terror")
