Phaser coolant

Phaser coolant leaks from the seal

Phaser coolant was a substance used to prevent shipboard phasers from overheating. It came in the form of a toxic gas, and prolonged exposure to it could be fatal.

In 2266, the phaser coolant seal on the USS Enterprise was breached due to damage caused by an attacking Romulan Bird-of-Prey, causing coolant to fill the phaser control room. Both crewmembers manning the phasers, Lieutenants Tomlinson and Stiles, were rendered unconscious by the fumes. Although Stiles was saved by Spock, Tomlinson died as a result of his exposure. (TOS: "Balance of Terror")

In the revised final draft script of "Balance of Terror", the phaser coolant was described firstly as a "blue-greenish gas" and, later in the teleplay, "green", despite the substance actually being colored purple in the final version of the episode.
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