Verterium
Sensor sweep for verterium isotope concentration
Verterium was a type of isotopic substance that, in its gaseous form, was invisible with a distinctive smell. The gas was poisonous to humanoid life, whereas exposure could cause a quick death without the use of a filter mask or the reintroduction of oxygen; Changelings, however, were seemingly immune to its effects.
Under normal circumstances, verterium gas was monitored by atmospheric diagnostics aboard Federation starships, and its isotopic concentration could be measured by a standard sensor sweep. (PIC: "Seventeen Seconds")
Verterium was used aboard Federation Intrepid-class starships in the form of verterium cortenide, which was a densified composite material used in the construction of the inner layer of warp coils. When irradiated, such as by a plasma burst, this inner layer of the warp coils can be burned away. The verterium cortenide composite, itself, was made from polysilicate verterium and monocrystal cortenum. (VOY: "Investigations")
Similarly, verterium gas was used aboard Constitution III-class starships for insulating warp coils contained within valve controlled pipes. (PIC: "Seventeen Seconds")
The planet Hemikek, located in the Delta Quadrant, was "quite rich in minerals" according to Neelix, who, in 2372, thought that USS Voyager could acquire some verterium and cortenum from the consortium of non-aggressive people who owned the mining rights on that planet. (VOY: "Investigations")
A sabotaged verterium gas line
In 2401, the Changeling saboteur disguised as the Starfleet Ensign Eli Foster, sabotaged the USS Titan-A by disabling the atmospheric diagnoistics then phasering a verterium gas line allowed the ship to be surreptitiously tracked by Vadic, from aboard the Shrike, with a mass spectrometer. (PIC: "Seventeen Seconds")