Argon (Ar) in plasma

Argon runs bright: in a flask the ring burns a saturated white with a faint lavender cast, dense enough to light the whole vessel and most of the bench. The same gas in a 40 cm glass torus, run much thinner, does the opposite trick: the ring breaks into standing striations, a chain of evenly spaced luminous clumps set by the plasma's own instability rather than by anything in the glass. Backed off in the flask it drops the white altogether and runs frankly pink, which is what its own line list predicts. All three regimes are in the gallery.

Ring becomes unstable over 30 torr