| Answer | Crossword Clue |
|---|---|
| XENON | Hard-to-combine gas |
| getters | any substance introduced into a partial vacuum, as the interior of a vacuum tube or an incandescent lamp, to combine chemically with the residual gas in order to increase the vacuum |
| getter | any substance introduced into a partial vacuum, as the interior of a vacuum tube or an incandescent lamp, to combine chemically with the residual gas in order to increase the vacuum |
| INERT | Hard to combine |
| INERT | Hard to combine, chemically |
| NEON | Gas that's hard to ignore |
