Saltpetre — Americans spell it saltpeter — is a white, salty-looking crystal, mostly what chemists call potassium nitrate. The old name means salt of the rock, because it was found as a crust on stone and damp walls. For centuries people needed it to make gunpowder (mixed with charcoal and sulphur) and to help matches catch fire; it was also used to preserve meat and, later, to feed fields. You will meet the word in history, old recipes, and chemistry, not at the dinner table. British spelling keeps the -re ending, as in theatre. It is not the same as table salt, even though English speakers reshaped the first part to look like salt — and, as it happens, English salt and the Latin word for salt in the old name really are relatives.