1911 Encyclopædia Britannica/Crucial

CRUCIAL (from Lat. crux, a cross), that which has the form of a cross, as the “crucial ligaments” of the knee-joint, which cross each other, connecting the femur and the tibia. From Francis Bacon’s expression instantia crucis (taken, as he says, from the finger-post or crux at cross-roads) for a phenomenon which decides between two causes which have each similar analogies in its favour, comes the use of “crucial” for that which decides between two alternatives, hence, generally, as a synonym for “critical.” The word is also used, with a reference to the use of a “crucible,” of something which tests and tries.