Word History: In a Latin work dating from 1620, the English philosopher and essayist Francis Bacon used the phrase instantia crucis, "crossroads instance," to refer to something in an experiment that proves one of two hypotheses and disproves the other. The word crucis in Bacon's phrase is the genitive form of the Latin word crux. Crux originally meant "cross" but had also developed the meaning "a guidepost that gives directions at a place where one road becomes two" and hence was suitable for Bacon's metaphor. Both Robert Boyle, often called the father of modern chemistry, and Isaac Newton used a similar Latin phrase, experimentum crucis, for an experiment that determines which of two hypotheses is valid. When English equivalents for these phrases were created in the 19th century, they became crucial instance and crucial experiment. Through the influence of these phrases, the adjective crucial, which before this time had meant simply "cross-shaped," acquired the sense "vital to the determination of an outcome."