It was not until the 1930s that anaesthetic agents were developed in the light of some understanding of structure-activity relationships. Probably the first to be investigated with this principle in mind was divinyl ether when, in 1930, Leake and Chen thought the compound might combine some of the advantages of diethyl ether and ethylene [4]. Previously, it had been chance observation, as much as any other factor, which brought a new compound to the attention of chemists and physicians. For instance, Arno Luckhardt first suspected that the gas ethylene might have anaesthetic properties when learning that the botanists, Crocker and Knight, had shown that it prevented carnation buds from opening [36]. The anaesthetic potential of halogens had long been known, but the particular advantages of fluorine compounds were perceived much earlier than is often appreciated. In a prophetic introduction to their studies on fluorine derivatives of chloroform, published in 1932, Harold Booth and E. May Bixby of Western Reserve University, Cleveland stated [1]: "A survey of the properties of 166 known gases suggested that the best possibility of finding a new noncombustible anesthetic gas lay in the field of organic fluoride compounds. Fluorine substitution for other halogens lowers the boiling point, increases stability, and generally decreases toxicity". Fluorine, the halogen with the lowest atomic weight (18.998; chlorine = 35.453, bromine = 79.904 and iodine = 126.904) was first identified in 1771 by the Swedish chemist, Karl Scheele, but it was not isolated until 1886, when Henri Moissan used platinum electrodes at low temperatures to separate fluorine from hydrogen fluoride. Fluorine is very active chemically and although it KEY WORDS Anaesthetics, volatile: desflurane, sevoflurane.