Just before the time of Darwin's legendary work, a major breakthrough occurred in chemistry which would eventually have profound implications for biology. In 1834, the French chemist Jean Dumas stated for the first time his "law of substitution." This edict stated that under the right conditions of temperature and pressure, a variety of chemicals, including fluorine, chlorine, bromine, and others, could take the place of hydrogen in any organic compound. This made it possible to synthesize, in the ensuing years, an enormous array of other types of materials, including a number that were indispensable to living organisms. An illustration of the phenomenon of hydrogen substitution is the well-known refrigerant freon, found in many freezing units. Chemists construct the freon molecule by substituting fluorine and chlorine atoms for the hydrogen atoms in a molecule of methane gas.