given simple written labels, such as O for oxygen, or Fe for iron, with proportions denoted by numbers—is the same basic system used today. The only difference is that instead of the subscript number used today (e.g., H2O), Berzelius used a superscript (H2O). Berzelius is credited with identifying the chemical elements silicon, selenium, thorium, and cerium. Students working in Berzelius's laboratory also discovered lithium and vanadium. Berzelius developed the radical theory of chemical combination, which holds that reactions occur as stable groups of atoms called radicals are exchanged between molecules. He believed that salts are compounds formed of acids and bases, and discovered that the anions in acids were attracted to a positive electrode (the anode), whereas the cations in a base were attracted to a negative electrode (the cathode). Berzelius did not believe in the Vitalism Theory, but instead in a regulative force which produced organization of tissues in an organism. Berzelius is also credited with originating the chemical terms "catalysis", "polymer", "isomer", and "allotrope", although his original definitions differ dramatically from modern usage. For example, he coined the term "polymer" in 1833 to describe organic compounds which shared identical empirical formulas but which differed in overall molecular weight, the larger of the compounds being described as "polymers" of the smallest. By this long-superseded, pre-structural definition, glucose (C6H12O6) was viewed as a polymer of formaldehyde (CH2O). New elements and gas laws English chemist Humphry Davy was a pioneer in the field of electrolysis, using Alessandro Volta's voltaic pile to split up common compounds and thus isolate a series of new elements. He went on to electrolyse molten salts and discovered several new metals, especially sodium and potassium, highly reactive elements known as the alkali metals. Potassium, the first metal that was isolated by electrolysis, was discovered in 1807 by Davy, who derived it from caustic potash (KOH). Before the 19th century, no distinction was made between potassium and sodium. Sodium was first isolated by Davy in the same year by passing an electric current through molten sodium hydroxide (NaOH). When Davy heard that Berzelius and Pontin prepared calcium amalgam by electrolyzing lime in mercury, he tried it himself. Davy was successful, and discovered calcium in 1808 by electrolyzing a mixture of lime and mercuric oxide. He worked with electrolysis throughout his life and, in 1808, he isolated magnesium, strontium and barium. Davy also experimented with gases by inhaling them.