Valency. The atoms of different elements have not all the same combining value; an atom of sodium, for instance, will unite with one of chlorine to form common salt; an atom of calcium needs two atoms of chlorine to form its chloride, while one of phosphorus will combine with either three or five. Frankland, in 1672, pointed out, as a deduction from his work on organo-metallic compounds, that metals combine with a definite number of atoms, and the same law holds if, instead of "atoms," we write "radicles." Further, radicles, which can be regarded as forming a complex body by substituting one or more atoms of hydrogen in a simpler body, are also found to possess the same combining power as those atoms of hydrogen. Thus, if a radicle can replace two atoms of hydrogen, it can combine with two atoms of chlorine, just as those two atoms of hydrogen can. Hence the substituting value of an atom or radicle is the same as its combining value, and this is known as its valency. An atom of oxygen or sulphur will either replace or combine with two atoms of hydrogen; so oxygen and sulfur are divalent. An atom of chlorine will replace or unite with only one atom of hydrogen; so it is monovalent, while for a similar reason carbon is regarded as tetravalent. The valency of an element is, however, not a fixed property as is its atomic weight. One atom may possess the combining powers of a different number of hydrogen atoms. Thus carbon in carbon monoxide replaces only two hydrogen atoms, and may be regarded as divalent, while in marsh gas it unites with four atoms, and is therefore tetravalent. Cases of disputed valency are by no means rare; nitrogen in ammonia is undoubtedly trivalent, but in ammonium chloride it is considered by some to be pentavalent, while others regarded as trivalent, the idea of "molecular" compounds being introduced to explain this latter view.