that of nitrogen and oxygen molecules in air. Hence, xenon vibrates more slowly in the vocal cords when exhaled and produces lowered voice tones (low-frequency-enhanced sounds, but the fundamental frequency or pitch doesn't change), an effect opposite to the high-toned voice produced in helium. Specifically, when the vocal tract is filled with xenon gas, its natural resonant frequency becomes lower than when it's filled with air. Thus, the low frequencies of the sound wave produced by the same direct vibration of the vocal cords would be enhanced, resulting in a change of the timbre of the sound amplified by the vocal tract. Like helium, xenon does not satisfy the body's need for oxygen, and it is both a simple asphyxiant and an anesthetic more powerful than nitrous oxide; consequently, and because xenon is expensive, many universities have prohibited the voice stunt as a general chemistry demonstration. The gas sulfur hexafluoride is similar to xenon in molecular weight (146 versus 131), less expensive, and though an asphyxiant, not toxic or anesthetic; it is often substituted in these demonstrations. Dense gases such as xenon and sulfur hexafluoride can be breathed safely when mixed with at least 20% oxygen. Xenon at 80% concentration along with 20% oxygen rapidly produces the unconsciousness of general anesthesia (and has been used for this, as discussed above). Breathing mixes gases of different densities very effectively and rapidly so that heavier gases are purged along with the oxygen, and do not accumulate at the bottom of the lungs. There is, however, a danger associated with any heavy gas in large quantities: it may sit invisibly in a container, and a person who enters an area filled with an odorless, colorless gas may be asphyxiated without warning. Xenon is rarely used in large enough quantities for this to be a concern, though the potential for danger exists any time a tank or container of xenon is kept in an unventilated space. Water-soluble xenon compounds such as monosodium xenate are moderately toxic, but have a very short half-life of the body — intravenously injected xenate is reduced to elemental xenon in about a minute. See also Buoyant levitation Noble gases Penning mixture References External links Xenon at The Periodic Table of Videos (University of Nottingham) USGS Periodic Table – Xenon EnvironmentalChemistry.com – Xenon Sir William Ramsay's Nobel-Prize lecture (1904) Chemical elements Noble gases Dissociative drugs General anesthetics Glycine receptor agonists