[Gk. hydor, water; and the root of gennao, to produce.] The lightest of the chemical elements. Hydrogen is a colorless and, when pure, tasteless gas. It is very inflammable, gives little light, but its flame is one of the hottest known. It is never found alone, but is always present in water. Pure hydrogen gas is about 141/2 times lighter than atmospheric air. It is generated by pouring dilute sulphuric acid upon zinc and collecting the gas in a receiver over water.