The primary tests that are applied to gasoline are intended to measure the following qualities: volatility, knock characteristics, tetraethyl lead content, gum content, and sulfur content. The tests relating to these qualities are discussed below in the order named. Other existing methods of test that have official standing, but which are of secondary importance in respect to gasoline, or which are infrequently applied, and so are not included in this discussion, are intended to measure the properties of color, gravity, autogenous ignition temperature, thermal value, viscosity, and content of unsaturated hydrocarbons (acid heat).