The predominant use of hydrocarbons is as a combustible fuel source. Methane is the predominant component of natural gas. The C6 through C10 alkanes, alkenes and isomeric cycloalkanes are the top components of gasoline, naphtha, jet fuel and specialized industrial solvent mixtures. With the progressive addition of carbon units, the simple non-ring structured hydrocarbons have higher viscosities, lubricating indices, boiling points, solidification temperatures, and deeper color. At the opposite extreme from methane lie the heavy tars that remain as the "lowest fraction" in a crude oil refining retort. They are collected and widely utilized as roofing compounds, pavement composition (bitumen), wood preservatives (the creosote series) and as extremely high viscosity shear-resisting liquids.