A gaseous hydrocarbon, C₄H₁₀; d. 0.58 g?cm^-3; m.p. –138°C; b.p. 0°C. Butane is obtained from petroleum (from refinery gas or by cracking higher hydrocarbons). The fourth member of the alkane series, it has a straight chain of carbon atoms and is isomeric with 2-methylpropane (CH₃CH(CH₃)CH₃, formerly called isobutane). It can easily be liquefied under pressure and is supplied in cylinders for use as a fuel gas. It is also a raw material for making buta-1,3-diene (for synthetic rubber).