This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
Petroleum ether ( aka petroleum spirits ) is a narrow alkane hydrocarbon
distillate fraction from crude oil. The names "ether" and "spirit" refer
to the very volatile nature of the solvent, and petroleum ether does not
have the ether ( Cx-O-Cy ) linkage, but solely consists of hydrocarbons.
Petroleum ethers are defined by their boiling range, and that is typically
20C. Typical fractions are 20-40C, 40-60C, 60-80C, 80-100C, 100-120C etc.
up to 200C. There are specially refined grades that have any aromatic
hydrocarbons removed, and there are specially named grades, eg pentane
fraction (30-40C), hexane fraction (60-80C, 67-70C). It is important to
note that most "hexane" fractions are mixtures of hydrocarbons, and pure
normal hexane is usually described as "n-hexane".