John McPhee on firewood

Excerpt from “A Reporter at Large: Firewood” in The New Yorker, 25 March 1974. The full essay is in McPhee’s collection Pieces of the Frame.

Science was once certain that firewood was full of something called phlogiston, a mysterious inhabitant that emerged after kindling and danced around in the form of light and heat and crackling sound – phlogiston, the substance of fire. Science, toward the end of the eighteenth century, erased that beautiful theory, replacing it with certain still current beliefs, which are related to the evident fact that green wood is half water. Seasoning, it dries down until, typically, the water content is twenty per cent. Most hardwoods – oak, maple, cherry, hickory – will season in six months. Ash, the firewood of kings, will season in half the time. When firewood burns, it makes vapor of the water. The rest of the log is (almost wholly) carbon, hydrogen, and oxygen – the three components of cellulose, also of starch and sugar. When a log is thrown on the fire, the molecules on the surface become agitated and begin to move vigorously. Some vibrate. Some rotate. Some travel swiftly from one place to another. The cellulose molecule is long, complicated, convoluted – thousands of atoms like many balls on a few long strings. The strings have a breaking point. The molecule, tumbling, whipping, vibrating, breaks apart. Hydrogen atoms, stripping away, snap onto oxygen atoms that are passing by in the uprushing stream of air, forming even more water, which goes up the chimney as vapor. Incandescent carbon particles, by the tens of millions, leap free of the log and wave like banners, as flame. Several hundred significantly different chemical reactions are now going on. For example, a carbon atom and four hydrogen atoms, coming out of the breaking cellulose, may lock together and form methane, natural gas. The methane, burning (combining with oxygen), turns into carbon dioxide and water, which also go up the flue. If two carbon atoms happen to come out of the wood with six hydrogen atoms, they are, agglomerately, ethane, which burns to become, also, carbon dioxide and water. Three carbons and eight hydrogens form propane, and propane is there, too, in the fire. Four carbons and ten hydrogens – butane. Five carbons…pentane. Six…hexane. Seven…heptane. Eight carbons and eighteen hydrogens – octane. All these compounds come away in the breaking of the cellulose molecule, and burn, and go up the chimney as carbon dioxide and water. Pentane, hexane, heptane, and octane have a collective name. Logs burning in a fireplace are making and burning gasoline.