Bitumen is a general term for a number of natural flammable pitchy or oily substances, consisting of hydrocarbons, generally to some extent oxygenated, and sometimes containing a little nitrogen. The liquid forms are called naphtha when thin and light-coloured, petroleum when less fluid and dark yellow or blackish brown, and maltha when very viscid. The solid forms are known under the general name asphalt (q.v.). They apparently originate, at least in some cases, from the natural distillation of organic matter, the petroleum of Pennsylvania coming from Old Red Sandstone or Silurian rocks, the most limpid and volatile oils from the deepest borings. The asphalt of Trinidad is derived from lignite beds in underlying clay. As colourless naphtha flows from the ground it partly evaporates, takes up oxygen and becomes brown and thick petroleum, or ultimately solid glassy asphalt. Related minerals are elaterite, elastic bitumen or mineral caoutchouc; albertite, a brittle black asphalt; ozokerite, a native paraffin; hatchettine, or mineral tallow; and torbanite, or boghead coal. Solid paraffin and other pure hydrocarbons are obtainable from all these substances by fractional distillation. Solid bitumen was used by Niepce in his photographic printing process, which depended on the fact that after long exposure to light the bitumen became insoluble in its ordinary solvents, as oil of lavender.