- substances and preparations which may become hot and finally catch fire in contact with air at ambient temperature without any application of energy, or - solid substances and preparations which may readily catch fire after brief contact with a source of ignition and which continue to burn or to be consumed after removal of the source of ignition, or - liquid substances and preparations having a very low flash-point, or - substances and preparations which, in contact with water or damp air, evolve extremely flammable gases in dangerous quantities