What every seventh grader knows today was not at all trivial at the time. On the contrary, it was a great discovery that buried the theory of phlogiston forever – a special fiery substance that is contained in all combustible substances and released from them when burning. The discovery was not easy – Lavoisier was busy with the study of hydrogen, or, as they said back then, “flammable air,” for almost ten years. Simultaneously with Lavoisier, water as a product of combustion of hydrogen was also discovered by English physicist Henry Cavendish. But he understood the opposite – he decided that hydrogen consists of water and phlogiston.