In 1804 the chemist John Dalton put forward the following idea. It is sometimes called 'Dalton's Law'. 'When two elements combine with each other to form more than one compound, the ratios of the masses of one element that combine with a fixed mass of the other element are simple whole numbers.' A student used the apparatus shown to find out if Dalton's Law is true for three oxides of lead. Methane gas reduced the heated lead oxides to lead. [Figure of apparatus showing porcelain boats with lead oxides A, B, C, heated by methane gas, with excess methane burning] Lead and oxides of lead are harmful by inhalation and if swallowed. They are very toxic to aquatic organisms and may cause long-term damage in the aquatic environment.