The Chinese are believed to have made use of natural gas as early as the 3rd century AD and its use was common in the USA in the second half of the 19th century. Natural gas was imported into Britain by ship in liquid form from 1959, for use in the new reformer plants. After large quantities natural gas were discovered off the coast of Yorkshire in 1965, the gas industry decided to supply this gas direct to consumers rather than use it to make manufactured gas. However, natural gas, which is predominantly methane, has very different burning properties from manufactured gas (mainly hydrogen and carbon dioxide). It was therefore necessary to adapt or replace every gas appliance in Britain, of which there were around 20 million, in a conversion process which started in 1967 and took ten years to complete. Many of the old appliances which National Gas Museum has were collected because they were could not be converted to use natural gas.