We have seen, in Chapter L, how the idea of an " element " as a constituent of compounds gradually became more defined. As fresh discoveries were made, it was found that certain substances could not further be decomposed, yielding new constituents. But it is not easy always to determine whether or no a substance is an element. For certain compounds are very stable, that is, are very difficult to decompose ; and it has happened several times that such compounds were mistaken for elements. A remarkable instance is a copper-coloured body, found in the debris left in the hearth of an old iron furnace, which was for long supposed to be the element titanium ; more careful investigation, however, proved it to be a compound of titanium with nitrogen and carbon.