Born in Paris, France, Cagniard de la Tour (kanyar de la toor) was educated at the Ecole Polytechnique and then spent his time as an amateur inventor. In 1819 he invented the disk siren, in which the sound is produced by air blowing through holes in a rotating disk, the pitch being determined by the speed of rotation. He made his most famous discovery in 1822; when he heated certain liquids in sealed tubes he observed that at a particular temperature and pressure the meniscus dividing liquid from vapor disappeared. Under these conditions – known as the critical state – the densities of liquid and vapor become the same and the two are identical.