The theory of gravitational separation of elements under the combined influence of gravity and electric fields (Schatzman 1958) has been very successful in explaining the general mono-elemental composition observed in most hot white dwarf atmospheres. Today the exceptions to the rule - DO and DAO with mixed compositions (Wesemael et al.1985), DA with traces of helium (Kahn et al. 1984, Petre et al. 1986, Jordan et al.1987, Paerels 1987), DAB (Liebert et al. 1984), and DBA (Shipman et al. 1987) - pose a problem, because they seem to demand a mechanism that counteracts gravitational separation. Such a mechanism that satisfies the observational constraints is very hard to find, and this has led in recent years to the growing conviction that we indeed observe the equilibrium state of diffusion, but in white dwarfs with extremely thin hydrogen layers that remain transparent (at least in the EUV) in many objects (Jordan and Koester 1986, Liebert et al.1987, Vennes et al. 1987, Fontaine et al.1988, Vennes et al.1988).