  A short review of the problems which arise in the generalization of the
Lifshitz theory of van der Waals force in the case of forces inside dielectric
media is presented, together with some historical remarks. General properties
of the stress tensor of equilibrium electromagnetic field in media are
discussed, and the importance of the conditions of mechanical equilibrium is
stressed. The physical meaning of the repulsive van der Waals interaction
between bodies immersed in a liquid is discussed.
