With "field-valence forces" Müller probably means "Van der Waals's mean-field approximation". The way in which Van der Waals had taken account of the attractive forces between particles of a liquid was to assume that each molecule experiences a uniform average attractive force due to its interactions with all the others. Because of these attractions, the particles are pulled together like stars in a galaxy, and so the outwards pressure that they exert on the walls of a container is less than it would be if there were no attractive forces at all. This way of treating the attractive forces is valid only if the force exerted by each particle acts over much larger distances than the average separation between them. Only then does each particle 'feel' enough others to experience a uniform mean attraction. Today, an assumption like this is called a mean-field approximation, meaning that the individual components of the system are assumed to experience a uniform mean field of attraction due to all the others.