A discussion will be given of the properties of a dissolved atom, with particular reference to those properties which can be assigned to the exact many-electron wave-function of the metal and those which can be defined only in terms of an approximate model. For example, as Anderson has emphasized, one can ask whether there is an uncompensated spin moment in the neighbourhood of a dissolved atom or whether there is none. The concept of bound states is discussed in this connection ; the writer believes there to be no exact division between the situation'in which there is a bound state present and that in which there is not. In terms of models, the nature of the wave-functions round a dissolved atom is discussed, particularly with reference to...