IN searching for evidence that would enable a decision to be made as to the exact distribution of the electrons responsible for the series of fifteen rare earth elements from lanthanum to lutecium, the colour of their salts appears to be of prime importance, particularly as it is known that colour is intimately associated with movements of electrons in atoms. The fact that many of the rare earths yield coloured salts has led to considerable uncertainty as to their proper place in the periodic classification, and many attempts have been made to allocate them to appropriate periodic groups on the grounds of valency, isomorphism, and colour analogies.