The discovery of the closed-cage fullerene and related structures raises numerous questions in relation to the properties, both electrical and magnetic, of amorphous carbons prepared by the thermal decomposition of organic materials in an inert atmosphere. It is suggested that non-planar structures containing pentagonal rings, which are closely related to fullerenes, can arise in amorphous carbons, and free radical material, in particular, probably odd-alternate n-radicals, may be trapped by the cage environment provided by non-planar carbon networks. Such an occurrence leads to a density of states profile by which the electrical behaviour of carbons can be rationalized. A half-filled band of localized mid-gap states is predicted to give rise to a metal-insulator transition at a spin concentration and minimum metallic conductivity of the same magnitude as is measured experimentally.