THE slow neutrons leaving a block of paraffin wax, containing a source of fast neutrons, mostly possess thermal energies. The existence of such thermal neutrons has been established by the discovery1 that the temperature T of the paraffin has an influence on the activities induced in various elements. Since the neutrons penetrating a screen of cadmium do not show this effect2, cadmium may be used to discriminate between the slowest neutrons (C-neutrons3) and the faster ones.