The thorium emanation is given off more or less freely from the compounds of thorium, the best source, according to Professor Soddy, being a preparation of radiothorium in a moist condition. The important members of this group from the medical point of view are mesothorium, thorium X and its emanation.

The presence of the emanation can by shown by the ionising effect in discharging an electroscope, by the phosphorescence it imparts to zinc sulphide, and by its behaviour as a gas. The ionising effect can be produced at a considerable distance, and in consequence after a certain lapse of time. At a distance of ten feet there is a delay of about half a minute before the leaves begin to fall; when once the fall has begun, it continues steadily until the leaves are discharged.