Although the officinal substances included in this chapter differ widely from each other in many respects, yet their relations to oxygen form a connecting link between them. Sulphur belongs to the same chemical group as oxygen. The chief action of charcoal is its power of oxidising organic substances by means of oxygen which it has condensed in its pores. The halogens probably owe their disinfecting properties in great measure to their power of liberating oxygen from water in the presence of organic matter which they thus oxidise and destroy.