Osmium is one of the precious metals and was discovered in 1803 by Tenant. It occurs naturally in close association with iridium. Osmiridium is an exceptionally hard alloy with a high melting point (2700? C.) and for this reason it is used extensively for tipping fountain-pen nibs. It is also used for electrical contacts, as a catalyst in the preparation of synthetic ammonia, and for measur ing the rapidity of explosion of gun-cotton. Before the introduction of tungsten, electric lamp filaments were sometimes made with it. Osmium is also used in the taking of finger-prints, and osmic acid which is a watery solution of osmium tetroxide is used for activating solutions of chlorate of potas sium; hardening and colouring certain prepara tions and in histology for the fixing and staining of nerves and fat. The precious metals are likely to be used increasingly in the future and it is felt that the possible health risks in the use of their deriva tives should be known at the outset.